# Current source with low-side LOAD

Why does this work?

simulate this circuit – Schematic created using CircuitLab

But this doesn't.

simulate this circuit

In the second schematic, V2 is adjustable.

Is there a better (not very complicated) way to make an adjustable current source where the LOAD is between Source (transistor) and GND (not with the sense resistor connected to GND and the LOAD on the high-side)?

Does the op-amp have to be powered with a dual power supply?

• What current are you trying to get through the 100 ohm load? Commented Feb 16, 2016 at 8:34
• Actually neither circuit works because the LM358 doesn't work properly with its inputs close to its positive rails. You should also have V2 referenced to the positive rail - it should not be ground referenced because THIS is not how this sort of current generator is intended to work. Commented Feb 16, 2016 at 9:14
• @SpehroPefhany I'd like to get a few amps if possible. The 100 Ohms is just a placeholder. LOAD will be from short (0 ohms) to whatever (until the voltage supply isn't able to provide the set current through the LOAD). Commented Feb 16, 2016 at 11:33
• With your 1 Ohm current-sense resistor, the current will be set according to voltage on the (+) opamp input, where I = V1 - input. If the input = V1, the current will be zero. If the input is V1 - 1V (or 9V), the current will be 2A. Etc. This holds until the voltage across the load becomes so high that the FET cannot provide the current -- the Vgs needs to be a few volts for the FET to function (look at the FET specs).
– Paul
Commented Feb 17, 2016 at 7:22
• And then there's the opamp. It will probably not need dual supplies, but you need to be sure that the inputs can operate up to the V1 voltage.
– Paul
Commented Feb 17, 2016 at 7:28