Simple current source

[timelessbeing]

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.

 

[alec_t]

Extremely high voltage driving extremely-low-resistance load via extremely-high-value resistor.
Not very practical in general, though. Simple isn't always best.

 

[timelessbeing]

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.

 

[crutschow]

If you are not concerned about efficiency then this LM317 based constant-current source is very simple and quite stable. The current is approximately 1.2V/R

 

[timelessbeing]

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.

 

[crutschow]

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.

 

[timelessbeing]

Thanks. That page has some helpful information.

Also, I have some salvaged opamps and precision references in my junk bin. Would those be helpful?

 

[crutschow]

Thanks. That page has some helpful information.

Also, I have some salvaged opamps and precision references in my junk bin. Would those be helpful?

They might but I thought you were worried about cost in production??

 

[timelessbeing]

They're salvage so they're free

 

[crutschow]

So you have enough parts in salvage to go into production?

 

[timelessbeing]

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?

 

[cowboybob]

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?

Perhaps more detail than you asked for, but:

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

 

[MikeMl]

Post #3 in this thread.

Post #7 in this thread

 

[crutschow]

I beg your pardon, but I'm not interested in production logistics.

........................

Well pardon my confusion, but in post #5 you stated

LM317 circuits start to get costly if you want to build a large number of them

so I assumed you were interested in production quantities.

 

[timelessbeing]

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

Post #3 in this thread.

Post #7 in this thread

Good stuff. I'll have a read. Thanks very much.