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

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.

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..

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

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.

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

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

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.

Hello Mike,

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

Regards, Joerg

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.

Source means current flowing out.

Sink means current flowing in.

Graham

Pooh Bear wrote: [...]

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

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