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LM317T

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Sceptre

New Member
hey guys..i'm using a lm317 to regulate a voltage. i was hoping to go from 48vdc to 10vdc...can i do this with the lm317? only thing i seen in specs was input-output diff max was 40

would this setup be ok?

TIA
 

Exo

Active Member
Hmm thats a big voltage drop. You have to remember all the unused energy has to be dissipated by the lm317. So if you draw 100mA from it times 38Volts = 3.8Watts... It should work but cool it well
 

Optikon

New Member
No, I wouldnt try it.. it REALLY stresses the internal components to that device. I have successfully used them at 35V in but thats about as far as I'll take it. You might consider placing a power zener in series with the input to drop some of the volts for you (to get the input down to say 25V or so..) That way you will distribute the power dissipation and cause less stress on the 317. Since it is adjustable, make sure that at no load, your voltage setting network maintains at least a couple of mA to keep the zener biased-on. I vaguely recall doing a test in which I found that 42V on the input made the part stop regulating (over volt protection built in-??)


Alternatively, I recall seeing somewhere a part called LM317HV which allows 50V input (and I think National makes-check it out) But for the power dissipation reasons, I personally prefer to drop some of the volts prior to the input with a series zener.


just my .02
 

Exo

Active Member
National datasheet says "It can regulate voltage upto several hundred's of volts as long as the input-output differential does not cross 40V"

but maybe this is only so for national parts
 

Sebi

Active Member
Yes, Exo right, but what happen when output shorted? I think need additional circuit for short-protecting.I recommend to build it with discrete component, or use simple switcher from NSC. HV-types can handle up to 60V DC input.
BTW: what is the output amperage?
 

KFar

New Member
> hey guys..i'm using a lm317 to regulate a voltage.
> i was hoping to go from 48vdc to 10vdc...can i do this with the lm317? > only thing i seen in specs was input-output diff max was 40
> would this setup be ok?

The LM317HVT will do this with heatsinking, as it will run warm.
(conservative limit is 1 watt PDmax; voltage drop * current)
Might consider a prereg arrangement; i.e. a 317 to knock the voltage down by 1/2 first, then a final.
Have had success doing this in the past.
Another thought is to use the T0-3 case.

Hope this helps...
 
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