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FrozenGate by Avery

Dropping 9v down before an lm317 driver

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member02

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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?
 





member02 said:
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?
Click to expand...

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.
 
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.
 
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.

...lazer.... ;D ;D ;D
 
;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?
 
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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