Help! How i do a test-load?

[daniloxx]

i was reading the "first build guide" here in the forum, and i did not understood how to prepare a test-load and how to proceed with it.

also i didn't understood how to regulate the current on a driver.

hope someone can help me, thanks a lot !!!

 

[Bionic-Badger]

Before you burn out a lot of parts, you should read up on some basic electronics. Here's a link to a tutorial. Others may have better suggestions.

 

[Benm]

To test a test load, i'd suggest this:

Get a 9V battery and a 100 ohm resistor. Connect the negative end of the testload and the battery to eachother. Connect the 100 ohm resistor between the positive pole of the battery and the other side of it to the test load.

Now, using a multimeter, measure the voltage over the test load. If it uses 3 diodes and a 1 ohm resistor, this should be in the order of 2.2 volts. Then measure the voltage over the 1 ohm resistor. Since its powerd by 9-2.2 = 6.8 volts over a 100 ohm resistor, the current should be about 68 mA (i.e. the measured voltage should be 68 mV).

The current is only valid if the battery is actually 9 volts, and may be a bit lower if its not a brand new battery. Don't worry if it reads 50 mV or something like that!

 

[Leodahsan]

Haha after reading Benm's explaining, I'm confuse too.

J/K

I've read that the diodes vary from laser to laser.. reddies use 3, blu goes up to 7. We should take the diode forward voltage and divide it by 0.6 (diode dropout) to result in the quantity of diodes?

 

[Coherent Light]

Lazeerer uses 4 diodes in His test loads for 405/445, and that's what I started using with no problems as of yet. I do however back off on the ma just a bit using this setup just in case is not accurate enough to push the current to the max.

 

[Benm]

Something like that. I'd go for 0.9 or so for the dropout at higher currents.

To simulate a red, use 3 1n4001 diodes and one 1 ohm resistor in series.

To simulate a bluray, use 5 or 6 1n4001 diodes and one 1 ohm resistor.

It's not overly important that the dummy load matches the laser diode exactly, the main purpose of using a dummy load is to test if the driver was assembled correctly, and to set the current. Since drivers are constant current sources, it does not matter if the dummy load is off by half a volt or something.

 

[Leodahsan]

Oh now I understand. But what about the 2W 445s? 1N5408? ^^

 

[Coherent Light]

Yes use the 1N54XX series of Diodes for a high current loads

 

[daniloxx]

ok, i understood that i need those 1n4001 for the test, i put them in series to reach the voltage of the real diode i will use, then in the end of this serie of diodes i put a 1ohm resistor then i put the positive in one side of the ohm resistor and the negative on the other side of it right?

YouTube - Part 1 - Tutorial - Measuring Current on the rkcstr MICRO-DRIVE Laser Diode Driver

thats what i understood from this video.

i just didn´t understood where to connect the parts for the test, the batteries are connected directly to the driver? where he soldered them? then there are some couple of wires soldered to the driver and to the switch and then to the diodes+resistor, that´s not very clear to me, someone please make or send me a schematic on where to solder the parts to the test, even if draw it in paint, doesnt matter xD


thanks again for you guys that already answered me and for any incoming help

 

[Bionic-Badger]

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

 

[Leodahsan]

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).

 

[Bionic-Badger]

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.

 

[Leodahsan]

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.

I'm always learning, thats incredible.
Thanks BB.

 

[Benm]

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.

That alone is a good reason never to use a multimeter in series with the laser diode. The connection can be unreliable even is you don't switch current ranges. Most multimeter buttons are just some pieces of copper tracking over a printed circuit board below. Just moving the meter around can give a brief interupption, especially on cheaper ones.