Using MOSFETs (or not) with Basic stamps & BS2 outputs.
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Posts: 46,084
> Secondly with regards to transistors I was wondering if anyone could
> help me select a sutiable transtor (incl base resistor...) that
> draws less than 5ma Base current and allows me to control a load
> that requires about 100-200 ma.
Yes, let's do that. The MOSFETS are great for switching larger currents
(and yes, I admit that I also sell them), but for your load of 100 to 200
mA, there may be a much better solution...
Three very popular small switching transistors are the 2N3904, 2N4401, and
2N7000. The first two are NPN junction transistors, and the 2N7000 is an
N-channel MOSFET. All are readily available in the plastic TO-92 case. As
you will see, one of these is a great match for your application, and the
other two aren't...
The NPN junction transistors use a small (base) current to switch a larger
load (collector) current, and the MOSFET uses a voltage applied to the gate
to control the load current.
The "gain" of the NPN transistors is an important concept, and gives an idea
how much base current it will take to effectively turn on or "saturate" the
transistor for a certain load current. More load current will require more
base current, and the amount depends on the gain.
Using a single gain figure for a given transistor can be misleading. For
example, take a look at the gain figures for a 2N3904 on the second page of
this datasheet:
http://www.fairchildsemi.com/ds/2N/2N3904.pdf
Notice in the ON CHARACTERISTICS section that the DC current gain is listed
for a number of different collector (load) currents. As you can see, the
gain peaks at around 10 mA of collector current, and goes down as the
collector current is increased or decreased. At your load current of 100 mA
or more, this device will definitely not be at its best.
Now take a look at the 2N4401 data sheet (also on page 2 in the same place):
http://www.fairchildsemi.com/ds/2N/2N4401.pdf
How about that? Peak gain at 150 mA -- right in the middle of your target
load current! This might be the one, but let's dig a bit deeper...
I just breadboarded the circuit with a 2N4401 connected in a common-emitter
configuration. In other words, the emitter is grounded, the base is
connected to the I/O pin through a resistor, and the collector is connected
to the load. The other side of the load -- which is a 25 ohm resistor -- is
connected to +5 volts. If the voltage drop across the transistor is
reasonably low when it's on or "saturated" then the load current will be
nearly 200 mA. I used a 1.0 k ohm base resistor, which will supply just
over 4 mA of base current given the base voltage of about .85 volts and the
I/O pin output of just under 5 volts.
With the transistor on, the voltage drop across the collector-emitter
(generally called Vce) is a nice 0.186 volts, which means that most of the
power is getting to your load, and not too much is turning into heat in the
transistor. Since power = volts x current, the wattage being "dissipated"
(turned into heat) in the transistor is 0.186V x 200 mA = 0.04 W -- well
under the 0.5 W allowed at ambient temperatures up to 50 deg C, as shown on
the lower graph on page 4 of the datasheet.
So, what have we seen so far? The 2N3904 is a very popular and useful
device, but for switching much more than 10 to 25 mA, it won't be as
efficient in terms of base current vs. load current as the 2N4401. The
2N4401 is nice up to 200 mA, and perhaps a bit more -- but then it will
begin to go downhill like the 2N3904 did at the lower current. It will no
longer be in its "sweet spot."
What's next if the load current is well above 200 mA? That's where the
IRL520, IRL530, IRL540, etc. MOSFETs begin to look very nice. High-gain NPN
Darlington transistors like the TIP120 are also popular, because they can
switch much larger currents without needing much base current.
Unfortunately, they have a substantial built-in voltage drop between the
collector and emitter (Vce, again) that cannot be eliminated no matter how
much base current you use. That means that there is less power available to
your load, and more heat being dissipated in the transistor. I posted a
detailed comparison between the IRL520 and TIP120 on this list last
February, and the short version is that the IRL520 blew away the TIP120 for
common low-speed switching applications at the 1,000 mA load current used
for testing.
But what about cost -- that usually needs to be considered. The 2N3904 and
2N4401 can be had at 12 for 98 cents. TIP120s for .39 each and IRL520s for
.68 each. Before the TIP120 starts looking better than the IRL520, consider
that in the test I posted last February, the TIP 120 was dissipating *three
times* the heat of the IRL520 due to that inescapable high Vce in the
Darlingtons -- so you may well need a heat sink for the TIP120 but not for
the IRL520. And that changes the economics in favor of the IRL520.
And, what about the 2N7000? If you study the data sheet:
http://www.fairchildsemi.com/ds/2N/2N7000.pdf
-- particularly Fig. 2 on the upper right corner of page 4 -- you'll see
that 5 volts at the gate doesn't turn it on nearly as well as 10 volts.
According to the graph, with a gate voltage (Vgs) of 5 volts and a load
current (Id) of 200 mA (0.2 A), the on resistance will be about 1.5 ohms --
which is not too good for this application. Based on these numbers, there
would be a voltage drop across the 2N7000 of about 0.3 V in my test circuit
above -- and that's much worse than the less expensive 2N4401. Remember
that any voltage drop across the transistor represents power that the load
isn't getting -- since the available voltage is being divided between the
load and the switching device. Ideally, the switching device would have no
voltage drop when on, but that doesn't happen. Lower is better, as long as
you aren't spending a lot of $ to make it lower than necessary.
Whew... There is a lot more to this subject, but I think this is
long-winded enough. Hope it helps!
