Mosfet circuit
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Posts: 46,084
Can someone tell me if the following mosfet circuit will work. I want to
turn a battery operated circuit on with a momentary ON pushbutton and have the
Stamp keep it on.
Battery plus to mosfet drain
Battery neg to system ground
Mosfet source to VR In
Mosfet gate to drain via pushbutton. Gate has 10K pulldown.
Press PB for 1 or 2 seconds, gate goes high, mosfet conducts, supplies power
to VR, then to Stamp. Stamp Pin 15 is held high by program, goes to gate and
keeps mosfet conducting. Later P15 goes low, turns off mosfet, turning off
battery.
Could this possibly work?
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
turn a battery operated circuit on with a momentary ON pushbutton and have the
Stamp keep it on.
Battery plus to mosfet drain
Battery neg to system ground
Mosfet source to VR In
Mosfet gate to drain via pushbutton. Gate has 10K pulldown.
Press PB for 1 or 2 seconds, gate goes high, mosfet conducts, supplies power
to VR, then to Stamp. Stamp Pin 15 is held high by program, goes to gate and
keeps mosfet conducting. Later P15 goes low, turns off mosfet, turning off
battery.
Could this possibly work?
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
Comments
laurasdog@w... writes:
> Even with a logic level FET, you will need to pull
> the gate several volts higher than the source to
> turn it on. If the source is connected to VR, where
> will the drive signal come from that is a higher
> voltage than that? Not from a stamp output pin...
The drive signal will come from the battery when I press the pushbutton.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
The gate needs to be several volts higher than the source to keep the device
on, just as Steve mentioned.
Dave recently brought up this subject. I thought he settled on a voltage
regulator with an enable pin.
Ken
===================
Sid,
No I don't think that will work...
Even with a logic level FET, you will need to pull
the gate several volts higher than the source to
turn it on. If the source is connected to VR, where
will the drive signal come from that is a higher
voltage than that? Not from a stamp output pin...
You can put the FET on the ground side of the
circuit and make it work.
Steve
[noparse][[/noparse]Non-text portions of this message have been removed]
laurasdog@w... writes:
> But when you release the button, the drive will come from a stamp
> pin that is less than 5 volts above ground. If the FET source is
> connected to V+ on the stamp, than the gate will need to be
> higher than V+ by several volts to stay on.
> Perhaps I'm not visualizing the circuit you propose correctly
I think you see the circuit correctly. I did not know that the gate voltage
has to be X volts higher that V+.
Yoiu said something about using it to complete the ground circuit. How would
I do that?
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
you are running a stamp the voltage regulator is 5 volts, correct?
And what is the voltage of the battery supply? Nine volts? 12 volts?
If the battery voltage is high enough to provide a MINIMUM of about 10.5
volts (5 volts plus Vce sat) one possible solution:
Of course, the pushbutton momentary switch is needed and I don't see any
problem there.
To keep the gate of the logic level mosfet at 10 volts you could have the
stamp ground the base lead of a PNP (2n3907).
Emmitter of the 3907 to battery plus, collector of 3907 to the gate of the
mosfet and the base of the 3907 to at stamp output pin that will go low to keep
it on.
I wonder though, when you make the stamp go high to turn things off, how it
will perform.......hmmmm I am going to build one up right now.....if it works I
will post a shematic in the files section of the basic stamps group.
Ken
Can someone tell me if the following mosfet circuit will work. I want to
turn a battery operated circuit on with a momentary ON pushbutton and have
the
Stamp keep it on.
[noparse][[/noparse]Non-text portions of this message have been removed]
3906
k
To keep the gate of the logic level mosfet at 10 volts you could have the
stamp ground the base lead of a PNP (2n3907).
Emmitter of the 3907 to battery plus, collector of 3907 to the gate of the
mosfet and the base of the 3907 to at stamp output pin that will go low to
keep
[noparse][[/noparse]Non-text portions of this message have been removed]
smartdim@a... writes:
> Dave recently brought up this subject. I thought he settled on a voltage
> regulator with an enable pin.
