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Old 07-29-2010, 05:20 AM #1
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Default LM317 getting hot in <10sec.. Short in diode wiring?

Hi Everyone. First post.

I have been working on a lpc815/lm317t setup for the past month (in between some repairs and shipments), and I got it working with a cheap diode and I measured the current with the multimeter. Read 220. added a resistor for 350. So I've put my diode in a o-like housing and for some reason the lpc815 that was once bright would get dimmer and the housing would getting warm. So I replaced the schotty diode thinking that might do something (so naive I know) and now the beam is constant, however weak. The brand new lm317 is getting hot fast. So my thinking is that maybe there's a short... I'm guessing through the diode itself. The multimeter still reads 354mA. Any takers?

Also, how do you tell if there's something wrong with an open can diode... my issue is that when I focus the very weak beam I get Uranus.

I'm TOO LAZY to build a test load. I used a crappy diode.

Thanks guys.



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Old 07-29-2010, 05:37 AM #2
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

should maybe try a test load

and perhaps post a picture of your driver

but to be honest if everything is working fine, just glue a penny to your lm317 or w.e =]
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Old 07-29-2010, 05:46 AM #3
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

Yeah maybe try a test load. Only takes about 1$ in total value to make. No big deal. 6 1N4001 and 1 ohm resistor or 2ohm (just multiply the mV by two to get current, only if 1ohm is unavailable) wired in series is completely easy. Solder it all up, and you're done. I like easy things such as this.
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Old 07-29-2010, 06:04 AM #4
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?


The dimness occurs even when the one half of the driver is out of black case, so it's not a case short.




could the capacitor be bad?

I took out the pot because I didn't think I needed it at the moment and I wanted to eliminate the pot from the problem bin.
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Old 07-29-2010, 06:07 AM #5
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

so what am I checking for with the test? voltage? I already have read 354mA using this diode. I'm missing something. (noob)

and the diode isn't fried now so I don't see why I would need the test (another trip to the radioshack) and it's more than 1 dollar. more like 5 dollars.

I don't think everything is working fine, nosebleed, because the thing does not produce a beam brighter than the gas station pointers....

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Old 07-29-2010, 09:31 AM #6
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

the lm317 does get hot quick, heak sinks for them can be brought from radio shack in T220 package, small or large.
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Old 07-29-2010, 10:17 AM #7
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

The laser diodes decrease their output if they too hot to preserve self.

You did not heatsink the diode well, it needs to stay locked into the housign all the time.

You risk damaging it, if one single particle of dust lands on the laser die itself while it's hot, it's gonna crisp there and ruin your diode.

Leave the diode in the housing.

Your driver can be greatly simplified, leave out the potenciometer and all resistors.

Place only one single 3.3 ohm resistor between OUT and ADJ pins. That's give you 380 mA to the diode,
No test loads and no trimming potenciometers.

LM317 dissipates all unneeded power, like excess voltage supplied.

What is your source (batteries)?
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Old 07-29-2010, 03:33 PM #8
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

good to know about dust. It's powered by fully charged 6xAAs. I only ran it outside diode to show how bright it was. It is heatsinked well in the casing. SHould I use compound between diode and heatsink? How do you tell if a capacitor isn't working? Do I need the capacitor still?

Can I used compressed air can on the diode?

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Old 07-29-2010, 03:42 PM #9
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

Quote:
Originally Posted by lam0r View Post
good to know about dust. It's powered by fully charged 6xAAs. I only ran it outside diode to show how bright it was. It is heatsinked well in the casing. SHould I use compound between diode and heatsink? How do you tell if a capacitor isn't working? Do I need the capacitor still?

Can I used compressed air can on the diode?
Capacitor is needed to soften start up spikes, without it diode won't life too long.
You can't tell if it's not working.

Leave the diode alone - if it's working , don't mess with it, there is no use of compress air on it, you can only destroy it.
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Old 07-29-2010, 03:45 PM #10
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

6x1.5 = 9 volts on a red diode? That should be ok for a blue. I think this is your problem too much voltage being converted into heat.
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Old 07-29-2010, 04:14 PM #11
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

maybe your lm317 is touching your host? lol
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Old 07-29-2010, 04:33 PM #12
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

the AAs are 1.2 last I checked.

Does it matter if the capacitor is 35V or 16V? The "it can be done" thread said 16v but pictured a 35v. the 16v is a yellow blob a little smaller than the blue cylinder.

I went to radioshack and found no 3.3 ohm resistors. However I found smaller 10 ohm resistors that are rated at 1/4 watt (smaller in size). The ones I was using before were half watt.

I also got the heatsink and compound.. though I'm not sure I really need the compound.
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Old 07-29-2010, 04:43 PM #13
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

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the AAs are 1.2 last I checked.

Does it matter if the capacitor is 35V or 16V? The "it can be done" thread said 16v but pictured a 35v. the 16v is a yellow blob a little smaller than the blue cylinder.

I went to radioshack and found no 3.3 ohm resistors. However I found smaller 10 ohm resistors that are rated at 1/4 watt (smaller in size). The ones I was using before were half watt.

I also got the heatsink and compound.. though I'm not sure I really need the compound.
The total output current of linear current regulators is determined by resistance on their voltage reference.

Voltage reference of 1.25 V is developed between LM317's OUT and ADJ pin.
Resistance there, total resistance (does not matter how you achieved it) determines the current.

Get your potenciometer, measure it, set resistance to 3.3 (3.0-3.5 will do), solder it up and fire it up. If you can, test output current with test load.

Widely known schematic for LM317 driver is overly complicated and stupid. You need three components, LM317, capacitor (any in uF range does good job), and resistor which you calculate:
1.25/ desired current in A = Resistance in ohms.
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Old 07-29-2010, 06:53 PM #14
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

I just replace the resistors with 1/4 watt 10 ohm x3 in parallel for 3.3 ohms. What I don't understand is why I need to use the potentiometer. I used it before and I could fluctuate between 0 and 360ish mA. so what. This diode can take up to 600. I'm not interested in taming it right now.

I replaced the capacitor also. I still get dim beam. no heat really. added heatsink anyways. replaced batteries. when the switch is off I tested the out and adj and it said 3.4 (while off, strangely... is that from the capacitor?). I'm having trouble testing it while on and got frustrated holding the leads in place. will try again later if it matters w hether it's off or on
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Old 07-29-2010, 07:00 PM #15
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

Quote:
Originally Posted by lam0r View Post
I just replace the resistors with 1/4 watt 10 ohm x3 in parallel for 3.3 ohms. What I don't understand is why I need to use the potentiometer. I used it before and I could fluctuate between 0 and 360ish mA. so what. This diode can take up to 600. I'm not interested in taming it right now.

I replaced the capacitor also. I still get dim beam. no heat really. added heatsink anyways. replaced batteries. when the switch is off I tested the out and adj and it said 3.4 (while off, strangely... is that from the capacitor?). I'm having trouble testing it while on and got frustrated holding the leads in place. will try again later if it matters w hether it's off or on
How dim? If it'l LED kind of glow, not lasing , then your diode is dead, and it's time to order new one.

Isolate anything that has a chance of shorting next time. Brass lock could short the diode pins.

I don't understand why people use potenciometers in DIY drivers too. I just put fixed resistance and use the laser, you are not going to use same driver in all your lasers so it needs to be adjustable.
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Old 07-29-2010, 07:43 PM #16
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Default Re: LM317 getting hot in <10sec.. Short in diode wiring?

Yeah I'd say it's less than the LED kind of glow. I'm going to try to get some from the dude selling them for $5 a shot. minimum quantity of 5
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