Randy
www.glitchbuster.com
PS -- there are many more good transistors available that work well in these
applications. I used the part numbers above because they are very
well-known, and represent a good value in the common switching applications
that are often discussed on this list.
> help me select a sutiable transtor (incl base resistor...) that
> draws less than 5ma Base current and allows me to control a load
> that requires about 100-200 ma.
Yes, let's do that. The MOSFETS are great for switching larger currents
(and yes, I admit that I also sell them), but for your load of 100 to 200
mA, there may be a much better solution...
Three very popular small switching transistors are the 2N3904, 2N4401, and
2N7000. The first two are NPN junction transistors, and the 2N7000 is an
N-channel MOSFET. All are readily available in the plastic TO-92 case. As
you will see, one of these is a great match for your application, and the
other two aren't...
The NPN junction transistors use a small (base) current to switch a larger
load (collector) current, and the MOSFET uses a voltage applied to the gate
to control the load current.
The "gain" of the NPN transistors is an important concept, and gives an idea
how much base current it will take to effectively turn on or "saturate" the
transistor for a certain load current. More load current will require more
base current, and the amount depends on the gain.
Using a single gain figure for a given transistor can be misleading. For
example, take a look at the gain figures for a 2N3904 on the second page of
this datasheet:
http://www.fairchildsemi.com/ds/2N/2N3904.pdf
Notice in the ON CHARACTERISTICS section that the DC current gain is listed
for a number of different collector (load) currents. As you can see, the
gain peaks at around 10 mA of collector current, and goes down as the
collector current is increased or decreased. At your load current of 100 mA
or more, this device will definitely not be at its best.
Now take a look at the 2N4401 data sheet (also on page 2 in the same place):
http://www.fairchildsemi.com/ds/2N/2N4401.pdf
How about that? Peak gain at 150 mA -- right in the middle of your target
load current! This might be the one, but let's dig a bit deeper...
I just breadboarded the circuit with a 2N4401 connected in a common-emitter
configuration. In other words, the emitter is grounded, the base is
connected to the I/O pin through a resistor, and the collector is connected
to the load. The other side of the load -- which is a 25 ohm resistor -- is
connected to +5 volts. If the voltage drop across the transistor is
reasonably low when it's on or "saturated" then the load current will be
nearly 200 mA. I used a 1.0 k ohm base resistor, which will supply just
over 4 mA of base current given the base voltage of about .85 volts and the
I/O pin output of just under 5 volts.
With the transistor on, the voltage drop across the collector-emitter
(generally called Vce) is a nice 0.186 volts, which means that most of the
power is getting to your load, and not too much is turning into heat in the
transistor. Since power = volts x current, the wattage being "dissipated"
(turned into heat) in the transistor is 0.186V x 200 mA = 0.04 W -- well
under the 0.5 W allowed at ambient temperatures up to 50 deg C, as shown on
the lower graph on page 4 of the datasheet.
So, what have we seen so far? The 2N3904 is a very popular and useful
device, but for switching much more than 10 to 25 mA, it won't be as
efficient in terms of base current vs. load current as the 2N4401. The
2N4401 is nice up to 200 mA, and perhaps a bit more -- but then it will
begin to go downhill like the 2N3904 did at the lower current. It will no
longer be in its "sweet spot."
What's next if the load current is well above 200 mA? That's where the
IRL520, IRL530, IRL540, etc. MOSFETs begin to look very nice. High-gain NPN
Darlington transistors like the TIP120 are also popular, because they can
switch much larger currents without needing much base current.
Unfortunately, they have a substantial built-in voltage drop between the
collector and emitter (Vce, again) that cannot be eliminated no matter how
much base current you use. That means that there is less power available to
your load, and more heat being dissipated in the transistor. I posted a
detailed comparison between the IRL520 and TIP120 on this list last
February, and the short version is that the IRL520 blew away the TIP120 for
common low-speed switching applications at the 1,000 mA load current used
for testing.
But what about cost -- that usually needs to be considered. The 2N3904 and
2N4401 can be had at 12 for 98 cents. TIP120s for .39 each and IRL520s for
.68 each. Before the TIP120 starts looking better than the IRL520, consider
that in the test I posted last February, the TIP 120 was dissipating *three
times* the heat of the IRL520 due to that inescapable high Vce in the
Darlingtons -- so you may well need a heat sink for the TIP120 but not for
the IRL520. And that changes the economics in favor of the IRL520.
And, what about the 2N7000? If you study the data sheet:
http://www.fairchildsemi.com/ds/2N/2N7000.pdf
-- particularly Fig. 2 on the upper right corner of page 4 -- you'll see
that 5 volts at the gate doesn't turn it on nearly as well as 10 volts.
According to the graph, with a gate voltage (Vgs) of 5 volts and a load
current (Id) of 200 mA (0.2 A), the on resistance will be about 1.5 ohms --
which is not too good for this application. Based on these numbers, there
would be a voltage drop across the 2N7000 of about 0.3 V in my test circuit
above -- and that's much worse than the less expensive 2N4401. Remember
that any voltage drop across the transistor represents power that the load
isn't getting -- since the available voltage is being divided between the
load and the switching device. Ideally, the switching device would have no
voltage drop when on, but that doesn't happen. Lower is better, as long as
you aren't spending a lot of $ to make it lower than necessary.
Whew... There is a lot more to this subject, but I think this is
long-winded enough. Hope it helps!
Randy
www.glitchbuster.com
PS -- there are many more good transistors available that work well in these
applications. I used the part numbers above because they are very
well-known, and represent a good value in the common switching applications
that are often discussed on this list.