>
I know, but that won't work. A user could reset the device anytime he wants
without waiting for a mandatory 10 minutes pause, so we gave that up.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
==============
Keep one thing in mind. If the current that runs through the mosfet ground
path (mosfet) rises enough, there will be a voltage drop across the FET and then
the ground potential of the stamp is no longer at ground. A mosfet with low
on resistance will eliminate that potential.
Example, a IRL520 on resistance is about .25 ohms and at .5 amps that will
raise the ground potential .125 volts.
Depending how the external components are configured that may or may not
cause a problem.
One other point to consider is with the cicuit will be "at positive
potential" when not enabled and any kind of ground that contacts the circuit where it
is "live" could lead to smoke.
Ken
[noparse][[/noparse]Non-text portions of this message have been removed]
laurasdog@w... writes:
> I use a lot of 30N06L FET's (digi-key), but any of the IRL series FET's
> will
> work. But not the IRF series....
>
>
Thanks, Steve.
The 30N06L is a 30A TO-220. Do you know of something in the 1A TO-92
style tht would work? What about the 2N7000?
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
With the stamp powered off, the impeadance of the stamp pin that was to
control the 2n3906 is low enough to activate the circuit.
Ken
Emmitter of the 3906 to battery plus, collector of 3906 to the gate of the
mosfet and the base of the 3906 to at stamp output pin that will go low to
keep
it on.
I wonder though, when you make the stamp go high to turn things off, how it
will perform.......hmmmm I am going to build one up right now.....if it works
I
will post a shematic in the files section of the basic stamps group.
Ken
Can someone tell me if the following mosfet circuit will work. I want to
turn a battery operated circuit on with a momentary ON pushbutton and have
the
Stamp keep it on.
[noparse][[/noparse]Non-text portions of this message have been removed]
smartdim@a... writes:
> Keep one thing in mind. If the current that runs through the mosfet ground
> path (mosfet) rises enough, there will be a voltage drop across the FET and
> then
> the ground potential of the stamp is no longer at ground. A mosfet with low
> on resistance will eliminate that potential.
>
> Example, a IRL520 on resistance is about .25 ohms and at .5 amps that will
> raise the ground potential .125 volts.
>
I would think our max current will be about 70ma.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
to turn the circuit on the first time?
When you say reset the circuit, do you mean to turn it on with the momentary
switch?
And if so, what prevents the useer from resetting the device anytime he wants.
I know, but that won't work. A user could reset the device anytime he wants
without waiting for a mandatory 10 minutes pause, so we gave that up.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
alw@a... writes:
> Keep in mind that not all FETs will fully switch on with 5V Vgs (the IRL
> series will, however).
>
>
Thanks, Al. I think now I understand how to use the Mosfet - I'll just have
to be careful about which one if pick.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
smartdim@a... writes:
> Doesn't you current proposed set up also have a momentary pushbutton switch
>
> to turn the circuit on the first time?
>
We'e down to pushing a button to turn on the mosfet to turn on the Stamp.
The Stamp program has a 10 minute pause, after which it takes the gate low
and turns its self off. The system can then be restarted.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
If you activate the mosfet by pushing a momentary switch, the stamp then
takes over and keeps the mosfet turned on for 10 minutes. After 10 minutes has
past, the user can again push the momentary switch to start over. Is my
understanding correct?
If my understanding is correct.....have the user turn on the voltage
regulator with a momentary switch, then the first command for the stamp is to send a
high (through 4.7k or so) to the same enable pin of the voltage regulator.After
10 mnutes has past, disable the voltage regulator enable pin via the stamp.
Am I missing something? I don't see any difference between enabling a mosfet
or enabling a voltage regulator?
Ken
================
We'e down to pushing a button to turn on the mosfet to turn on the Stamp.
The Stamp program has a 10 minute pause, after which it takes the gate low
and turns its self off. The system can then be restarted.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
laurasdog@w... writes:
The IRL520 is rated (I think) at 100 volts and has a fairly high Rds(on)
=======================
0.18 ohms
[noparse][[/noparse]Non-text portions of this message have been removed]
No I don't think that will work...
