48V PSU - Fried trimpot

[lesabb]

Hello all,

I've built a PSU to power up a mic pre. Attached schematic. Meter reads 24VDC and ~68VDC on the doubler (48pk*1.4 I guess). If I try to adjust the voltage on the 317, I fry the trimpot. I'm using a 3296 KT, 28 turn 5k (0.5W) is that a problem? Also, what is the purpose of C1 and C2 if they are directly hooked up to AC, before the bridge. Thanks

Luc

[void_error]

It looks like you've got a bit of learning to do... then to go back to the drawing board (it would be nice to also include component values on the schematic).

Other than that your LM317 for the +48V rail is probably fried if you set the output voltage too low. You actually have a thread on this project so I suggest going back and taking a look at what other people suggested. No need to start a new thread.

[lesabb]

I am in the learning phase. Just looking for troubleshooting advice here. Taking into account previous suggestions (from previous thread), the output voltage on the 317 is 68V, which makes me think the 317 is fried since it does not respond to trimpot and has fried it

[void_error]

I'd suggest you get away from the LM317 for the +48V rail since, as it was previously said, it will be fried in case you accidentally short it to ground. You could easily avoid this by using discrete parts (transistors) - it's easier to find transistors which can withstand 70V and above than a regulator IC and you can even get short circuit protection by adding one transistor and one resistor.

I assume the preamp you intend to power only draws a couple of miliamps at most so it shouldn't be too difficult to find a suitable discrete circuit. Googling 'discrete voltage regulator schematic' might give you a few hints. If I'm not lazy enough I'll take a picture from a book about voltage regulators dating back to 1983, google results aren't good enough.

[mij59]

You need a high voltage regulator like this http://www.ti.com/lit/ds/symlink/tl783.pdf
Remove R2 and C4, and connect the - of C5 to the + of B2.

[Zero999]

Yes, it sounds like you've fried the LM317.

It's true it's a floating regulator and it's the difference between the input and output voltage which is important but the capacitors on the output charging will easily fry it. You could put a 39V zener diode between the input and output terminals and that will ensure the maximum voltage rating isn't exceeded but it will still blow up when short circuited, unless another current limiting circuit is added.

You could also use the LM317HV which is rated to 60V.