Comments
thnx 4 tha info
Arridh
--- In basicstamps@yahoogroups.com, "Randy Jones" <randyjones@w...>
wrote:
> > Secondly with regards to transistors I was wondering if anyone
could
> > help me select a sutiable transtor (incl base resistor...) that
> > draws less than 5ma Base current and allows me to control a load
> > that requires about 100-200 ma.
>
>
> Yes, let's do that. The MOSFETS are great for switching larger
currents
> (and yes, I admit that I also sell them), but for your load of 100
to 200
> mA, there may be a much better solution...
>
> Three very popular small switching transistors are the 2N3904,
2N4401, and
> 2N7000. The first two are NPN junction transistors, and the
2N7000 is an
> N-channel MOSFET. All are readily available in the plastic TO-92
case. As
> you will see, one of these is a great match for your application,
and the
> other two aren't...
>
> The NPN junction transistors use a small (base) current to switch
a larger
> load (collector) current, and the MOSFET uses a voltage applied to
the gate
> to control the load current.
>
> The "gain" of the NPN transistors is an important concept, and
gives an idea
> how much base current it will take to effectively turn on
or "saturate" the
> transistor for a certain load current. More load current will
require more
> base current, and the amount depends on the gain.
>
> Using a single gain figure for a given transistor can be
misleading. For
> example, take a look at the gain figures for a 2N3904 on the
second page of
> this datasheet:
>
> http://www.fairchildsemi.com/ds/2N/2N3904.pdf
>
> Notice in the ON CHARACTERISTICS section that the DC current gain
is listed
> for a number of different collector (load) currents. As you can
see, the
> gain peaks at around 10 mA of collector current, and goes down as
the
> collector current is increased or decreased. At your load current
of 100 mA
> or more, this device will definitely not be at its best.
>
> Now take a look at the 2N4401 data sheet (also on page 2 in the
same place):
>
> http://www.fairchildsemi.com/ds/2N/2N4401.pdf
>
> How about that? Peak gain at 150 mA -- right in the middle of
your target
> load current! This might be the one, but let's dig a bit deeper...
>
> I just breadboarded the circuit with a 2N4401 connected in a
common-emitter
> configuration. In other words, the emitter is grounded, the base
is
> connected to the I/O pin through a resistor, and the collector is
connected
> to the load. The other side of the load -- which is a 25 ohm
resistor -- is
> connected to +5 volts. If the voltage drop across the transistor
is
> reasonably low when it's on or "saturated" then the load current
will be
> nearly 200 mA. I used a 1.0 k ohm base resistor, which will
supply just
> over 4 mA of base current given the base voltage of about .85
volts and the
> I/O pin output of just under 5 volts.
>
> With the transistor on, the voltage drop across the collector-
emitter
> (generally called Vce) is a nice 0.186 volts, which means that
most of the
> power is getting to your load, and not too much is turning into
heat in the
> transistor. Since power = volts x current, the wattage
being "dissipated"
> (turned into heat) in the transistor is 0.186V x 200 mA = 0.04 W --
well
> under the 0.5 W allowed at ambient temperatures up to 50 deg C, as
shown on
> the lower graph on page 4 of the datasheet.
>
> So, what have we seen so far? The 2N3904 is a very popular and
useful
> device, but for switching much more than 10 to 25 mA, it won't be
as
> efficient in terms of base current vs. load current as the
2N4401. The
> 2N4401 is nice up to 200 mA, and perhaps a bit more -- but then it
will
> begin to go downhill like the 2N3904 did at the lower current. It
will no
> longer be in its "sweet spot."
>
> What's next if the load current is well above 200 mA? That's
where the
> IRL520, IRL530, IRL540, etc. MOSFETs begin to look very nice.
High-gain NPN
> Darlington transistors like the TIP120 are also popular, because
they can
> switch much larger currents without needing much base current.
> Unfortunately, they have a substantial built-in voltage drop
between the
> collector and emitter (Vce, again) that cannot be eliminated no
matter how
> much base current you use. That means that there is less power
available to
> your load, and more heat being dissipated in the transistor. I
posted a
> detailed comparison between the IRL520 and TIP120 on this list last
> February, and the short version is that the IRL520 blew away the
TIP120 for
> common low-speed switching applications at the 1,000 mA load
current used
> for testing.
>
> But what about cost -- that usually needs to be considered. The
2N3904 and
> 2N4401 can be had at 12 for 98 cents. TIP120s for .39 each and
IRL520s for
> .68 each. Before the TIP120 starts looking better than the
IRL520, consider
> that in the test I posted last February, the TIP 120 was
dissipating *three
> times* the heat of the IRL520 due to that inescapable high Vce in
the
> Darlingtons -- so you may well need a heat sink for the TIP120 but
not for
> the IRL520. And that changes the economics in favor of the IRL520.
>
> And, what about the 2N7000? If you study the data sheet:
>
> http://www.fairchildsemi.com/ds/2N/2N7000.pdf
>
> -- particularly Fig. 2 on the upper right corner of page 4 --
you'll see
> that 5 volts at the gate doesn't turn it on nearly as well as 10
volts.
> According to the graph, with a gate voltage (Vgs) of 5 volts and a
load
> current (Id) of 200 mA (0.2 A), the on resistance will be about
1.5 ohms --
> which is not too good for this application. Based on these
numbers, there
> would be a voltage drop across the 2N7000 of about 0.3 V in my
test circuit
> above -- and that's much worse than the less expensive 2N4401.