Even with a logic level FET, you will need to pull
the gate several volts higher than the source to
turn it on. If the source is connected to VR, where
will the drive signal come from that is a higher
voltage than that? Not from a stamp output pin...
You can put the FET on the ground side of the
circuit and make it work.
Steve
At 06:38 AM 6/12/2004, you wrote:
>Can someone tell me if the following mosfet circuit will work. I want to
>turn a battery operated circuit on with a momentary ON pushbutton and have
>the
>Stamp keep it on.
>
>Battery plus to mosfet drain
>Battery neg to system ground
>Mosfet source to VR In
>Mosfet gate to drain via pushbutton. Gate has 10K pulldown.
>
>Press PB for 1 or 2 seconds, gate goes high, mosfet conducts, supplies power
>to VR, then to Stamp. Stamp Pin 15 is held high by program, goes to gate and
>keeps mosfet conducting. Later P15 goes low, turns off mosfet, turning off
>battery.
>
>Could this possibly work?
>
>Sid
>
>
>[noparse][[/noparse]Non-text portions of this message have been removed]
>
>
>
>To UNSUBSCRIBE, just send mail to:
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>
>
>
>
[noparse][[/noparse]Non-text portions of this message have been removed]
>The drive signal will come from the battery when I press the pushbutton.
>
>Sid
But when you release the button, the drive will come from a stamp
pin that is less than 5 volts above ground. If the FET source is
connected to V+ on the stamp, than the gate will need to be
higher than V+ by several volts to stay on.
Perhaps I'm not visualizing the circuit you propose correctly...
Got a schematic?
Steve
[noparse][[/noparse]Non-text portions of this message have been removed]
Look at http://www.al-williams.com/fetrly.htm
Keep in mind that not all FETs will fully switch on with 5V Vgs (the IRL
series will, however).
Regards,
Al Williams
AWC
* Add floating point math to any processor (now on sale):
http://www.awce.com/pak12.htm
Original Message
From: Newzed@a... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=xQ7C8gWVXI2GA-WZe2k6BZlFI26D5l8Gvs4213tNocyvdIZo1uEnDcJrQC60zOpncpsAWOZzwx4]Newzed@a...[/url
Sent: Saturday, June 12, 2004 9:58 AM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] Mosfet circuit
In a message dated 6/12/2004 10:54:21 AM Eastern Daylight Time,
laurasdog@w... writes:
> But when you release the button, the drive will come from a stamp pin
> that is less than 5 volts above ground. If the FET source is connected
> to V+ on the stamp, than the gate will need to be higher than V+ by
> several volts to stay on. Perhaps I'm not visualizing the circuit you
> propose correctly
I think you see the circuit correctly. I did not know that the gate voltage
has to be X volts higher that V+.
Yoiu said something about using it to complete the ground circuit. How
would
I do that?
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
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.
Yahoo! Groups Links
>In a message dated 6/12/2004 10:54:21 AM Eastern Daylight Time,
>laurasdog@w... writes:
>
>
> > But when you release the button, the drive will come from a stamp
> > pin that is less than 5 volts above ground. If the FET source is
> > connected to V+ on the stamp, than the gate will need to be
> > higher than V+ by several volts to stay on.
> > Perhaps I'm not visualizing the circuit you propose correctly
>
>I think you see the circuit correctly. I did not know that the gate voltage
>has to be X volts higher that V+.
>
>Yoiu said something about using it to complete the ground circuit. How would
>I do that?
>
>Sid
Connect the battery + to the stamp V+ like normal.
Connect the source to battery ground. Connect the drain to the stamps ground.
Now turning the FET on will complete the ground path to turn the stamp on.
If you use a logic level FET, you will need a gate signal that goes to 3-5
volts
^above ground* to turn it on, which the stamp can provide.
