Driver Current/ Test Load Question

[cosmiceleven]

I have always had DTR set my current for me when I order driver boards from him but I have a couple adjustable drivers from him that I'd like to change the current on. If I purchase the 20A test load from survival lasers I read in the instructions that I need to choose number of diodes to simulate my the laser diode I'll be using but when I order from DTR he doesn't know what diode I'm using the driver for so I'm just trying to figure out he sets the current for a situation like this. So is a dummy load required for something like this, if so what setting would I put it on in order to, lets say, set a blackbuck 8 to 4.5a? Any help would be appreciated. Thanks!

 

[CynicalBrad]

Yes, a dummy load is required for this. You set the number of diodes on the test load using the jumper to simulate the diode you are setting the driver for. You also need to take voltage drop across the resistor into account. Noone here can accurately answer what bridge setting to use without knowing what diode you are setting the driver up for. That would depend on the forward voltage of the chosen laser diode. The pdf on the page for that test load, while not formatted the best, also provides some useful info.

 

[paul1598419]

Since these drivers are constant current regulated, you can set them up without knowing what diode will be used if you know something about the driver. For bucking and linear drivers, the forward voltage of the diode will be less than the supply to the diode, so 5 or 6 diodes plus the voltage drop across the resistor should work fine. Keep in mind that each diode is ~0.7 volts. If it is a boost driver, the forward voltage must be higher than the supply voltage. If the supply is 4.2 VDC, 6 to 8 diodes should work. Then there are bucking/boost drivers. These will chage regulation depending on the conditions. If you don't know the diode here, I would split the difference and use 6 or 7 diodes. Hope this helps.

 

[Benm]

The idea of the dummy load is to test what a driver will do to an actual laser diode by simulating it's electrical characteristics as well as possible - usually with a number of silicon diodes and a resistor in series.

For boost regulators you actually require such a load to test them, although you could also use a resistor that matches the voltage and current of the diode you intend to use.

For buck and linear regulators it depends - some will drive a dead short with no problem so you can just use a multimeter in the 10/20A range as a load and get an accurate result.

That only goes for ideal drivers though, some may have some current variation depending on dropout voltage.

A well built linear current regulator should not give a darn what it's load is, as long as it's low resistance enough to take the required current at the supply voltage. I prefer opamp based designs personally, and those really don't give a different current if the load is a laser diode or a crowbar.

The reason for that is that the output element is a big transistor or mosfet, but the reference source is a small voltage reference or diode not heated by that output driver very much.

Upside of the approach is also that it prevents thermal runaway: If somehow the whole circuit heats up, the voltage across the reference diodes goes slightly down, resulting in lower output current.

You could purposely expand that effect a bit further by having an NTC resistor pulling down the reference such that current falls off with system temperature. I've never needed to build that, but for a handheld laser that has insufficient heatsinking to run continiously it could be a good feature.

 

[cosmiceleven]

Thanks for the replies everyone! You've all helped me out a great deal, I really appreciate it