Remember
> that any voltage drop across the transistor represents power that
the load
> isn't getting -- since the available voltage is being divided
between the
> load and the switching device. Ideally, the switching device
would have no
> voltage drop when on, but that doesn't happen. Lower is better,
as long as
> you aren't spending a lot of $ to make it lower than necessary.
>
> Whew... There is a lot more to this subject, but I think this is
> long-winded enough. Hope it helps!
>
> Randy
> www.glitchbuster.com
>
> PS -- there are many more good transistors available that work
well in these
> applications. I used the part numbers above because they are very
> well-known, and represent a good value in the common switching
applications
> that are often discussed on this list.
As I mentioned in my original post, the IRL part is overkill in this case.
However, I've taken to using almost nothing else for two reasons. First, the
switching characteristics of the FET are almost perfect. The on resistance
is very low and the gate isolation is very high. Second, I don't have to
keep a bunch of different things on hand. The IRL part will switch up to 9A
so I can use it for almost anything we normally handle. Yeah, it might be an
extra buck -- and if I were building 50 units with 32 switches I might be
worried about that buck -- but it is sure handy to have one part that
handles almost anything I normally do. We even have standard "switch boards"
that let you prototype more rapidly.
Below 100mA I still favor the 2N2222. I buy those like jelly beans. But I've
largely abandoned bipolar for switching. Darlingtons, as you mention, have
way too many problems. Even single transistors can be problematic since the
beta reduces under several circumstances and you wind up drawing more
current from the Stamp.
The 2N7000 is good, although I still favor the IRL series because the gate
turns on at 5V (as opposed to the IRF which Radio Shack sells). Of course,
you can use a bipolar to switch the gate voltage, but that reduces the
benefit and increases the cost.
So I agree that if you are designing something where you need to mass
produce, you ought to make a more exacting pick. But for one offs, the big
MOSFET switches are cheap enough that you can use them without a lot of
thought process. Since you use more of them, you can buy more of them and
drive the price down even further.
Just my thoughts. By the way, the Stamp FAQ has two pretty good articles on
switching that covers some of this material. Go to
http://www.wd5gnr.com/stampfaq.htm or more specifically:
http://www.wd5gnr.com/stampfaq.htm?article=12
And
http://www.wd5gnr.com/stampfaq.htm?article=13
Note that article #12 mentions the IRF510 (which is available at Radio Shack
although last time I looked they had them marked IFR510). This is not an IRL
part so it does not fully switch on at 5V which is why we switched to the
IRL part.
Regards,
Al Williams
AWC
* Floating point math for the Stamp:
http://www.al-williams.com/pak1.htm
>
Original Message
> From: Randy Jones [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=fITNl_nOtptmuNPdkIrTA_iaA6hl49vyEJ7ewWtis64het225YzJ8WfoiiW3PZnkMmiLqI4RmLI9fygwYCgR0iNt-s_SeA]randyjones@w...[/url
> Sent: Tuesday, December 09, 2003 1:49 AM
> To: basicstamps@yahoogroups.com
> Subject: Re: [noparse][[/noparse]basicstamps] Using MOSFETs (or not) with Basic stamps & BS2
> outputs.
>
> > Secondly with regards to transistors I was wondering if anyone could
> > help me select a sutiable transtor (incl base resistor...) that
> > draws less than 5ma Base current and allows me to control a load
> > that requires about 100-200 ma.
>
>
> Yes, let's do that. The MOSFETS are great for switching larger currents
> (and yes, I admit that I also sell them), but for your load of 100 to 200
> mA, there may be a much better solution...
>
> Three very popular small switching transistors are the 2N3904, 2N4401, and
> 2N7000. The first two are NPN junction transistors, and the 2N7000 is an
> N-channel MOSFET. All are readily available in the plastic TO-92 case.
> As
> you will see, one of these is a great match for your application, and the
> other two aren't...
>
> The NPN junction transistors use a small (base) current to switch a larger
> load (collector) current, and the MOSFET uses a voltage applied to the
> gate
> to control the load current.
>
> The "gain" of the NPN transistors is an important concept, and gives an
> idea
> how much base current it will take to effectively turn on or "saturate"
> the
> transistor for a certain load current. More load current will require
> more
> base current, and the amount depends on the gain.
>
> Using a single gain figure for a given transistor can be misleading. For
> example, take a look at the gain figures for a 2N3904 on the second page
> of
> this datasheet:
>
> http://www.fairchildsemi.com/ds/2N/2N3904.pdf
>
> Notice in the ON CHARACTERISTICS section that the DC current gain is
> listed
> for a number of different collector (load) currents. As you can see, the
> gain peaks at around 10 mA of collector current, and goes down as the
> collector current is increased or decreased. At your load current of 100
> mA
> or more, this device will definitely not be at its best.
>
> Now take a look at the 2N4401 data sheet (also on page 2 in the same
> place):
>
> http://www.fairchildsemi.com/ds/2N/2N4401.pdf
>
> How about that? Peak gain at 150 mA -- right in the middle of your target
> load current! This might be the one, but let's dig a bit deeper...
