I'm trying to design a 0 to 300 mA controlled current source, the problem is that the supply voltage is a normal adapter with a 12v output. I'v already tried several op-Amp based circuits but the voltage drop over the load exceeds the power supply in all of them. suggestions for a circuit?
Load is a constant 40ohm device.
Thank you all
Electrical – Designing 0- 300 mA current source with 12v Voltage supply (load = constant 40ohm)
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You mention that you want fast switching. None of the devices will be able to do that. Any the feedback circuits internally are designed to be slow so that they don't to much incorporate noise. And in fact some of the feedback is thermal feedback. Put another way, the dominant pole is low, 100's of KHz is typically the the fastest response time. This is true whether you are using a disable pin or connecting/opening the load to the source.
The best way to fix this is that you build an external circuit that current steers between two limbs. On one limb will be your LED chain and on the other a dummy load that draws the same amount of current (and hopefully also has similar load characteristics). An example of such a circuit is a differential pair. You should be able to get 10's of ns switching time then.
The challenge with the two limbs will be that the load characteristic differences may disturb the feedback of the LM317 etc. which then reacts slowly. If you can't match the two limbs then I'd suggest building a current mirror that decouples the current output the LM317 etc. so that it only sees a constant load the mirror transistor sees the variability.
You should be able to simulate all of this in LTSPice or similar before building.
The other factors you mention above not show stoppers, so pick what you want.
I've successfully used Vcc tied loads like this in constant current circuits: -
simulate this circuit – Schematic created using CircuitLab
The op-amp has negative feedback from the emitter and therefore the emitter voltage must be at the same potential as the input. This means the current through the BJT = \$\dfrac{V_{IN}}{R1}\$
You can also use a PNP BJT and an op-amp mirrored up to the positive supply rail for ground coupled loads.
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Best Answer
You say you want a 0-300 mA current source, but that the load is a constant 40 Ω. There is therefore no distinction between a 0-300 mA current source and a 0-12 V voltage source.
The biggest problem you have is that your circuit can't drop any voltage at full output since you only have 12 V to start with. That's not possible, so what you asked can't actually be done. However, you can get reasonably close. Of course a better answer would be to start with a power supply a little above 12 V so that the pass element is allowed to drop some voltage at full load.
To minimize the voltage drop across the pass element, use a MOSFET. It's not hard to find devices that can go down to a few mΩ with full gate drive. 20 mΩ, for example, is 0.05% of your 40 Ω load. That would be within measurement error for most purposes.
You didn't give any constraints on how this current source is supposed to be controlled, so we can pick something simple. A pot setting the gate voltage of the MOSFET provides control over the current.
Here is a circuit that meets all your specs, except that it doesn't quite put out 12 V or 300 mA with exactly 12 V in:
This particular MOSFET can handle up to 20 V and goes down to less than 10 mΩ with 10 V on the gate. Only you can say whether that is close enough. Again, your specs are actually impossible to meet.
The control is achieved by rotating the pot (R1).