Open collector and open drain...
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Posts: 46,084
Hi All,
Can someone explain to me what these two terms mean? I see them on
device datasheets and read them in magazine articles all the time. I
do understand what a collector is on a BJT and a drain on an FET. But
what do these terms mean and what is their significance when hooking
up devices whose datasheets say things like "This output is open
drain, or open collector" at which point I'm supposed to say "Oh,
that means I need to do thus and such". Thanks.
-Dave
Can someone explain to me what these two terms mean? I see them on
device datasheets and read them in magazine articles all the time. I
do understand what a collector is on a BJT and a drain on an FET. But
what do these terms mean and what is their significance when hooking
up devices whose datasheets say things like "This output is open
drain, or open collector" at which point I'm supposed to say "Oh,
that means I need to do thus and such". Thanks.
-Dave
Comments
connect to is connected to the collector of
a transistor on the chip -- but nothing else.
The chip can turn that transistor on and off,
so what you are getting is a high resistance
to ground (off) (aka 'high-impedance'),
or a low resistance to ground (on)(aka 'a short').
This can be very useful in 'wired-or' configurations.
Since the chip never 'drives' the line HIGH,
multiple output pins can be wired together.
You then put a single resistor (10K ohm typical)
to +5 volts on the line. If all the pins are
'off', then the line 'floats' to +5. If ONE
of the pins turn on, it will pull the line
to 0 volts -- but won't damage the other
drivers, which are still 'floating'.
So, open-collector outputs let you do
this 'party-line' thing with less hardware.
You just have to add your own resistor.
Note the 'normal' TTL output is a 'push-pull'
output -- the output pin gets connected to
+5 through a low-resistance path to 'push' the
pin high, or to ground through a low-resistance
path to 'pull' the pin low. If you connect two
of THESE drivers together, and one tries to 'push'
while the other 'pulls', if there isn't some
small resistor (like 220 ohms) between them,
one of the drivers on one of the pins is going
to act like a fuse and burn out.
--- In basicstamps@yahoogroups.com, "nuclearspin2000"
<nuclearspin2000@y...> wrote:
> Hi All,
>
> Can someone explain to me what these two terms mean? I see them on
> device datasheets and read them in magazine articles all the time.
I
> do understand what a collector is on a BJT and a drain on an FET.
But
> what do these terms mean and what is their significance when
hooking
> up devices whose datasheets say things like "This output is open
> drain, or open collector" at which point I'm supposed to say "Oh,
> that means I need to do thus and such". Thanks.
>
> -Dave
Open collector means that the device (op amp, TTL chip, any kind of module)
has a transistor where the collector or drain terminal is brought out to a pin,
and there is nothing connected to the collector.
For example, take an op amp comparitor running on 12 volts, but you want the
output of the op amp to be a 0 to 5 volt TTL signal.
You would connect a resistor (10k 4.7k etc) from the open collector output to
+5 volts.
When the comparitor is in one state, the output will read 5 volts, and the
opposite state 0 volts.
Picture something like a simple 2N3904 transitor inside the IC or device, and
the collector terminal of the transistor is brought out to a pin that the
user can access. I have only seen NPN type open collector devices........not
sure if there is PNP types....
Hope this helps more than confuses.
ken
In a message dated 1/13/2004 4:23:35 PM Pacific Standard Time,
nuclearspin2000@y... writes:
Hi All,
Can someone explain to me what these two terms mean? I see them on
device datasheets and read them in magazine articles all the time. I
do understand what a collector is on a BJT and a drain on an FET. But
what do these terms mean and what is their significance when hooking
up devices whose datasheets say things like "This output is open
drain, or open collector" at which point I'm supposed to say "Oh,
that means I need to do thus and such". Thanks.
-Dave
[noparse][[/noparse]Non-text portions of this message have been removed]
Close. For the SN7405, which is the Hex inverter but with its outputs
as that, it means that they do not have an internal pull up resistor.
The CMOS version of that device, has its outputs arranged in roughly
the same fashion. To use them, you need to supply a proper pull up
resistor, a 2.2K one usually does that. They are normally used for
relay drivers.
Gregg C Levine hansolofalcon@w...