Many "regular" FETs require 8-10 volts of gate drive to turn fully on, but
Logic
Level FETs work at standard 5v logic level, perfect for use on a stamp.
I use a lot of 30N06L FET's (digi-key), but any of the IRL series FET's will
work. But not the IRF series....
Steve
[noparse][[/noparse]Non-text portions of this message have been removed]
device. FETs with lower voltage ratings will likely have lower Rds(on)
The IRL520 is rated (I think) at 100 volts and has a fairly high Rds(on)
The 30N06L is rated for 30 volts and has a Rds(on) of less
than 0.05ohm. So at 1 amp you're only looking at a drop of 0.05 volts
across the FET.
Sorry, I don't have any part #'s for TO92 sized FET's... when I need
a FET I'm usually looking to control something big and nasty.. (-:
Look for logic level FETs with a nice low Rds(on).
Steve
At 09:22 AM 6/12/2004, you wrote:
>In a message dated 6/12/2004 12:12:08 PM Eastern Daylight Time,
>smartdim@a... writes:
>
>
> > Keep one thing in mind. If the current that runs through the mosfet ground
> > path (mosfet) rises enough, there will be a voltage drop across the FET
> and
> > then
> > the ground potential of the stamp is no longer at ground. A mosfet with
> low
> > on resistance will eliminate that potential.
> >
> > Example, a IRL520 on resistance is about .25 ohms and at .5 amps that will
> > raise the ground potential .125 volts.
> >
>
>I would think our max current will be about 70ma.
>
>Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
at 100V. Just select the device with the Rds you need:
IRL520N: Rds = 0.18
IRL530N: Rds = 0.10
IRL540N: Rds = 0.044
The above Rds values are for 10V on the gate. At 5V gate voltage the Rds is
slightly higher (about 20%).
-Randy
www.glitchbuster.com
> As a general rule, Rds(on) is related to the voltage rating of the
> device. FETs with lower voltage ratings will likely have lower Rds(on)
>
> The IRL520 is rated (I think) at 100 volts and has a fairly high Rds(on)
>
> The 30N06L is rated for 30 volts and has a Rds(on) of less
> than 0.05ohm. So at 1 amp you're only looking at a drop of 0.05 volts
> across the FET.
>
> Sorry, I don't have any part #'s for TO92 sized FET's... when I need
> a FET I'm usually looking to control something big and nasty.. (-:
>
> Look for logic level FETs with a nice low Rds(on).
>
> Steve
For the same Vds, why wouldn't you always just select the device with the
lowest Rds? What are the tradeoffs? Different max current? When would you
want a high Rds? Or?
Thanks!
Steve
//////////////////////////////////
At 11:29 AM 6/12/04, you wrote:
>The IRL520 is a member of a family with varying Rds values. All are rated
>at 100V. Just select the device with the Rds you need:
>
>IRL520N: Rds = 0.18
>IRL530N: Rds = 0.10
>IRL540N: Rds = 0.044
>
>The above Rds values are for 10V on the gate. At 5V gate voltage the Rds is
>slightly higher (about 20%).
>
>-Randy
>www.glitchbuster.com
>
>
>
> > As a general rule, Rds(on) is related to the voltage rating of the
> > device. FETs with lower voltage ratings will likely have lower Rds(on)
> >
> > The IRL520 is rated (I think) at 100 volts and has a fairly high Rds(on)
> >
> > The 30N06L is rated for 30 volts and has a Rds(on) of less
> > than 0.05ohm. So at 1 amp you're only looking at a drop of 0.05 volts
> > across the FET.
> >
> > Sorry, I don't have any part #'s for TO92 sized FET's... when I need
> > a FET I'm usually looking to control something big and nasty.. (-:
> >
> > Look for logic level FETs with a nice low Rds(on).
> >
> > Steve
>Question:
>For the same Vds, why wouldn't you always just select the device with the
>lowest Rds? What are the tradeoffs? Different max current? When would you
>want a high Rds? Or?
>Thanks!
>Steve
>//////////////////////////////////
A lot of times the trade off is cost. Within the same family, the lower
RDs(on) units are usually more expensive.
