# need to understand source and sink currents w.r.t output buffers

Discussion in 'Electronic Design' started by topgun, Jun 17, 2005.

1. ### topgunGuest

Hi all,

Can someone please explain the terms sink current and source
currents with respect to output buffer in a design.

When does the output buffer source current and when does it sink
current?? (i.e relationship to the applied voltages..)

I am digital engineer and I am trying to understand and expand my
knowledge related to analog and other fields....

Thanks

topgun, Jun 17, 2005

2. ### Tim WescottGuest

topgun wrote:

> Hi all,
>
> Can someone please explain the terms sink current and source
> currents with respect to output buffer in a design.
>
> When does the output buffer source current and when does it sink
> current?? (i.e relationship to the applied voltages..)
>
> I am digital engineer and I am trying to understand and expand my
> knowledge related to analog and other fields....
>
>
>
> Thanks
>

Do this thought experiment:

Take a capacitor that's charged to 0.75V and connect it to a dry cell
battery. The capacitor voltage will rise to 1.5V because the battery
sourced current.

Now take the capacitor, again charged to 0.75V, and connect it to a
resistor. The capacitor voltage will drop to 0V because the resistor
sunk current.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott, Jun 17, 2005

3. ### JoergGuest

Hello Topgun,

With modern CMOS logic you can obtain a rough estimate by finding the ON
resistance of the upper FET and that of the lower FET from the data
sheet. Sometimes they are the same but not always. That, Ohm's law and
the voltage of the load at a given time will provide an estimate of the
current the device can sink and source at a given VCC.

But be aware that it isn't always allowed to sink or source as much as
these resistances would provide for.

For some logic chips the ON resistance isn't stated explicitly. Then
check the drops from VCC and GND respectively for various load currents.
That allows to estimate the ON resistances.

Regards, Joerg

Joerg, Jun 17, 2005
4. ### Mike MonettGuest

topgun wrote:
>
> Hi all,
>
> Can someone please explain the terms sink current and source
> currents with respect to output buffer in a design.
>
> When does the output buffer source current and when does it sink
> current?? (i.e relationship to the applied voltages..)
>
> I am digital engineer and I am trying to understand and expand my
> knowledge related to analog and other fields....
>
> Thanks

That might be a dangerous question to ask in this newsgroup. A recent
thread discussin DC vs AC went over 300 posts. But this one should be
simple so I'll take a crack.

Consider a 74HC device. When the output is logic one, it is at +5V. Any
load to ground draws current, which must be supplied by the device. In
this case, the device is sourcing the current.

OTOH, when the output is logic zero, it is at 0V. A load connected to +5V
will draw current from the +5, though the device to gnd. In this case,
the device is sinking the current.

The same concept applies to op amps.

Hope that helps.

Mike Monett

Mike Monett, Jun 17, 2005
5. ### BanGuest

topgun wrote:
> Hi all,
>
> Can someone please explain the terms sink current and source
> currents with respect to output buffer in a design.
>
> When does the output buffer source current and when does it sink
> current?? (i.e relationship to the applied voltages..)
>
> I am digital engineer and I am trying to understand and expand my
> knowledge related to analog and other fields....
>

Well, as a digital engineer you will know the states high(2-5V) and
low(0-0.8V). These are TTL-levels for 5V logic. The device has to source
current when the level is high. So if the device can source 20mA it means
that with a load that consumes 20mA i.e. a LED the output voltage is higher
than 2V. In many cases it will be higher than 4V even.
When the level is low, the buffer sucks in the current from the output. It
sinks for example 50mA, so when 50mA are flowing into the output, the
voltage is maintained lower than 0.8V. You could switch on two paralleled
LEDs with 20mA each.
Usually a device can sink more current than source, and also has more
voltage loss and dissipation when sourcing, what can be seen on the specs
(0.8V vs. 3V across the device).
With modern CMOS parts the source and sink abilities are almost equal and
also the transition point is in the middle of the voltage range.
--
ciao Ban
Bordighera, Italy

Ban, Jun 17, 2005
6. ### John PopelishGuest

topgun wrote:
> Hi all,
>
> Can someone please explain the terms sink current and source
> currents with respect to output buffer in a design.
>
> When does the output buffer source current and when does it sink
> current?? (i.e relationship to the applied voltages..)
>
> I am digital engineer and I am trying to understand and expand my
> knowledge related to analog and other fields....

These descriptions are based on the fluid pressure analogy for
voltage, with more positive voltage being analogous to more pressure.
So currents driven from a positive voltage through a driver control
circuit through load to a more negative voltage are referred to as
sourced currents. This is because the fluid analogy would be a source
of fluid being pushed through valve and then through a resistance
(garden hose) and draining out on the ground (zero volts). Sinked
currents are analogous to fluid sinking down a drain valve on the
floor. So load currents that are driven by an external positive
voltage, and passing through the driver to a more negative (or zero)
voltage are referred to as sinked currents.

Sourced = flow controlled on the positive voltage side of the load.
Sinked = flow controlled on the negative voltage side of the load.

John Popelish, Jun 17, 2005
7. ### JoergGuest

Hello Mike,

> That might be a dangerous question to ask in this newsgroup. A recent
> thread discussin DC vs AC went over 300 posts. ...

And I always thought AC DC was an Australian rock band.

