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LM317 power supply problem

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mgeno216

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So i have put together a single lm317 circut using the following schematic:
(sorry for bad pic)
**broken link removed**

I am trying to do the same, but with four 317 in paralell to accomodate the laptop power supply I am using for ac to dc. The laptop supply's output is 19.5volts and 3.5amps. The output of my 317 circut is a constant .7v. Does anyone know why this is. It is currently only breadboarded and check that all my connections were pushed in properly.

Thanks,
Mike
 
Are you attempting to parallel four 317 to supply the current? If so, that will not work, as one will always try to deliver all the current which will cause it to go into its thermal shutdown mode. Even if you could get them to share the load, they will be supplying ~1A each, so each one would have to be bolted to a large heatsink.
 
I think I read on these forums that connecting the 317 improperly can cause a .7 volt output. Re-check the pin layout of the 317. It is also important that the 2 small capacitors be very close to the 317 chip.
 
Thanks guys. I was over on the DutchForce forum, and they came to the conclusion that lm317 are fine in paralell. They also said to be safe an put a 1Ω or less on each lm317 to make sure they share current. I am not saying that you guys are wrong, but they are all really smart over there. I will check my wiring again and look at that datasheet.

Thanks for the help


Mike
 
Last edited:
Alright just got it working. I made a stuiped misktake. When I was breadboarding I bridged the pot. Thanks to everyone who posted.

Mike
 
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