The goal of the test load is to mimic the voltage the laser diode will drop, while giving you something to measure the current with. So if your laser diode's forward voltage drop (V_f) is 5.2V, and you estimate that you want 800mA, and your test load's resistor is 0.6 ohm (just to throw out numbers) then:
Voltage dropped by the resistor (V = IR): 0.800A * 0.6ohm = 0.48V
To get in the ballpark of 5.2V, with 1N4001 diodes that drop about 0.7V each, you need:
(5.2V - 0.48V) / 0.7V = 6.7 diodes
Round up to 7 and you should be okay (6 is okay too, since it's a ballpark).
Put all the diodes in series with the resistor (for six diodes):
(-) --|>|----|>|----|>|----|>|----|>|----|>|----/\/\/\/\-- (+)
Measure across the resistor to see what the voltage is, which shows you how much current the driver will be supplying to your laser when you hook it up:
Current = Measured Voltage / 0.6ohm
I always had a question.. Why not just wire the driver to a DMM in miliamps config? They supply a load and show the current draw..
mine only have 10A config, so teh smallest number he can show is 00.1A (100mA).
A few reasons:
1) The DMM isn't the same kind of load. For one, you're not mimicking the voltage drop of the laser diode; some drivers respond differently at different voltage drops. Also you could damage or even destroy your driver if you suddenly lose the connection or if the DMM doesn't provide a sufficient load like in those Lavadrives expect.
2) You can blow out your DMM, and at the higher fuse ratings often have lower precision like you noticed above. Also why risk damaging your DMM or possibly need to find a new fuse for it when you can just rig up a cheap dummy load? The dummy load takes your DMM out of the driver system so it doesn't affect it and doesn't get affected by it.
3) You can blow out your laser diode! I've put my DMM inline with a low-powered 635nm diode run at about 15mA. That's a current rating that wouldn't affect a DMM. However, I stupidly switched the switch while it was on and the resulting change killed the laser diode. Fortunately the meter was okay. If I had put a resistor in there or something to measure upon it would have never happened. Oh well.
You can blow out your laser diode! I've put my DMM inline with a low-powered 635nm diode run at about 15mA. That's a current rating that wouldn't affect a DMM. However, I stupidly switched the switch while it was on and the resulting change killed the laser diode. Fortunately the meter was okay. If I had put a resistor in there or something to measure upon it would have never happened. Oh well.