Regards, Joerg

Joerg, Jun 17, 2005
8. ### topgunGuest

Hi All,

Thanks for the good insight into current sourcing and sinking
...

I guess now I have much better visulaization...of the whole concept..

thanks again..

topgun, Jun 17, 2005
9. ### Reg EdwardsGuest

> I am digital engineer and I am trying to understand and expand my
> knowledge related to analog and other fields....
>

The first things to do are to discard your chip manufacturer's data
books, go back to square one, and swot up on Ohms Law and elementary
circuit analysis.

I say this with the nicest of intentions. It will require a little
hard work on your part to catch up.
----
Reg.

Reg Edwards, Jun 17, 2005
10. ### Pooh BearGuest

topgun wrote:

> Hi all,
>
> Can someone please explain the terms sink current and source
> currents with respect to output buffer in a design.
>
> When does the output buffer source current and when does it sink
> current?? (i.e relationship to the applied voltages..)
>
> I am digital engineer and I am trying to understand and expand my
> knowledge related to analog and other fields....

Source means current flowing out.

Sink means current flowing in.

Graham

Pooh Bear, Jun 17, 2005
11. ### Tam/WB2TTGuest

"topgun" <> wrote in message
news:...
> Hi all,
>
> Can someone please explain the terms sink current and source
> currents with respect to output buffer in a design.
>
> When does the output buffer source current and when does it sink
> current?? (i.e relationship to the applied voltages..)
>
> I am digital engineer and I am trying to understand and expand my
> knowledge related to analog and other fields....
>
>
>
> Thanks
>

BS aside, if you connect a resistor from output to ground, you are SOURCING
current into the load; If you connect a resistor from output to VCC, you are
SINKING current.

Tam

Tam/WB2TT, Jun 17, 2005
12. ### Mike MonettGuest

Pooh Bear wrote:
[...]

> Source means current flowing out.
>
> Sink means current flowing in.
>
> Graham

In metals, like pcb traces, current flow is by means of electrons.

So when the electrons are flowing out, do you mean the device is sourcing
current, and when electrons are flowing in, it is sinking?

Mike Monett

Mike Monett, Jun 18, 2005
13. ### MacGuest

On Fri, 17 Jun 2005 19:54:20 -0400, Mike Monett wrote:

> Pooh Bear wrote:
> [...]
>
>> Source means current flowing out.
>>
>> Sink means current flowing in.
>>
>> Graham

>
> In metals, like pcb traces, current flow is by means of electrons.
>
> So when the electrons are flowing out, do you mean the device is sourcing
> current, and when electrons are flowing in, it is sinking?
>
> Mike Monett

Mike, why are you asking this? Is there some kind of context that has
gone before (which I missed)? Do you have something against Pooh Bear?

For the record, current and electrons flow in opposite directions by
convention. Blame Ben Franklin for guessing wrong before anybody knew
what current really was. Or at least that is the legend I heard.

--Mac

Mac, Jun 18, 2005
14. ### JamieGuest

topgun wrote:

> Hi all,
>
> Can someone please explain the terms sink current and source
> currents with respect to output buffer in a design.
>
> When does the output buffer source current and when does it sink
> current?? (i.e relationship to the applied voltages..)
>
> I am digital engineer and I am trying to understand and expand my
> knowledge related to analog and other fields....
>
>
>
> Thanks
>

yes i know its tricky, those terms have been used so loosely in
various docs that i have seen.
this is how i understand it.
Source Current is a device that supplies the common/Negitive rail
which in turn would pull something to ground, Sink current is the
other way around where it supplies voltage into a device that is
going to the common/negitive rail side.
if you look at it this way, Source current is something that is needed
for a voltage source to create current, thus you would be looking at
a active device for example that pulls to the common.

maybe i am wrong here but that is how i have always dealt with it.

Jamie, Jun 20, 2005
15. ### Rich GriseGuest

On Sun, 19 Jun 2005 16:49:51 -0700, Jamie wrote:

> topgun wrote:
>
>> Hi all,
>>
>> Can someone please explain the terms sink current and source
>> currents with respect to output buffer in a design.
>>
>> When does the output buffer source current and when does it sink
>> current?? (i.e relationship to the applied voltages..)
>>
>> I am digital engineer and I am trying to understand and expand my
>> knowledge related to analog and other fields....
>>
>>
>>
>> Thanks
>>

> yes i know its tricky, those terms have been used so loosely in
> various docs that i have seen.
> this is how i understand it.
> Source Current is a device that supplies the common/Negitive rail
> which in turn would pull something to ground, Sink current is the
> other way around where it supplies voltage into a device that is
> going to the common/negitive rail side.
> if you look at it this way, Source current is something that is needed
> for a voltage source to create current, thus you would be looking at
> a active device for example that pulls to the common.
>
> maybe i am wrong here but that is how i have always dealt with it.

No, you got it exactly backwards. Remember, we're talking about
conventional current here, which goes the opposite direction from
electron flow. Sourcing current means conventional current is
coming out of your source, like water out of a faucet.

But this is conventional current - the flow of charge from positive
to negative. The electrons really only pass the charge along, and
it doesn't really matter which direction it's flowing - the electrons
themselves, individually, migrate at a few inches an hour or so. Toward
the current source. ;-)

That's right, kiddies, the current flow in an electron beam is upstream.
;-P

Cheers!
Rich

Rich Grise, Jun 20, 2005

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