No point in buying a $0.99 FET when a $0.89 one will do... :-)
Steve
[noparse][[/noparse]Non-text portions of this message have been removed]
cost. So cost is the tradeoff. For example:
IRL520N: Rds = 0.18 -- $0.67
IRL530N: Rds = 0.10 -- $0.87
IRL540N: Rds = 0.044 -- $1.17
For many Stamp applications even the IRL520N is overkill, but if you're
looking for even lower Rds the IRL530N has just over 1/2 the IRL520N's Rds
for only a slightly higher price.
Another interesting thing to note is that the devices with the lower Rds
will drop less voltage and dissipate less heat for any given load current.
Using the IRL540N in place of one of the others with higher Rds could
eliminate the need for a heat sink in some higher current applications, and
end up costing less overall. In addition, more of the supply voltage gets
to the load device instead of dropping in the transistor.
But... for many applications the IRL520N without a heat sink is more than
enough.
-Randy
www.glitchbuster.com
Original Message
From: "Steve Roberts" <sroberts@s...>
To: <basicstamps@yahoogroups.com>
Sent: Saturday, June 12, 2004 12:21 PM
Subject: Re: [noparse][[/noparse]basicstamps] Mosfet circuit
> Question:
> For the same Vds, why wouldn't you always just select the device with the
> lowest Rds? What are the tradeoffs? Different max current? When would you
> want a high Rds? Or?
> Thanks!
> Steve
> //////////////////////////////////
>
> At 11:29 AM 6/12/04, you wrote:
> >The IRL520 is a member of a family with varying Rds values. All are
rated
> >at 100V. Just select the device with the Rds you need:
> >
> >IRL520N: Rds = 0.18
> >IRL530N: Rds = 0.10
> >IRL540N: Rds = 0.044
> >
> >The above Rds values are for 10V on the gate. At 5V gate voltage the Rds
is
> >slightly higher (about 20%).
> >
> >-Randy
> >www.glitchbuster.com
> At 12:21 PM 6/12/2004, you wrote:
>
> >Question:
> >For the same Vds, why wouldn't you always just select the device
with the
> >lowest Rds? What are the tradeoffs? Different max current? When
would you
> >want a high Rds? Or?
> >Thanks!
> >Steve
> >//////////////////////////////////
>
>
>
> A lot of times the trade off is cost. Within the same family, the
lower
> RDs(on) units are usually more expensive.
> No point in buying a $0.99 FET when a $0.89 one will do... :-)
In reading the origional post, it appears that the situation is for a
battery operated unit and that by turning the FET ON the device is
powered.
So, if you have a high resistance on the FET, the unit will stop
working at a higher battery voltage. the difference can easily be a
1/2 volt. And, if one is starting with 4 AA batteries, the dropout
will have a pretty big difference for that 1/2 volt.
I question I have is about the voltage and current needed on the gate
at these low power/low voltage uses. Is there anything special
needed ?
Dave
>battery operated unit and that by turning the FET ON the device is
>powered.
>
>So, if you have a high resistance on the FET, the unit will stop
>working at a higher battery voltage. the difference can easily be a
>1/2 volt. And, if one is starting with 4 AA batteries, the dropout
>will have a pretty big difference for that 1/2 volt.
Dave, most low voltage FET's you would use in a battery operated
circuit will have an Rds(on) of 0.2 ohms or less. There's *lots* of
good cheap FETs with Rds(on) of less than 0.05 ohm.
So even at 2 amps (which is a lot for 4 AA batteries) the voltage
lost across the FET would likely be less than 0.1 volts. That's
certainly better than using a bipolar device...
>I question I have is about the voltage and current needed on the gate
>at these low power/low voltage uses. Is there anything special
>needed ?
Not much required to drive a FET gate. The voltage on the gate
(relative to the source) for logic level devices just needs to be a
few volts to turn fully on, and usually can't exceed around 10 volts
without possible gate damage. Check the data sheet for your
device, and all that...
