Re: Constant current dummy load - home etched, single sided
You may have already figured this out, but the limiting factor in this circuit isn't the opamp supply voltage. The range is more dependent on the sense resistor. For instance, with a 1 ohm 5 Watt resistor, you're going to dissipate I^2 watts. You'll exceed 5 Watts at 2.24 Amps. Any higher than that you'll burn up your resistors.
Now, before I move on, one consideration is that the OpAmp must be able to supply enough bias voltage to the mosfet base to reach saturation. An amp like the LM324 can go from 0 to Vsupply - 1.5 volts, so you need to look at the datasheet for your mosfet to get the saturation voltage and make sure to drive your opamp at Vsaturation + 1.5 minimum.
For the sake of argument, let's say you're still using a 1 ohm resistor and expect up to 6.5 volts across it, giving you 6.5 amps. At that current your sense resistor will need to dissipate over 42 watts. Yikes. The way to reduce this is to use a smaller value for your sense resistor. In fact, I built this circuit using 40 half-watt 10 ohm resistors in parallel for 0.25 ohms. This way I am able to sink currents up to 8.9 amps. I also added a toggle switch that bypasses 30 of these resistors to leave me with a 1 ohm sense resistor. This way I can switch between a "high" range (about 8ma to 8.9 Amps) and a "low" range of about 2ma to 2.2 Amps.