How do I make a constant current load?

You'll need a MOSFET to handle 5A at 0.8V (Ron < 0.15 ohm), assuming that combination is in your area of operation. A couple MOSFETs paralleled would be nice to spread out the heat, although one can handle the 15V * 5A = 75W maximum you want to dissipate. Such MOSFETs are cheap, you'll spend a few bucks at most.

A small source resistor to detect current, an op-amp with output connected to MOSFET gate, inverting input connected to source and noninverting input connected to a voltage that determines current (to be exact, Iout = Vin / Rsource). You'll need a 10V supply for the op-amp, and around a 100 ohm resistor in series with the MOSFET's gate to prevent oscillation. Remember to get an op-amp that can operate with its inputs at very low voltage.

Don't forget the source resistor subtracts from the MOSFET's available voltage. You might want as low as 0.05 ohm here. That leaves 0.15 - 0.05 =

0.1 ohm for the MOSFET, which is still easy enough. 0.05 ohm puts your input at 0 to 0.25 volts for full range.

Tim

-- Deep Fryer: a very philosophical monk. Website:

There has recently been a design idea in EDN Europe for a similar device

august 2005, a programmable active load) but you'll have to modify it to go down to 0.8V.

--DF

Also (much simpler, referenced by the other article)

--DF