BJT constant current - Page 1
Forum section: electronics / projects

author: Kirigozo
author_rank: Contributor
timestamp: October 24, 2013, 11:36:27 am
content: 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.

author: w2aew
author_rank: Super Contributor
timestamp: October 24, 2013, 01:39:13 pm
content: 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.

author: Kirigozo
author_rank: Contributor
timestamp: October 24, 2013, 11:54:44 pm
content: 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.

author: dannyf
author_rank: Super Contributor
timestamp: October 25, 2013, 02:30:50 am
content: 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.

author: Kirigozo
author_rank: Contributor
timestamp: October 25, 2013, 05:02:08 am
content: Quote from: dannyf on October 25, 2013, 02:30:50 amDannyf, 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.

author: dannyf
author_rank: Super Contributor
timestamp: October 25, 2013, 06:36:04 am
content: QuoteProbably not.QuoteThe transistor will always "control" (as in "adjust" or "regulate") the current down to ua levels. The (potential) issues with the designs are that the forward current through the led, thus the reference voltage for the CCS, depends on 1) the supply voltage, and 2) the Vbe of the transistor (which changes with die temperature).Those issues are more pronounced when the LED is lightly biased (as in this case), or the Vbe changes a lot (if a darlington is used, or if heavy load / voltage drop is required). Because of that, this type of CCS generally performs poorly - which, depending on your application, may still be ok.You can perform a parametric sweep or a temperature sweep to get some sense if the issues are severely for your application to justify a redesign.

author: tszaboo
author_rank: Super Contributor
timestamp: October 25, 2013, 02:49:39 pm
content: Quote from: Kirigozo on October 25, 2013, 05:02:08 am#DEFINE PRECISIONWithout this, we cannot do anything. For you Precision can mean 1%, for me it can be 1PPM. As the other said, a real current source often uses precision resistors, reference voltages and OPAMPs. If you think that the above circuit is fine...At least put a feedback in it somehow.

author: Marco
author_rank: Super Contributor
timestamp: October 25, 2013, 02:58:03 pm
content: The control resistor is a form of negative feedback ...A second PNP resistor used in cascode can reduce the load dependence.

author: codeboy2k
author_rank: Super Contributor
timestamp: October 25, 2013, 03:00:07 pm
content: What nandblog said is true... you have to define what you expect from the circuit. You asked is it stable? What does that mean to you and your application? stable over what input voltage range? stable over what load? stable over what temperature range? stable over what time period?? and for each time I have said stable, you need to put a number to that, like 1%, .1%, .01% , 100ppm. etc...Using a LED like a voltage regulator is fine, it works, but it has a temperature drift, time drift, PSU drift, etc. But if all these drifts are within the performance specifications that you need, then its OK for you.If not, then you need to start throwing more parts into it to stabilize whatever needs to be improved, i.e. as nandblog said, precision resistors with good tempco, and voltage references, opamps, FETS.... by the time you are done, you might have a discrete circuit as complex as an integrated circuit current source, but without ever reaching the performance of a single chip IC CCS.Finally, some basic improvements you can do right away:(1) use a TL431, LT1009 or LM339-2.5 voltage reference instead of the LED ;(2) If jaycar has none of these references, as you said it must be in jaycar's catalogue, then use another transistor to provided a constant current to the LED ;(3) better still, raise your input voltage and use a standard 6.2V zener and a 1N914 diode in series to provide a near 0 tempco zener, or use a temperature compensated 6.2V zener (which has the series diode on the same substrate) ;(4) want even better ? use a transistor to provide a constant current to the 6.2V zener;(5) use a transistor in cascode to buffer the load, as Marco said previously.