Another PSU problem

[jeroddumas]

I just built up one of keef's SSL PSU. I am using a 22v-0-22v toriod transfo. I am testing unloaded. At the +48v I am getting +44.9 which I assume is acceptable. On the +18v I am getting around +17.8 which is acceptable, but on the -18v, I am getting -29v. There is a problem here. he transfo out is putting out about 27vac. Can someone help me here. I am using his PCB with lm7818, lm7918, and lm317.

 

[PRR]

> I am testing unloaded.

That may not work. Put about 1K on each 18V output and see what happens.

> On the +18v I am getting around +17.8 which is acceptable

The regulators and resistors are rarely specced better than 5%, so 17V to 19V better be acceptable for "18V". I'd call 17.8V "right on", or close-enuff for rock-n-roll. The +44.9 for "48V" is 7% low, which could be accumulated 5% errors. It is within spec for P48 power, but just barely. If I happened to know off-hand which design is "keef's SSL PSU" I might know which resistor wants trimming. But again, you should probably load it, 10K is probably good here.

 

[jeroddumas]

well I hope I didn't destroy the regulator.

 

[smilinfu]

jeroddumas
Did loading the power supply help?

I am having a similar problem with a power supply using a 7824 and 7924. I tried the circuit under load (just kept uping the resistance hoping it would make a difference) but I still had the same problem you mention. The positive rail is fine, but the negative is way over what it should be.

I was using a 22v-0-22v at 50VA, and I think even with the load, it was putting out a lot more than 22V (like 25v) and was getting 36V going into the 7924. Even though the specs are the same, I think the 7924 I can not take as high a voltage as the 7824 (or I got some bad ones)

I am going to try a lower voltage transformer, as well as a lower VA, but please post if you solve your problem.