Do these numbers make sense?

Howdy, I've been lurking here for a few months and I'm onto my 2nd build now. I went a bit more DIY this time and built my own test load to set the driver current:



My diode is an Osram PL450B that I'm going to feed 200-250ma from a Micro Boost driver (fed by 1 li-ion battery). According to this thread, the PL450B drops about 6 volts at my target current: http://laserpointerforums.com/f65/osram-pl450-450nm-80mw-power-chart-71645.html

Based on my calculations, I should hook up my driver to the test load with 8 of the 1N4007 diodes in the circuit (0.7 x 8 = 5.6) to come close to the 6 volt drop of the PL450B. I just wanted to ask you experts if these numbers make sense since most of the info I read about the test loads suggests using either 4 or 6 diodes in the circuit.

If you have a bench power supply you can test the voltage drop at the current you will be using. I believe those are rated 0.7V drop @ 1A so they could be dropping a lot less than that at lower currents. With the blueray setting on my test load it only drops 4V @ 300mA.

Thanks, DTR! Unfortunately, I don't have a bench power supply yet. I'm just getting started in this cool hobby and I'm ramping things up slowly with each build I do.

I've got a couple extra 1N4007 diodes loose - do you know of any way to test one with some basic equipment? Batteries, multimeter, etc...

I'm not going to be running my laser diode near its max so as long as I can use my test load to get the Micro Boost in the ball park in terms of current, I think I'd be OK. Then I'll fine tune once the actual laser diode is connected.

Run your test load at the current you are interested in and use your DMM to measure the voltage across one of the diodes. This will give you an idea how much each drops at that particular current. All diodes aren't created equal so some of them may read a slightly different voltage.

Start connecting the microboost to the load using 6 diodes and measure the voltage on all of diodes. You can measure in the series, not needed to measure one by one and sum. Just remember the output for this driver is minimum Vsource, maximum 13v, so don't feed it more than the test load drop...

But something you said sounds strange...

Stereo Joe said:

I'm not going to be running my laser diode near its max so as long as I can use my test load to get the Micro Boost in the ball park in terms of current, I think I'd be OK. Then I'll fine tune once the actual laser diode is connected.

Click to expand...

Ball park? The diodes on the load sets a voltage drop, it don't go more or less accurate if you miss the actual quantity of diodes.
What would give you a "ball park" value would be a 10% tolerance 1ohm resistor.

When in doubt, you can always connect your DMM in current mode, "between" the whole test load and driver.

And you said "fine tune"... How will you do that? Turning the pot? lol. Thats >$100 you're playing with... beware.

edit:
@DTR
The drop vary for each diode. Most are rated 1.1V drop @ 1A.