>
> I just breadboarded the circuit with a 2N4401 connected in a common-
> emitter
> configuration. In other words, the emitter is grounded, the base is
> connected to the I/O pin through a resistor, and the collector is
> connected
> to the load. The other side of the load -- which is a 25 ohm resistor --
> is
> connected to +5 volts. If the voltage drop across the transistor is
> reasonably low when it's on or "saturated" then the load current will be
> nearly 200 mA. I used a 1.0 k ohm base resistor, which will supply just
> over 4 mA of base current given the base voltage of about .85 volts and
> the
> I/O pin output of just under 5 volts.
>
> With the transistor on, the voltage drop across the collector-emitter
> (generally called Vce) is a nice 0.186 volts, which means that most of the
> power is getting to your load, and not too much is turning into heat in
> the
> transistor. Since power = volts x current, the wattage being "dissipated"
> (turned into heat) in the transistor is 0.186V x 200 mA = 0.04 W -- well
> under the 0.5 W allowed at ambient temperatures up to 50 deg C, as shown
> on
> the lower graph on page 4 of the datasheet.
>
> So, what have we seen so far? The 2N3904 is a very popular and useful
> device, but for switching much more than 10 to 25 mA, it won't be as
> efficient in terms of base current vs. load current as the 2N4401. The
> 2N4401 is nice up to 200 mA, and perhaps a bit more -- but then it will
> begin to go downhill like the 2N3904 did at the lower current. It will no
> longer be in its "sweet spot."
>
> What's next if the load current is well above 200 mA? That's where the
> IRL520, IRL530, IRL540, etc. MOSFETs begin to look very nice. High-gain
> NPN
> Darlington transistors like the TIP120 are also popular, because they can
> switch much larger currents without needing much base current.
> Unfortunately, they have a substantial built-in voltage drop between the
> collector and emitter (Vce, again) that cannot be eliminated no matter how
> much base current you use. That means that there is less power available
> to
> your load, and more heat being dissipated in the transistor. I posted a
> detailed comparison between the IRL520 and TIP120 on this list last
> February, and the short version is that the IRL520 blew away the TIP120
> for
> common low-speed switching applications at the 1,000 mA load current used
> for testing.
>
> But what about cost -- that usually needs to be considered. The 2N3904
> and
> 2N4401 can be had at 12 for 98 cents. TIP120s for .39 each and IRL520s
> for
> .68 each. Before the TIP120 starts looking better than the IRL520,
> consider
> that in the test I posted last February, the TIP 120 was dissipating
> *three
> times* the heat of the IRL520 due to that inescapable high Vce in the
> Darlingtons -- so you may well need a heat sink for the TIP120 but not for
> the IRL520. And that changes the economics in favor of the IRL520.
>
> And, what about the 2N7000? If you study the data sheet:
>
> http://www.fairchildsemi.com/ds/2N/2N7000.pdf
>
> -- particularly Fig. 2 on the upper right corner of page 4 -- you'll see
> that 5 volts at the gate doesn't turn it on nearly as well as 10 volts.
> According to the graph, with a gate voltage (Vgs) of 5 volts and a load
> current (Id) of 200 mA (0.2 A), the on resistance will be about 1.5 ohms -
> -
> which is not too good for this application. Based on these numbers, there
> would be a voltage drop across the 2N7000 of about 0.3 V in my test
> circuit
> above -- and that's much worse than the less expensive 2N4401. Remember
> that any voltage drop across the transistor represents power that the load
> isn't getting -- since the available voltage is being divided between the
> load and the switching device. Ideally, the switching device would have
> no
> voltage drop when on, but that doesn't happen. Lower is better, as long
> as
> you aren't spending a lot of $ to make it lower than necessary.
>
> Whew... There is a lot more to this subject, but I think this is
> long-winded enough. Hope it helps!
>
> Randy
> www.glitchbuster.com
>
> PS -- there are many more good transistors available that work well in
> these
> applications. I used the part numbers above because they are very
> well-known, and represent a good value in the common switching
> applications
> that are often discussed on this list.
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
> Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
Forced Beta.
Normally to insure a proper saturation (lowest Vcesat) is recommended
to drive the base with a current ten times the specs current gain.
Also must be noted that when a BJT device is in full saturation, the
storage times increases considerably.
Yep, the IRL520 is wonderful... I use them here too even when they're
overkill. It sure is nice to grab a "no-brainer" switching device that is
perfectly happy with a 5 volt "no current" input and works beautifully with
so many moderate sized loads. And it's easy to substitute the IRL530 or
IRL540 if a lower on resistance is worth the slightly higher cost.
The 2N2222 sure has been a workhorse for years. Interestingly, in the
Fairchild data sheets for the 2N4401 and PN2222 (a 2N2222 in a plastic TO-92
housing, for those who might not be familiar with it), all the performance
graphs for the two devices appear to be 100% identical. Makes me wonder if
they make one device that meets the extremely similar specs for both part
numbers, and sell it under the two numbers. I replaced the 2N4401 with a
PN2222 in that breadboarded circuit and tried some different base and load
currents, with nearly identical readings.
The 100 to 200 mA range seems like the place where it makes sense to go from
a TO-92 part to the IRL520N or something along those lines. I wasn't trying
to contradict your good suggestion about the IRL520N, and hope it didn't
appear that way. The question about selecting and using a transistor seems
fairly common, and I thought I'd try to address some of the considerations
for selecting one.
BTW, I like your new PCB book -- just picked up a copy!
Randy
www.glitchbuster.com
Original Message
From: "Al Williams" <alw@a...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, December 09, 2003 5:27 AM
Subject: RE: [noparse][[/noparse]basicstamps] Using MOSFETs (or not) with Basic stamps & BS2
outputs.
