Measuring Laser Diode powersupply - Page 1
Forum section: electronics / repair

author: kolbep
author_rank: Frequent Contributor
timestamp: August 21, 2015, 03:02:38 pm
content: HiSo I bought a dual head laser from a local pawn broker. I negotiated them down because the red laser diode output is very faintThe unit has a red and green laser diode in it. The Chinese lies, I mean specs, say that the green laser is 60mW and the red laser is 120mW.I hooked the red laser diode up in place of the green one, and it was still barely visible. The green laser is perfect. Leads me to believe that the powersupply is fine, and it is just the diode that is faultyNow the question is, how can I tell what the proper power for the red diode is, so that I can get the proper replacement.Can i somehow measure by using a resistor, and work out the current the supply is giving out???

author: acbern
author_rank: Frequent Contributor
timestamp: August 21, 2015, 03:31:57 pm
content: to be safe, you should consult the data sheet of the pwer supply and search for an appropriate diode then.

author: kolbep
author_rank: Frequent Contributor
timestamp: August 21, 2015, 08:38:09 pm
content: Here is a shot of the Laser,as well as the powersupply board that drives the lasers.This board has on the left hand side, an AC Input, as well as inputs for the micro to switch the lasers on and off.On the right hand side is the outputs to the laser diodes.I measured the outputs to the Diodes. I was using a cheap multimeter, and also did not take into account that it was possibly pulsing the lasers.I got a voltage of 3.7v, and the current being drawn by the green laser diode was 0.78A. Does that sound right.Oh, and I did change all the resistors at the bottom, because they were rusted like crazyThere are no markings on the powersupply board

author: acbern
author_rank: Frequent Contributor
timestamp: August 21, 2015, 08:59:30 pm
content: Try to identify the ICs and power switch transistors, that way you may be able to derive the appr. power capabilities. Anyway, this is not a very powerfull PSU.

author: kolbep
author_rank: Frequent Contributor
timestamp: August 22, 2015, 01:40:43 pm
content: Thanks.I will do that.It does not seem too complicated, so I might even be able to reverse engineer it.Do you think it could really be 60mW Green / 120mW Red, or is it overstated.When I measured the Voltage and Current for the Green Laser Diode, it was 3.7v and 0.78A, which means that it is operating the green laser at 2.88W?? (unless I am wrong about that...) I dont know how the diode is surviving, because if it is really a 60mW, then it should be toasted?The laser diode only has 2 wires, so it does not do any feedback..

author: e_sousa
author_rank: Contributor
timestamp: August 22, 2015, 02:12:12 pm
content: The green laser is not a simple diode, it is a dpss module(much less efficiency than a laser diode), it has a higher power ir pump diode, the power requirements are not the same as a simple red diode.

author: kolbep
author_rank: Frequent Contributor
timestamp: August 23, 2015, 09:18:34 am
content: What about putting a resistor on the output where the red lAser diode goesI can then measure the current flowing though it, to determine what laser diode I need (based on the voltage and current.)Or doesn't it work like that???

author: Psi
author_rank: Super Contributor
timestamp: August 23, 2015, 09:24:06 am
content: Quote from: kolbep on August 21, 2015, 03:02:38 pmI'd probably just wack in the red diode from a DVD burner.But maybe that's just meThey're usually 200~300 mw form a 16x DVD burner and cost like $20 for the whole burner

author: kolbep
author_rank: Frequent Contributor
timestamp: August 23, 2015, 03:35:10 pm
content: Thanks Greek Letter PSI.That Idea works beautifully.Unfortunately I did not realise that the red laser diode from the DVD burner did not have a lense assembly, and that it is part of the mirrors, etc, inside the pickup head.Tried just putting the diode in the existing lens assembly from the Laser, but just got a very wide beam.I then loosely put the diode back into the head from the dvd burner. Hold it at right angles, and it worked quite well. A bit dimmer than the Green, but I guess that is expected.I measured the current and voltage while running this new laser diode, and it was giving150mA Average2.65V Almost Constant.Which works out to almost 400mW.Do they class the Laser diodes based on the input power, or by the output.Will it last long using the DVD diode, or is it overdriving it.If I do want to get a Laser diode with built in lens, that works with this powersupply, what type must I get (spec wise)Thanks.P

author: Psi
author_rank: Super Contributor
timestamp: August 25, 2015, 12:30:55 pm
content: From memory, dvd burner diodes run much higher than 150mA input current.Pretty sure you are underdriving itLaser diodes are classed on the light output power.So a 200mW laser will draw much more than 200mW input.Probably more like 600mW inputYou should be able to use the existing lens.I'm not sure why that isnt working.

author: kolbep
author_rank: Frequent Contributor
timestamp: September 12, 2015, 03:12:38 pm
content: OkJust another point on this.Attached is the Final Stage of the Laser Powersupply (For the Green Laser Diode)I replaced a lot of the resistors in this section, but I did not have any 0,5ohm resistors on hand.For some reason they have 2 off 0,5 ohms in Parallel, which is in series with another 2 off 0,5 ohms in parallel.So basically between the 0v and the Emitter of the NPN transistor, you have a resistance of 0,5 ohms,but why would they have used the parallel/series combination. Is it in parallel for the Current expected?I did a test using a 1 ohm resistor (instead of the whole 0,5 ohm section), but the laser output was very dim.I bridged out this section, and the laser was bright (did a quick test only)..Does it need to have the 0,5 ohms in the base, what is it for?. If so, then I will need to get some 0,5 ohm 1/4 watt resistors??I definately do not want to overdrive the Laser diode, so I would rather be safe...ThanksP

author: SeanB
author_rank: Super Contributor
timestamp: September 12, 2015, 07:43:27 pm
content: The resistance is there to drive the laser diode with a constant current, as the transistor and the resistors form a constant current sink so the diode will pass a roughly constant current as it heats up. The resistor network is there instead of a high power resistor, it was probably a lot cheaper to put down the 4 low power resistors instead of buying in a 5W wire ended resistor that in any case would be a 10% part. Using the 4 SMD resistors instead gave them around 0.5W of power rating, or 1W Chinese rating, and as the diode runs at around 300-500mA the dissipation would be within the ratings of the resistors as a group, but would be over the 1/8W of a single one.