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Trouble with LM317? Answered

im having trouble in finding a heatsink for an LM317M chip and was looking for suggestions on where to find one that would be good for it. this is the datasheet for the chip...it should be the DCY Package (top view) http://www.ti.com/lit/ds/symlink/lm317m.pdf

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steveastrouk (author)2013-01-11

You haven't wired it right, I suspect you have no current limits to the LEDs.

Please draw your circuit and post it.

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diabloboy (author)steveastrouk2013-01-11

So here goes my setup. hopefully it makes it better to understand. so I don't know if you need to limit the current to 60ma because they are in series of 3 or 20mA because each is rated at 20mA.

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steveastrouk (author)diabloboy2013-01-11

Ah, as I suspected, completely missing current limiting.

You don't even need the regulator here. Put three diodes in SERIES and with a 90 ohm resistor in series with them. Make PARALLEL chains of three diodes, and one resistor, and just feed the 12V to them.

Job done.

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diabloboy (author)steveastrouk2013-01-11

I tried it with a resistor already but they arent as bright when i hook it up to the voltage regulator and im guessing it's because 500mA are running through them. so in this case i would like to control the current to see how bright they get without getting hot or they stay at a temperature of 80 Celsius or below because that is the recommended temperature for them to run fine.

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steveastrouk (author)diabloboy2013-01-12

Put the resistors in anyway, use 47 Ohm resistors, and then adjust your supply volts.

You should have 8 parallel chains carrying 20mA, that's 160mA nominal current through the regulator. What voltage are you supplying TO the regulator ?

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diabloboy (author)steveastrouk2013-01-13

12v. i want to know how what you just said, looks like in a diagram.

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frollard (author)diabloboy2013-01-13

There is nothing stopping ALL THE REGULATED CURRENT going into just 1 series strand of leds - if the internal semiconductor resistance of these is off by just a fraction of an ohm, you'll get a catastrophic chain reaction failure as each strand burns brighter until failing, opening the circuit, and burning the others out.

Do as steve says, use a resistor on each strand. If they're only 20mA leds you don't need it configured as a current regulator you need a voltage regulator..

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mpilchfamily (author)2013-01-11

How much power are you putting into it and how much is being pulled out of it?

You may not need a heat sync for it. With that package the copper pad and traces for the ouput are used as a heat sync. So take that into consideration as you design your board. Pin 4 (larger output pad) is what needs to be soldered down to the pad that will be the heat sync. If more that a copper trace is needed then you'll need to find a small heat sync and some thermal adhesive to glue it onto the regulator. But i doubt you would need to go that far.

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diabloboy (author)mpilchfamily2013-01-11

well it gets pretty hot because i have it hooked up to a 12 volt battery and i stepped down the voltage to 10.374V in order to drive 24 L.E.D's that are wired 3 LED's (Each single L.E.D needs 3.4V to power it) in series which need 10.2V so i basically have an array that is 3 by 8.The LED's also get hot and i think it's because the LM317 is giving it a current of 500ma. im not sure if they need 60ma or 20ma constant to get bright.

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mpilchfamily (author)diabloboy2013-01-11

Put a temp probe on it and see just how hot it's getting. Acording to the data sheet you'll be fine as long as it doesn't exceed 125C.

You need to know what kind of power your LEDs need so you can have the right driver for them or current limiting resistor. Otherwise they will pull as much as they can until the power supply can't give anymore or they burn out.

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diabloboy (author)mpilchfamily2013-01-11

i was thinking of adding another lm317 as a current regulator but Idk if because each LED needs about 20mA to work but they are in series of 3 that i need to add up the mA to 60mA (20mA * 3) therefore it needs to be regulated at 60mA or it just needs 20mA because each LED needs 20mA to work. basically I dont know if the mA increase if you wire leds in series.

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frollard (author)diabloboy2013-01-13

leds increase voltage in series, and current in parallel.

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iceng (author)2013-01-11

The 317 has a fascinating ability to protect it self.

You CANNOT damage this wonderful Regulator with over current 
or over heating because the IC simply turns itself OFF ! !

The Only Way to damage the LM317 is to exceed the 37 Voltage
or wire it wrong.

A

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steveastrouk (author)iceng2013-01-11

Nah, look at the application circuits. That diode's there for good reasons.

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iceng (author)steveastrouk2013-01-12

Very good reasons and
As I said,   the wiring mustn't be wrong...

When C1 gets large enough to try pushing current back to a source
because a load does not empty it soon enough.

It is then in fact the same as if it was wired backwards and
I stand by my previous comment ;-)

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steveastrouk (author)iceng2013-01-12