This instructable will show you how to make a current source with negligible current sag for loads with a resistance of less than 1.4k. In our example using a power supply, the current provided is 9.1 mA but can be adjusted. The advantage of a current source is that it provides a constant amount of current that is independent of the load’s resistance. Further, this circuit has the load grounded, which eliminates the danger of having two exposed connections with floating voltages.
One LF411 8 pin Operational Amplifier (Op Amp) ~$4
One PNP transistor model 2N3609 ~$2
Three resistors of values 10k, 1k, 150Ω <$1
A load of your choosing (3 LEDs in the above picture)
Power Supply. In this example, we used a +15 V and -15V commercial power supply.*
*Note: We were also able to power this circuit with two 9V batteries (one for each the + and the -), but circuit produced a current of only 5.4mA with a similar maximum load. Also, the negative terminal battery became very warm quickly, so the power supply is highly recommended. Power supplies can be made by converting an ATX power supply from an old computer such as this instructable