Diode, Test load and Multimeter questions?

So, I need a little bit of clarification. What exactly is the test load used for? I've set one up already, but I don't really know how to use it. Does it work to change current on a driver? Alter something on the laser diode?

Another question...I know diodes can get easily fried if you overpower them, even if for a tiny moment. How can I go about knowing the capabilities of a diode, and how can I make the current on a driver be select to output close to the diode's potential?

Lastly, a multimeter. How does this thing work with the test load?

the test load simulates the voltage drop from a diode while you measure the current of your driver. so it does two things- measure current(1ohm resistor and multimeter on dc volts) and simulate voltage drop(diodes) so you know if your driver and/or battery is good for your laser diode.
The capabilities of a unknown diode can be estimated from the speed of the drive it was from(8x,16x,22x,etc.) not sure on the values though, someone else here should know though.

Chris

Test loads: the simplest test load is a single resistor. However, for a given amount of current, a resistor can't simulate the same kind of voltage drop that your laser diode would produce. You'd have to tune the resistor for the amount of current and voltage each time. So for example, if you had a 4.5V laser diode that you wanted to test at an amp of power, you'd need a 4.5 ohm resistor that can endure 4.5W of power and cause a voltage drop of 4.5V. For some other laser you'd need a different valued resistor, usually ones that are more expensive (5W 4.5ohm resistors are relatively pricey). Instead, you could put a single 1 ohm resistor in series with some rectifier diodes that drop the voltage across the resistor to lower levels. By varying the number of diodes (which are easy to obtain and cheap), you can see what your driver needs to be set at.

Testing diodes: Buy a bunch and test them until they die. Barring that, a "rule of thumb" is to raise the current slowly until you detect a "knee" in the mW/mA graph. The "knee" is where you start getting less mW for a given mA. Prior to that point it is usually quite linear. Back off just before the knee and your diode will usually have reasonably long life.

Multimeters and test loads: One thing good about using a test load is that it keeps your multimeter out of the "loop" so to speak. In other words, rather than measuring the current using the current-measuring (mA) mode of the multimeter, you measure the voltage drop across a known resistor in your test load. So if you have a 1ohm resistor, the amount of current flowing through it will be proportional to the voltage you measure across it. Since the multimeter isn't in line with the circuit, you don't risk blowing out your multimeter, or frying your diode accidentally. Also while nearly all multimeters have wide and accurate voltage measurement ranges, many multimeters can't measure current over a wide range. Even expensive ones only go to 10A or so and require switching the cable between sockets; many cheaper ones top off at 200mA.