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Old 01-05-2008, 02:58 PM #1
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Default Dropping 9v down before an lm317 driver

I am trying to drop a 9v down a couple of volts before it gets to the lm317 to drive a red diode, I don't have room for a heatsink on the lm317, hence the reason to drop the voltage a bit.

I was told I had to put a large resistor (the electronics guy calculated 56ohm 1watter in series with the power and circuit) between the power and circuit, but i'm only trying to power a red LD at 25ma.

would the resistor affect the lm317s operation?


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Old 01-05-2008, 03:38 PM #2
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Default Re: Dropping 9v down before an lm317 driver

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I am trying to drop a 9v down a couple of volts before it gets to the lm317 to drive a red diode, I don't have room for a heatsink on the lm317, hence the reason to drop the voltage a bit.

I was told I had to put a large resistor (the electronics guy calculated 56ohm 1watter in series with the power and circuit) between the power and circuit, but i'm only trying to power a red LD at 25ma.

would the resistor affect the lm317s operation?
25mA...hmm. LM317 heat dissipation shouldnt be a concern at 25mA and 9V input. when the batt depletes, the additional drop across the external resistor will render LM317 incapable of sourcing current.
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Old 01-05-2008, 03:40 PM #3
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Default Re: Dropping 9v down before an lm317 driver

If you don't have room for a small heatsink, you probably don't have room for a 1W resistor. I'd expect the resistor to dissipate about the same heat that the LM317 would by running at the higher voltage. At only 25ma you shouldn't need a heatsink at all, especially if it's an LM317T package. I'm running my BluRay at higher current than that from a 9V with the smaller form-factor LM317 and it doesn't even get warm.


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Old 01-05-2008, 04:36 PM #4
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Default Re: Dropping 9v down before an lm317 driver

Um, do you mean a red ld like one from a dvd burner? If thats the case that is not enough ma to even get it to start lazing, it will start lazing around 120ma usualy.

If your trying to power a blu-ray that would be fine, but if its a dvd burner diode that wouldnt work because the 9v doesnt hold enough power so it would start sagging imideatly, i would use 6aa ni-mh batteries, they would be perfect to power it.

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Old 01-05-2008, 05:16 PM #5
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Default Re: Dropping 9v down before an lm317 driver

;D nooo, i'm using that 25ma because it's where my red doide lases. and you are sayign that there should be no heat issue doing that with the lm317t on 9v?
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Old 01-05-2008, 05:26 PM #6
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Default Re: Dropping 9v down before an lm317 driver

no heat issues whatsoever. just give it a go and see/feel for yourself. (the resistor between Adj and Out pins of LM317 is 50ohms, right?)
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