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Old 04-03-2011, 11:36 AM #1
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Unhappy 445nm DIY driver issue

I recently built a lab style 445nm laser @ 1.2A but as I turn it on the voltage starts to drop rapidly and the laser dims because of it. I use LM317 and a PC power supply 12V rail. Same happens with a 9V 800mAh battery. Current stays stable but the voltage drops. The circuit I'm using is constant current with 1Ohm resistor pack and a capasitor in front of the diode. LM317 is heatsinked and the diode stays cool. My resistor pack (5 resistors in paraller) is the only part which gets warm. Hapl pleees!


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Old 04-03-2011, 02:19 PM #2
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Default Re: 445nm DIY driver issue

What ohm resistors are you using for each of the 5 resistors in parallel and what wattage?
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I am going to be building a 1.5W 445nm laser in a few months. I am also building a 120mW green 532nm.



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Old 04-03-2011, 02:42 PM #3
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Default Re: 445nm DIY driver issue

5 1Ohm resistors in parallel = 0.2 Ohms.

1.25 / 0.2 = 7.5 A

I think you're toasting your diode with way too much current.
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Old 04-03-2011, 03:02 PM #4
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Default Re: 445nm DIY driver issue

Quote:
Originally Posted by rhd View Post

I think you're toasting your diode with way too much current.
If he is using 5x 1 ohm resistors in parallel that is why the diode is going dead. I would used 2x 0.5 2W ohm resistors in series this way both your wattage and ohms will be doubled, this means you will get 1ohm and 2W.
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I am going to be building a 1.5W 445nm laser in a few months. I am also building a 120mW green 532nm.



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Old 04-03-2011, 03:09 PM #5
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Default Re: 445nm DIY driver issue

I mean my 5 0.25W resistors make ~1.2ohm in total. I've measured everything and my current is stable ~1.2 amps but the voltage drops rapidly when I turn the laser on.
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Old 04-03-2011, 03:20 PM #6
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Default Re: 445nm DIY driver issue

So they are:

5x 0.24 Ohm - in series
or
5x 6 Ohm - in parallel

?

I'm trying to figure out what your resistors are first. Can you clarify very specifically:
1) What Ohm are they?
2) What Watt are they?
3) Series or Parallel?

(just the resistors - on their own - want to know what you actually used)
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Old 04-03-2011, 03:26 PM #7
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Default Re: 445nm DIY driver issue

5*4.7ohm 0.25W resistors in parallel. Attached to the board they give 1.6ohms in total. Checked and double checked.
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Old 04-03-2011, 03:44 PM #8
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Default Re: 445nm DIY driver issue

That's your problem:

- 5*4.7 ohm will give you 0.94 Ohms.
- Remember, at such low resistances, your meter's probes and wires skew the readings. In fact, my DMM's probe wires add pretty much exactly 0.6 Ohms, so my readings are always 0.6 ohms high.

So you're getting a resistance of 0.94 ohms, which should create current of 1330 mA. You're getting 1.25W out of the parallel resistors, but they're dissipating 1.66W of power as heat. In other words, you're using resistors of too low a wattage.

- Further, when you use the 12V rail, you're probably asking the 317 to dissipate way too much excess voltage as heat. That's a problem.
- When you're using a 9v "800mAh" battery, what kind of battery is that? Do you mean a typical 9V (square) battery?
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Old 04-03-2011, 04:00 PM #9
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Default Re: 445nm DIY driver issue

So I need higher wattage resistors. My multimeter is very high quality and it's wires give 0.1ohms (checked) and I added it to the reading I got from the resistors earlier. The LM317 is not heating up because I've got a huge heatsink for it. The battery is from an airsoft gun (I'm aware that a normal 9volt battery won't work). But I'll get some higher wattage resistors and report back if it helps. Thanks for the help!
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Old 04-04-2011, 12:09 PM #10
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Default Re: 445nm DIY driver issue

now I've got 3*0.47ohm 5Watt resistors in series and the laser worked for 10 seconds before dimming again. The voltage drops but much slower... No parts heating up
Edit: I measured the current and it's still stable. With these resistors 890mA which should be enough for a bright light rather than a blue(ish) glow. The voltage is just too low and keeps dropping.
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Old 04-04-2011, 03:41 PM #11
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Default Re: 445nm DIY driver issue

I think your diode is just toast.
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Old 04-04-2011, 03:49 PM #12
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Default Re: 445nm DIY driver issue

Yea... I think its a toast now. I had it running for 10 minutes and it was warm to hand and dim like my semi-dead blu-ray. DAMN it was expensive! I even bought safety glasses for this one. The diode was never working right. My first try was with ~500mA and it did the same thing: few seconds bright and then started to drop the voltage. No parts were heating up but it still dies... !!!!! pissed off but I learned something anyways. Thanks for your help, I'm off to build another red laser ->
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Old 04-04-2011, 04:13 PM #13
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Default Re: 445nm DIY driver issue

It happens to everyone.

You may have forgotten to discharge a capacitor, or allowed static to enter the equation.
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Old 04-04-2011, 04:17 PM #14
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Default Re: 445nm DIY driver issue

I had one close call with the cap when I detached the diode wires from the board but I managed to short it in time (I guess) after the diode died. I think I destroyed another diode(again) with static... Glad I still have healthy eyes tho ^^
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Old 04-04-2011, 04:35 PM #15
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Default Re: 445nm DIY driver issue

"short it in time" and "close call" don't really compute when talking about capacitor discharge. It would be instantaneous. You wouldn't have any time (practically speaking) to do anything reactive.
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Old 04-04-2011, 05:27 PM #16
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Default Re: 445nm DIY driver issue

Quote:
Originally Posted by rhd View Post
"short it in time" and "close call" don't really compute when talking about capacitor discharge. It would be instantaneous. You wouldn't have any time (practically speaking) to do anything reactive.
Well the diode was not in contact with the board when my cap was charged and I almost connected it. Thats a close call. BUT! This was a learning experience and my next 445nm will be made in an ESD protected area and I now have the know how. I will now start to work with my IR/Red diodes.
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