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Constant current source for a low Ohm load


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How would I go about creating a constant current source to supply a 20 Ohm load? I want the current to be variable from 0 to 120 mA.

So I need a circuit which supplies a constant current to a 20 Ohm load, in the range of 0 to 120 mA.



^ That is a circuit that I have designed already, however I believe it is a little unstable and the transistor may need a heatsink to suffice the circuit.

I was hoping that perhaps there is a way to achieve a circuit that is independent of temperature, so I won't need a heatsink or have to worry about possible thermal runaway.

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Erm, thanks for that indulis, but I think I'm going to need a few other values and device numbers to go with that.

Is that a precision constant current source circuit?

Edit: I decided to simulate the circuit using SIMetrix, with a few components:

http://vicious5id.zerodollars.com/eL-iSrc.GIF

^ Thats my version of the circuit. It works OK for a constant 20 Ohms, however the current will change if I replace the 20 Ohm load. Also I simulated it under varying temperatures and there's a drift of ~+5 mA, whereas in the circuit I have (with the appropriate heatsink in place) will drift by only ~+0.01 mA.

I do hope that its the way I have set it up thats not letting it work, I like the idea of using a MOSFET. I'd be grateful, indulis, if you could upload a schematic of your circuit working with those specifications I just made; perhaps you could adjust mine or change yours.

One final thing: it'd be really awesome if you could get a current source to supply 0 to 120 mA to a 20 Ohm load, which is independent of temperature change. That would be just kick-ass. I'm thinking about perhaps hi-power op-amps that allow upto 150 or 200 mA.

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I've never built that particular circuit, but have built very similar circuits. Unfortunatly, the easiest way to measure current it to put it through a resistor and measure the drop. You indirectly bring up the age old question of... how good is good enough!! Since there will ALWAYS be "some drift", how "little" is is "little" enough? Better precsion calls for better part's... i.e. a 20 ohm .01% 25PPM resistor would be nice, but is it practical? Same goes for the input offset voltage to the op-amp... how small is small enough? Then there is the reference voltage and the resistive divider... how good is good enough? The better it has to be the more it will cost, and the more complex it'll become. I didn't put any values in, exept the 20 ohms and divider nodal voltages, because they are the key to "operational concept"... 120mA and 20 ohm... Hmmmm that's 2.4V, so varing the voltage across the 20 ohm from 0 to 2.4V gives you 0-120mA. You could get fancy with a SUPER accurate and stable resistor and make a current mirror and have it work for any load within the compliance voltage of your source.

How "good" do you need it to be?

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If your resistor is really a constant 20 ohms, then you only need a variable voltage source with a range of zero to 2.4V, which is what Indulis's circuit does (although it may oscillate).
A current source provides constant current independent of the load. Does your load vary? If so, I can provide you with the schematic for a variable constant current source.

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Yes that's the whole point! The 20 Ohm load I've got begins to vary, but I don't want the current within it to change, it must stay constant.

I would be very grateful if you could let me see that schematic for a variable constant current source.



^ That's another version of the circuit I designed, I hoped the tighter spec LM224 would give better results or something.

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indulis, R1 and R5 are present to take some voltage away from the transistor so it dissipates as little power as possible, with the setup shown it only churns out ~400 mW.

The application is basically that I have a ~20 Ohm load resistor which I expect to change in value in accordance with its temperature, but the current that I apply through it should not change, even if the resistance were to change.

However I have tried it out a few times and the load does not look like its the problem, I think its more to do with the circuit, I have a funny feelind that the feedback might be messed up.

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How would I go about creating a constant current source to supply a 20 Ohm load? I want the current to be variable from 0 to 120 mA.

So I need a circuit which supplies a constant current to a 20 Ohm load, in the range of 0 to 120 mA.



^ That is a circuit that I have designed already, however I believe it is a little unstable and the transistor may need a heatsink to suffice the circuit.

I was hoping that perhaps there is a way to achieve a circuit that is independent of temperature, so I won't need a heatsink or have to worry about possible thermal runaway.


Please check at ELV.DE. As far as I know they offer a programmable current sink for a widespread band of operation. Don't sink too much current - otherwise You will have to buy an own nuclear power plant to cool the device properly.

The german word for this device is: Programmierbare Stromsenke

Regards

Herman the German
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