> Great Post Randy.
>
> As I mentioned in my original post, the IRL part is overkill in this case.
> However, I've taken to using almost nothing else for two reasons. First,
the
> switching characteristics of the FET are almost perfect. The on resistance
> is very low and the gate isolation is very high. Second, I don't have to
> keep a bunch of different things on hand. The IRL part will switch up to
9A
> so I can use it for almost anything we normally handle. Yeah, it might be
an
> extra buck -- and if I were building 50 units with 32 switches I might be
> worried about that buck -- but it is sure handy to have one part that
> handles almost anything I normally do. We even have standard "switch
boards"
> that let you prototype more rapidly.
>
> Below 100mA I still favor the 2N2222. I buy those like jelly beans. But
I've
> largely abandoned bipolar for switching. Darlingtons, as you mention, have
> way too many problems. Even single transistors can be problematic since
the
> beta reduces under several circumstances and you wind up drawing more
> current from the Stamp.
>
> The 2N7000 is good, although I still favor the IRL series because the gate
> turns on at 5V (as opposed to the IRF which Radio Shack sells). Of course,
> you can use a bipolar to switch the gate voltage, but that reduces the
> benefit and increases the cost.
>
> So I agree that if you are designing something where you need to mass
> produce, you ought to make a more exacting pick. But for one offs, the big
> MOSFET switches are cheap enough that you can use them without a lot of
> thought process. Since you use more of them, you can buy more of them and
> drive the price down even further.
>
> Just my thoughts. By the way, the Stamp FAQ has two pretty good articles
on
> switching that covers some of this material. Go to
> http://www.wd5gnr.com/stampfaq.htm or more specifically:
> http://www.wd5gnr.com/stampfaq.htm?article=12
> And
> http://www.wd5gnr.com/stampfaq.htm?article=13
>
> Note that article #12 mentions the IRF510 (which is available at Radio
Shack
> although last time I looked they had them marked IFR510). This is not an
IRL
> part so it does not fully switch on at 5V which is why we switched to the
> IRL part.
>
> Regards,
>
> Al Williams
> AWC
> * Floating point math for the Stamp:
> http://www.al-williams.com/pak1.htm
running several times the calculated base current based on the beta. Ten
times may be good insurance for variations between component batches or even
manufacturers when the lowest possible Vsat is required, but I've had good
success with 2.5 to 5 times in many cases. In the example I discussed, it
is probably about 4 times at a 100 mA load current, and about 2 times at 200
mA load current. Feeling a little uncomfortable at the 200 mA load current,
I tried doubling the base current -- but the Vsat only changed very slightly
so I stayed with the original 4+ mA. For a one-off project that would not
be operated under extreme conditions, I'd feel (barely) comfortable with
that -- but definitely not for a product that would be subject to greater
production and user variables.
Randy
www.glitchbuster.com
Original Message
From: "Albert Catano" <acatano2002@y...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, December 09, 2003 5:58 AM
Subject: [noparse][[/noparse]basicstamps] Re: Using MOSFETs (or not) with Basic stamps & BS2
outputs.
> One important parameter with JBTs used in the switched mode is the
> Forced Beta.
> Normally to insure a proper saturation (lowest Vcesat) is recommended
> to drive the base with a current ten times the specs current gain.
>
> Also must be noted that when a BJT device is in full saturation, the
> storage times increases considerably.
Any ideas on how much current the Backlight on a LCD display
actually draws? What sort of transistor and base resistor would I
use to drive this from a controller like a preprogrammed PIC?
BTW thanks for hte info on using mosfets and transistors. [noparse]:)[/noparse]
Arridh
--- In basicstamps@yahoogroups.com, "Randy Jones" <randyjones@w...>
wrote:
> I probably should have included a paragraph mentioning the
importance of
> running several times the calculated base current based on the
beta. Ten
> times may be good insurance for variations between component
batches or even
> manufacturers when the lowest possible Vsat is required, but I've
had good
> success with 2.5 to 5 times in many cases. In the example I
discussed, it
> is probably about 4 times at a 100 mA load current, and about 2
times at 200
> mA load current. Feeling a little uncomfortable at the 200 mA
load current,
> I tried doubling the base current -- but the Vsat only changed
very slightly
> so I stayed with the original 4+ mA. For a one-off project that
would not
> be operated under extreme conditions, I'd feel (barely)
comfortable with
> that -- but definitely not for a product that would be subject to
greater
> production and user variables.
>
> Randy
> www.glitchbuster.com
>
>
>
Original Message
> From: "Albert Catano" <acatano2002@y...>
> To: <basicstamps@yahoogroups.com>
> Sent: Tuesday, December 09, 2003 5:58 AM
> Subject: [noparse][[/noparse]basicstamps] Re: Using MOSFETs (or not) with Basic
stamps & BS2
> outputs.
>
>
> > One important parameter with JBTs used in the switched mode is
the
> > Forced Beta.
> > Normally to insure a proper saturation (lowest Vcesat) is
recommended
> > to drive the base with a current ten times the specs current
gain.
> >
> > Also must be noted that when a BJT device is in full saturation,
the
> > storage times increases considerably.
What kind of LCD are you using... size and/or part number? I've seen some
LCD backlights draw around 80 mA, as I recall, but yours could be quite a
bit more or less than that.
