Increasing sink current?

Thread Starter

Razor Concepts

Joined Oct 7, 2008
214
I have a IC that can sink up to 38ma. I would like to drive higher currents but was wondering how i could do this. Usually I'm accustomed to working with things that source current, so using a darlington array would be standard. But I am unsure of how to do this with sinking current. Thanks
 

Wendy

Joined Mar 24, 2008
23,415
A common use for current sinks is electronic loads for power supplies. I'm thinking of going with this...

 

Ron H

Joined Apr 14, 2005
7,063
I have a IC that can sink up to 38ma. I would like to drive higher currents but was wondering how i could do this. Usually I'm accustomed to working with things that source current, so using a darlington array would be standard. But I am unsure of how to do this with sinking current. Thanks
If you ground the emitter of an NPN Darlington (or a normal NPN), and drive the base with a current-limited voltage, the collector will sink current.
 

Thread Starter

Razor Concepts

Joined Oct 7, 2008
214
If you ground the emitter of an NPN Darlington (or a normal NPN), and drive the base with a current-limited voltage, the collector will sink current.
I don't understand - the chip can only sink current so how does it drive the base with a current limited voltage?
 

Wendy

Joined Mar 24, 2008
23,415
Has to do with a transistor being a current controlled device and beta. Ic=ß Ib

I prefer using the emitter resistor myself, with a controlled base voltage.
 

Ron H

Joined Apr 14, 2005
7,063
I don't understand - the chip can only sink current so how does it drive the base with a current limited voltage?
If your output can only sink current, you can make it possible to source current for the base of the Darlington (or plain NPN) by adding a pullup resistor to +V.

See attachment.

Note that if your circuit is analog, this won't work.
 

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Thread Starter

Razor Concepts

Joined Oct 7, 2008
214
Thank you, now I understand. I think I'll use a ULN2003A Darlington. Is there a "standard" value pull-up I should use to test things out? Thanks.
 
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