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Old 02-20-2011, 12:19 AM #17
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

Yeah it seems that the lm317 works best with "battery voltages" like 7.4v. Also much less of a voltage spike at startup too.


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Old 02-20-2011, 12:20 AM #18
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

Yeah. the LM series is a bit of a pain to work with because of its drop IMO. If you set it up just right, it can do the job just fine for a diode.
For anything high powered, I find linears just bad. the ammount of power they waste isnt that good when you cram several amps into a diode.

Id rather stick to buck/boost converters then linear regulators, alot more efficient.
btw, for a voltage spike, either use a TVS diode or a RC snubber (10K R + 100nf non-polar cap work fine).

Last edited by GBD; 02-20-2011 at 12:25 AM.
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Old 02-20-2011, 12:44 AM #19
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

I've used 12V from a PC PSU on a lm317 circuit, no problem at all, the IC does get
hot, but that's no issue if you heatsink it alright.
Overvoltage doesn't seem to be the issue here at all to me.

I too have killed 2 red diodes, being careless with the pots and resistors.
I have since gotten hold of a blue nichia diode, and that thing just laughs at any current
I throw at it, swallows it up and asks for seconds. Tough little bastard. You have to try
really hard if you want to kill it.
Once, at 1.9Amps, it got so hot that it faded quickly in brightness and almost went all dark, but
I beat it to that, switching it off right away!
*GULP*....
Switched it back on, the diode was there like nothing even happened.
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Old 02-20-2011, 05:22 PM #20
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

I dont get it though, the lm317 can handle up to 30 (well, 35 it begins to fail). According to what it does, you should be able to feed the regulator with 30 volts and it would output the same as it would with 7.4

Instead of using a TVS diode, can I use a 12v 1W zener diode? Those should be fast enough to stop spikes or what not along with a .1uF cap across the diode

Confusing
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Old 02-20-2011, 11:59 PM #21
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

Did you get a switching wall wart? Sometimes they can produce some huge amount of voltage beyond their rated amount. I've never had any problems with wall warts powering a LM317, but mine were all switching. At those power levels you should definitely heatsink the LM317l, since it needs to convert that excess voltage to heat. Perhaps put some rectifier diodes in series with the input to the LM317 so it doesn't need to drop so much voltage for a red diode which needs at most 3.6V (i.e. 6.6V at the LM317's input).
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Old 02-21-2011, 05:14 AM #22
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

LM317 can handle 12v fine. I feed one of my LM317 drivers powering a PHR 3x10440 which is almost 13v on a full charge. I have another in a 635nm pen host that runs off 2x10440s (8.4v) fine too. I suspect something is wrong in the way his caps are set up is why the diodes keep croaking. I haven't used any caps in the couple builds I've done (only on a breadboard test setup) but that's where I'd start looking.
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Old 02-21-2011, 11:39 AM #23
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

Something else must be wrong indeed. The basic LM317 circuit should have no problem handing higher input power, nor a big 50/60 Hz ripple. It was designed to remove that ripple, and it will do so readily as long as the input voltage never drops under the required minimum.

Switchmode wallwarts have a riplle at much higher frequency. An additional capacitor to filter that out is still recommened, though not always required.

The output capacitor of the driver is recommend by the manufacturer, and should not be omitted. Often nothing happens, but in some instances those LM317 can start oscillating, and the output is then very erratic.
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Old 02-21-2011, 06:26 PM #24
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

Well, I have it on a heatsink, and nothing gets warm. My wall transformer outputs 18 volts, but with little effort, it drops to its 12. Still though, the lm317 can take on up to 30 volts, and output the same.
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Old 02-21-2011, 08:17 PM #25
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

Nothing gets warm? Hmm... at a 15V difference, the chip should get modestly warm. Have you tried simply replacing the chip with a new one? Worth a shot I suppose! Please post pictures of your circuit so we can further examine it!

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Old 02-21-2011, 11:21 PM #26
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

Depending on how you're heatsinking the LM317 you may be indadvertedly shorting it. The metal back is attached to Vin or Vout IIRC so if that's touching anything in your host you could be creating a short.
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Old 02-22-2011, 09:46 AM #27
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Default Re: Powering lm317 driver with a 12v 1a wallwort?

Quote:
The metal back is attached to Vout
fixed
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