Simple current source

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timelessbeing

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What is the simplest (in terms of complexity) constant current source you can build with discrete components? Something on the order of 1A, with 0.25% stability. An example of an application would be a high power LED driver.
 
Extremely high voltage driving extremely-low-resistance load via extremely-high-value resistor.
Not very practical in general, though. Simple isn't always best.
 
Haha, yes that's one way. What about low voltage, say ... 12V

Simple isn't best, but it's easiest to understand, and build up from.
 
I know there are plenty of IC's that will do the job. I guess I should have said common components instead. Transistors, resistors etc.. LM317 circuits start to get costly if you want to build a large number of them.

I understand that low efficiency and heat comes with the linear territory, but switching regulators are a big jump in complexity/cost. I'm OK with that when it's not a mobile/battery operated application.
 
I don't know that a LM317 is much more than a power transistor as I see them for $0.71 in a TO-220 case, but here's a simple circuit that uses one BJT, one power MOSFET and a couple resistors. The temperature stability is determined by the base-emitter voltage of the BJT transistor which changes about 0.3% per degree C. R2 and R3 can be one resistor (don't know why they show two). The constant current is approximately 0.65V /R1. Note that, at 1A, you likely will have to heatsink the MOSFET with heatsink size depending upon the supply voltage and voltage drop across the MOSFET.
 
Thanks. That page has some helpful information.

Also, I have some salvaged opamps and precision references in my junk bin. Would those be helpful?
 
I beg your pardon, but I'm not interested in production logistics.

Getting back... I've seen references and opamps used in constant current circuits before, but I couldn't understand how they worked because there was extra stuff around it. I think these parts are at the heart of the circuit though. Can someone please explain how it works, or link a simplified diagram that distills the principle of operation?
 
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Perhaps more detail than you asked for, but:

http://microblog.routed.net/wp-content/uploads/2008/01/an587-d.pdf
 
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