The gate current requirement is essentially zero. Most MOSFETs
have gate input impedances that exceed 10's of Megohms, and
most exceed that by a great deal..
Steve
[noparse][[/noparse]Non-text portions of this message have been removed]
> circuit will have an Rds(on) of 0.2 ohms or less. There's *lots* of
> good cheap FETs with Rds(on) of less than 0.05 ohm.
My first selection was a 2N7000 MOSFET commonly used in place of a
2N2222 darlington.
the spec sheet from Fairchild lists it as 5 ohms.
I was surprized at the high resistance, thus the cause fro my post.
On the surface, it seems the TO-220 case may offer a larger area for
the junction and therefor much lower resistance.
Dave
> So even at 2 amps (which is a lot for 4 AA batteries) the voltage
> lost across the FET would likely be less than 0.1 volts. That's
> certainly better than using a bipolar device...
>
> >I question I have is about the voltage and current needed on the
gate
> >at these low power/low voltage uses. Is there anything special
> >needed ?
>
> Not much required to drive a FET gate. The voltage on the gate
> (relative to the source) for logic level devices just needs to be a
> few volts to turn fully on, and usually can't exceed around 10 volts
> without possible gate damage. Check the data sheet for your
> device, and all that...
>
> The gate current requirement is essentially zero. Most MOSFETs
> have gate input impedances that exceed 10's of Megohms, and
> most exceed that by a great deal..
>
> Steve
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
>with the
>> >lowest Rds? What are the tradeoffs? Different max current? When
>would you
>> >want a high Rds? Or?
>> >Thanks!
> > >Steve
Another option for your power-hold circuit might be one of the
photomos relays, example Aromat AQV212 in 6 pin DIP, rated 400 mA.
These have an LED input, and a mosfet output. The pushbutton via a
resistor could turn on the led, which powers up the Stamp, which
continues to power the led. You only need a couple of resistors in
addition to the AQV212. It would be more expensive than the circuit
using the plain mosfet, though.
I'm not sure if anyone mentioned this, but another factor in the
choice of a mosfet is switching speed. The higher current devices
often have very large junction area, and that translates into a large
input capacitance. For example, the capacitance of the IRL520 is
about 500 pf, while the capacitance of the higher current IRL540 is
1800 pf. The circuit that drives the gate has to charge up that
capacitance or discharge it every time it changes state. That is not
a limitation for the DC circuit you are talking about, but it could
be a factor in a PWM circuit or a switching power supply.
There are many applications for mosfets. For example, the mosfet
chosen for a high speed sample and hold circuit might have an ON
resistance of 5 ohms, but an input capacitance of 10 or 20 pf. A low
gate to channel capacitance tranlates to rapid switching and also low
"charge injection" from the digital to the analog circuitry.
-- Tracy
> choice of a mosfet is switching speed. The higher current devices
> often have very large junction area, and that translates into a
large
> input capacitance. For example, the capacitance of the IRL520 is
> about 500 pf, while the capacitance of the higher current IRL540 is
> 1800 pf. The circuit that drives the gate has to charge up that
> capacitance or discharge it every time it changes state. That is
not
> a limitation for the DC circuit you are talking about, but it could
> be a factor in a PWM circuit or a switching power supply.
I had mentioned the high capacatance of the MOSFET a few months ago,
but it was about the need to switch the MOSFET fast. if one tries to
trickle the unit to turn on, the current passing the junction will
destroy the chip as the small sections start to pass the current, the
current will overheat the sections and burn it up.
That is one of the reasons for using mosfet driver chips with high
amp ratings in really tiny packages.
Very interesting discussion this is becomming.
Dave
>
> There are many applications for mosfets. For example, the mosfet
> chosen for a high speed sample and hold circuit might have an ON
> resistance of 5 ohms, but an input capacitance of 10 or 20 pf. A
low
> gate to channel capacitance tranlates to rapid switching and also
low
> "charge injection" from the digital to the analog circuitry.
>
>
> -- Tracy