"The Force will be with you...Always." Obi-Wan Kenobi
"Use the Force, Luke."· Obi-Wan Kenobi
(This company dedicates this E-Mail to General Obi-Wan Kenobi )
(This company dedicates this E-Mail to Master Yoda )
>
Original Message
> From: goflo@p... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=Q08CneCbEI2iZrfSRVWGvrTLjgQaS_lh3lb5Zm_iIRKUmcTXXmXwLbFJ3W3NYxczrDEjdbGDINNz]goflo@p...[/url
> Sent: Tuesday, January 13, 2004 9:24 PM
> To: basicstamps@yahoogroups.com
> Subject: Re: [noparse][[/noparse]basicstamps] Open collector and open drain...
>
> It usually means that when the device is toggled "active" by
> base current or gate voltage the output will go to ground.
>
> Jack
>
> > Can someone explain to me what these two terms mean? I see them on
> > device datasheets and read them in magazine articles all the time.
I
> > do understand what a collector is on a BJT and a drain on an FET.
But
> > what do these terms mean and what is their significance when
hooking
> > up devices whose datasheets say things like "This output is open
> > drain, or open collector" at which point I'm supposed to say "Oh,
> > that means I need to do thus and such". Thanks.
>
On a transistor spec sheet you will see a term under maximum ratings, a term
like
Vebo
This gives the maximum voltage one can apply across the two labeled terminals
(emitter base in this example Vebo) and the unlabled last terminal, collector
in this case, is left open.
In a message dated 1/13/2004 4:23:35 PM Pacific Standard Time,
nuclearspin2000@y... writes:
Hi All,
Can someone explain to me what these two terms mean? I see them on
device datasheets and read them in magazine articles all the time. I
do understand what a collector is on a BJT and a drain on an FET. But
what do these terms mean and what is their significance when hooking
up devices whose datasheets say things like "This output is open
drain, or open collector" at which point I'm supposed to say "Oh,
that means I need to do thus and such". Thanks.
-Dave
[noparse][[/noparse]Non-text portions of this message have been removed]
In a message dated 1/13/2004 4:34:54 PM Pacific Standard Time,
allan.lane@h... writes:
'Open-collector' means the output pin you
connect to is connected to the collector of
a transistor on the chip -- but nothing else.
The chip can turn that transistor on and off,
so what you are getting is a high resistance
to ground (off) (aka 'high-impedance'),
or a low resistance to ground (on)(aka 'a short').
This can be very useful in 'wired-or' configurations.
Since the chip never 'drives' the line HIGH,
multiple output pins can be wired together.
You then put a single resistor (10K ohm typical)
to +5 volts on the line. If all the pins are
'off', then the line 'floats' to +5. If ONE
of the pins turn on, it will pull the line
to 0 volts -- but won't damage the other
drivers, which are still 'floating'.
So, open-collector outputs let you do
this 'party-line' thing with less hardware.
You just have to add your own resistor.
Note the 'normal' TTL output is a 'push-pull'
output -- the output pin gets connected to
+5 through a low-resistance path to 'push' the
pin high, or to ground through a low-resistance
path to 'pull' the pin low. If you connect two
of THESE drivers together, and one tries to 'push'
while the other 'pulls', if there isn't some
small resistor (like 220 ohms) between them,
one of the drivers on one of the pins is going
to act like a fuse and burn out.
[noparse][[/noparse]Non-text portions of this message have been removed]
base current or gate voltage the output will go to ground.
Jack
> Can someone explain to me what these two terms mean? I see them on
> device datasheets and read them in magazine articles all the time. I
> do understand what a collector is on a BJT and a drain on an FET. But
> what do these terms mean and what is their significance when hooking
> up devices whose datasheets say things like "This output is open
> drain, or open collector" at which point I'm supposed to say "Oh,
> that means I need to do thus and such". Thanks.
-Dave
--- In basicstamps@yahoogroups.com, "nuclearspin2000"
<nuclearspin2000@y...> wrote:
> Hi All,
>
> Can someone explain to me what these two terms mean? I see them on
> device datasheets and read them in magazine articles all the time.
I
> do understand what a collector is on a BJT and a drain on an FET.
But
> what do these terms mean and what is their significance when
hooking
> up devices whose datasheets say things like "This output is open
> drain, or open collector" at which point I'm supposed to say "Oh,
> that means I need to do thus and such". Thanks.
>
> -Dave