Do you have a multi-meter that you could use to measure the current? If
not, but you have a voltmeter, you could place a 1.0 ohm or 0.1 ohm resistor
in series with the power wire to the backlight and measure the voltage drop
across the resistor. Use ohms law to determine the current: I (current) =
V/R. So, if you read 0.095 volts across a 1 ohm resistor, you would have
0.095 V/1.0 ohm = 0.095 A (95 mA). The idea is to use a resistor that is
small enough so it isn't a big percentage of your main load resistance, but
not so small that you aren't able to get a good reading with your meter.
One ohm would probably be fine for this application.
If you're under 100 mA, a 1 k ohm base resistor with a 2N4401, PN2222, or
2N2222 should be fine. From 100 mA to 150 mA, you might want to increase
the base current by using a resistor around 500 ohms or so.
Randy
www.glitchbuster.com
Original Message
From: "arridh_shashank" <arridh_shashank@y...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, December 09, 2003 10:35 PM
Subject: [noparse][[/noparse]basicstamps] Re: Using MOSFETs (or not) with Basic stamps & BS2
outputs.
> Hey guys,
>
> Any ideas on how much current the Backlight on a LCD display
> actually draws? What sort of transistor and base resistor would I
> use to drive this from a controller like a preprogrammed PIC?
>
> BTW thanks for hte info on using mosfets and transistors. [noparse]:)[/noparse]
>
> Arridh
>
>
>
>
>
> --- In basicstamps@yahoogroups.com, "Randy Jones" <randyjones@w...>
> wrote:
> > I probably should have included a paragraph mentioning the
> importance of
> > running several times the calculated base current based on the
> beta. Ten
> > times may be good insurance for variations between component
> batches or even
> > manufacturers when the lowest possible Vsat is required, but I've
> had good
> > success with 2.5 to 5 times in many cases. In the example I
> discussed, it
> > is probably about 4 times at a 100 mA load current, and about 2
> times at 200
> > mA load current. Feeling a little uncomfortable at the 200 mA
> load current,
> > I tried doubling the base current -- but the Vsat only changed
> very slightly
> > so I stayed with the original 4+ mA. For a one-off project that
> would not
> > be operated under extreme conditions, I'd feel (barely)
> comfortable with
> > that -- but definitely not for a product that would be subject to
> greater
> > production and user variables.
> >
> > Randy
> > www.glitchbuster.com
> >
> >
> >
Original Message
> > From: "Albert Catano" <acatano2002@y...>
> > To: <basicstamps@yahoogroups.com>
> > Sent: Tuesday, December 09, 2003 5:58 AM
> > Subject: [noparse][[/noparse]basicstamps] Re: Using MOSFETs (or not) with Basic
> stamps & BS2
> > outputs.
> >
> >
> > > One important parameter with JBTs used in the switched mode is
> the
> > > Forced Beta.
> > > Normally to insure a proper saturation (lowest Vcesat) is
> recommended
> > > to drive the base with a current ten times the specs current
> gain.
> > >
> > > Also must be noted that when a BJT device is in full saturation,
> the
> > > storage times increases considerably.
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
Its cool I worked it out as you said
thnx
Arridh
--- In basicstamps@yahoogroups.com, "Randy Jones" <randyjones@w...>
wrote:
> Hi Arridh,
>
> What kind of LCD are you using... size and/or part number? I've
seen some
> LCD backlights draw around 80 mA, as I recall, but yours could be
quite a
> bit more or less than that.
>
> Do you have a multi-meter that you could use to measure the
current? If
> not, but you have a voltmeter, you could place a 1.0 ohm or 0.1
ohm resistor
> in series with the power wire to the backlight and measure the
voltage drop
> across the resistor. Use ohms law to determine the current: I
(current) =
> V/R. So, if you read 0.095 volts across a 1 ohm resistor, you
would have
> 0.095 V/1.0 ohm = 0.095 A (95 mA). The idea is to use a resistor
that is
> small enough so it isn't a big percentage of your main load
resistance, but
> not so small that you aren't able to get a good reading with your
meter.
> One ohm would probably be fine for this application.
>
> If you're under 100 mA, a 1 k ohm base resistor with a 2N4401,
PN2222, or
> 2N2222 should be fine. From 100 mA to 150 mA, you might want to
increase
> the base current by using a resistor around 500 ohms or so.
>
> Randy
> www.glitchbuster.com
>
>
>
Original Message
> From: "arridh_shashank" <arridh_shashank@y...>
> To: <basicstamps@yahoogroups.com>
> Sent: Tuesday, December 09, 2003 10:35 PM
> Subject: [noparse][[/noparse]basicstamps] Re: Using MOSFETs (or not) with Basic
stamps & BS2
> outputs.
>
>
> > Hey guys,
> >
> > Any ideas on how much current the Backlight on a LCD display
> > actually draws? What sort of transistor and base resistor would I
> > use to drive this from a controller like a preprogrammed PIC?
> >
> > BTW thanks for hte info on using mosfets and transistors. [noparse]:)[/noparse]
> >
> > Arridh
> >
> >
> >
> >
> >
> > --- In basicstamps@yahoogroups.com, "Randy Jones"
<randyjones@w...>
> > wrote:
> > > I probably should have included a paragraph mentioning the
> > importance of
> > > running several times the calculated base current based on the
> > beta. Ten
> > > times may be good insurance for variations between component
> > batches or even
> > > manufacturers when the lowest possible Vsat is required, but
I've
> > had good
> > > success with 2.5 to 5 times in many cases. In the example I
> > discussed, it
> > > is probably about 4 times at a 100 mA load current, and about 2
> > times at 200
> > > mA load current. Feeling a little uncomfortable at the 200 mA
> > load current,
> > > I tried doubling the base current -- but the Vsat only changed
> > very slightly
> > > so I stayed with the original 4+ mA. For a one-off project
that
> > would not
> > > be operated under extreme conditions, I'd feel (barely)
> > comfortable with
> > > that -- but definitely not for a product that would be subject
to
> > greater
> > > production and user variables.
