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BJT constant current

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Kirigozo:
Hi folks,

I've been looking at BJT constant current circuits like the one in the pic. I'm looking for a very large current range, from 30mA down to 10uA. Is it a reasonable expectation that a small bjt will be stable in this type of circuit? I've been modeling this with LTSpice and it indicates all is well but I'm just not sure. I'd appreciate any feedback from anyone who has had experience with this type of circuit at very low currents.

The control pot is 50k. The zenner will probably be an LED or something similar. Jelly-bean parts only.

w2aew:
It will be stable from a transient standpoint. However, it will have some variation with temperature because the Vbe and the diode each have their own variation with temperature. You can minimize this by making the voltage across the emitter resistor large.

Kirigozo:
Thank you for responding.

I think I can manage at least 3.3v but the going higher diminishes the range at the low current end of the scale. The old balancing options act, again.

Thank you.

dannyf:
Not sure of the particulr applications you intended for the CCS. Three suggestions:

1) use a real voltage reference, like TL431 or better, if you need better regulation / temperature drift performance;
2) you may consider something like a mosfet as the regulator;
3) or the use of a ccs to drive the led/Vref.

Kirigozo:

--- Quote from: dannyf on October 25, 2013, 02:30:50 am ---Not sure of the particulr applications you intended for the CCS. Three suggestions:

1) use a real voltage reference, like TL431 or better, if you need better regulation / temperature drift performance;
2) you may consider something like a mosfet as the regulator;
3) or the use of a ccs to drive the led/Vref.

--- End quote ---

Dannyf, thanks for the suggestions but ...

This project is designed for jelly-bean parts only. If it's not in the Jaycar catalogue or my junk box, it won't be used. Keeping everything operating well within a stable operating range and avoiding significant self heating are the only things that concern me.

In the circuit above, I realized a simple voltage divider would work fine in that section. The part I was concerned about was the transistor not controlling uA current levels in a stable manner. Problem solved.

Thanks.

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