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! :cryyy:
LPF Donation via Stripe | LPF Donation - Other Methods
Links below open in new window
ArcticMyst Security by Avery
445nm DIY driver issue
- Thread starter Hodajii
- Start date
itsmei
0
- Joined
- Dec 14, 2009
- Messages
- 151
- Points
- 0
What ohm resistors are you using for each of the 5 resistors in parallel and what wattage?
rhd
0
- Joined
- Dec 7, 2010
- Messages
- 8,475
- Points
- 0
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.
1.25 / 0.2 = 7.5 A
I think you're toasting your diode with way too much current.
rhd
0
- Joined
- Dec 7, 2010
- Messages
- 8,475
- Points
- 0
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)
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)
rhd
0
- Joined
- Dec 7, 2010
- Messages
- 8,475
- Points
- 0
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?
- 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?
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!
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.
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.
Last edited:
rhd
0
- Joined
- Dec 7, 2010
- Messages
- 8,475
- Points
- 0
I think your diode is just toast.
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 ->
Last edited:
rhd
0
- Joined
- Dec 7, 2010
- Messages
- 8,475
- Points
- 0
It happens to everyone.
You may have forgotten to discharge a capacitor, or allowed static to enter the equation.
You may have forgotten to discharge a capacitor, or allowed static to enter the equation.
rhd
0
- Joined
- Dec 7, 2010
- Messages
- 8,475
- Points
- 0
"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.