> > >
> > > Randy
> > > www.glitchbuster.com
> > >
> > >
> > >
Original Message
> > > From: "Albert Catano" <acatano2002@y...>
> > > To: <basicstamps@yahoogroups.com>
> > > Sent: Tuesday, December 09, 2003 5:58 AM
> > > Subject: [noparse][[/noparse]basicstamps] Re: Using MOSFETs (or not) with Basic
> > stamps & BS2
> > > outputs.
> > >
> > >
> > > > One important parameter with JBTs used in the switched mode
is
> > the
> > > > Forced Beta.
> > > > Normally to insure a proper saturation (lowest Vcesat) is
> > recommended
> > > > to drive the base with a current ten times the specs current
> > gain.
> > > >
> > > > Also must be noted that when a BJT device is in full
saturation,
> > the
> > > > storage times increases considerably.
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the
Subject and
> Body of the message will be ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> >
> >
in place of the 2N2222 and its mate -- works well.Hopefully they will
work on the BS1 serial adapter as well.
> The 2N2222 sure has been a workhorse for years. Interestingly, in the
> Fairchild data sheets for the 2N4401 and PN2222 (a 2N2222 in a plastic TO-92
> housing, for those who might not be familiar with it), all the performance
> graphs for the two devices appear to be 100% identical. Makes me wonder if
> they make one device that meets the extremely similar specs for both part
> numbers, and sell it under the two numbers. I replaced the 2N4401 with a
> PN2222 in that breadboarded circuit and tried some different base and load
> currents, with nearly identical readings.
>
> The 100 to 200 mA range seems like the place where it makes sense to go from
> a TO-92 part to the IRL520N or something along those lines. I wasn't trying
> to contradict your good suggestion about the IRL520N, and hope it didn't
> appear that way. The question about selecting and using a transistor seems
> fairly common, and I thought I'd try to address some of the considerations
> for selecting one.
>
> BTW, I like your new PCB book -- just picked up a copy!
>
> Randy
> www.glitchbuster.com
>
>
>
Original Message
> From: "Al Williams" <alw@a...>
> To: <basicstamps@yahoogroups.com>
> Sent: Tuesday, December 09, 2003 5:27 AM
> Subject: RE: [noparse][[/noparse]basicstamps] Using MOSFETs (or not) with Basic stamps & BS2
> outputs.
>
>
> > Great Post Randy.
> >
> > As I mentioned in my original post, the IRL part is overkill in this case.
> > However, I've taken to using almost nothing else for two reasons. First,
> the
> > switching characteristics of the FET are almost perfect. The on resistance
> > is very low and the gate isolation is very high. Second, I don't have to
> > keep a bunch of different things on hand. The IRL part will switch up to
> 9A
> > so I can use it for almost anything we normally handle. Yeah, it might be
> an
> > extra buck -- and if I were building 50 units with 32 switches I might be
> > worried about that buck -- but it is sure handy to have one part that
> > handles almost anything I normally do. We even have standard "switch
> boards"
> > that let you prototype more rapidly.
> >
> > Below 100mA I still favor the 2N2222. I buy those like jelly beans. But
> I've
> > largely abandoned bipolar for switching. Darlingtons, as you mention, have
> > way too many problems. Even single transistors can be problematic since
> the
> > beta reduces under several circumstances and you wind up drawing more
> > current from the Stamp.
> >
> > The 2N7000 is good, although I still favor the IRL series because the gate
> > turns on at 5V (as opposed to the IRF which Radio Shack sells). Of course,
> > you can use a bipolar to switch the gate voltage, but that reduces the
> > benefit and increases the cost.
> >
> > So I agree that if you are designing something where you need to mass
> > produce, you ought to make a more exacting pick. But for one offs, the big
> > MOSFET switches are cheap enough that you can use them without a lot of
> > thought process. Since you use more of them, you can buy more of them and
> > drive the price down even further.
> >
> > Just my thoughts. By the way, the Stamp FAQ has two pretty good articles
> on
> > switching that covers some of this material. Go to
> > http://www.wd5gnr.com/stampfaq.htm or more specifically:
> > http://www.wd5gnr.com/stampfaq.htm?article=12
> > And
> > http://www.wd5gnr.com/stampfaq.htm?article=13
> >
> > Note that article #12 mentions the IRF510 (which is available at Radio
> Shack
> > although last time I looked they had them marked IFR510). This is not an
> IRL
> > part so it does not fully switch on at 5V which is why we switched to the
> > IRL part.
> >
> > Regards,
> >
> > Al Williams
> > AWC
> > * Floating point math for the Stamp:
> > http://www.al-williams.com/pak1.htm
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and Body
of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
> [noparse][[/noparse]...] any voltage drop across the transistor represents power that the
> load isn't getting -- since the available voltage is being divided between
> the load and the switching device. Ideally, the switching device would
> have no voltage drop when on, but that doesn't happen.
>
Well, that's essentially the case with relays, but of course they deviate
from the ideal in other ways. Like costing more...
Gary