help required on regulated psu

[garbage]

hi all

i've built 2 regulated psu using pedga's schematic.
only difference is that i am using +12/-12 vac transformer secondaries to get a regulated +15/-15vdc. also, in place of the 2.2k resistor, i had a variable resistor. higher capacitance was also used. a diode was also placed in the direction of ADJ to OUT pin of lm338t.



one of them works fine, the other(built at a later date), does not seem to work correctly.

the voltage will fluctuate from 3+v to 8+v then drop to 3+v then back again. between the cycle, it will hold steady for a short while at 3+v and also at 8+v.

after rectifier stage is ok, measured close to 19vdc. somewhere from lm338t onwards, there seem to be a problem. i've checked my connections. they seem fine.

any ideas where i might have possibly gone wrong?

[li_gangyi]

could it be a bad potentiometer?? perhaps u're cycling it into some kinda protection...and what output volatges are u aiming for?

[janneman]

What voltage do you measure at the adj pin for the one that has the problem? If that fluctuates, try it with the adj cap disconnected, it can be leaky (or reversed). Also, check/remove temporarily any diodes at the adj pin to make sure they are not leaking or something. Does the reg get warm at all?

Jan Didden

[garbage]

i've taken out the variable resistors out for measurment, they seemed ok. i'm actually aiming for +/-15vdc.

the first circuit that i built worked, it is now powering my opa627/buf634. since i have 2 transformers of the same type, i wanted to build another one so that i can use one for each channel.

[garbage]

Originally posted by janneman
What voltage do you measure at the adj pin for the one that has the problem? If that fluctuates, try it with the adj cap disconnected, it can be leaky (or reversed). Also, check/remove temporarily any diodes at the adj pin to make sure they are not leaking or something. Does the reg get warm at all?

Jan Didden

i'll try out your suggestions.
thanks.

edit:
the regs do not get warm, and i've tried with the adj cap disconnected(no difference).
next to try is to remove adj cap and diode.

[janneman]

What's the voltage on the adj pin doing. Try to measure the voltage between the output pin and the adj pin of the reg. If that remains constant about 1.25V, than the reg is OK.
To measure is to know!
Did you change the divider on the rteg output, because for the 15V version you need to change it from what you showed in the schematic upstairs.

Jan Didden

[carlosfm]

Check the pinout of the regs.
Look at it and check if you made the right connections to the right pins.

[joensd]

With 2*12VAC you´ll get about +-18VDC maximum depending on your transformer.
Aiming for +-15VDC isn´t a problem in most cases (I´m using the same setup with 7815,7915 and it works nicely) but the voltage-headroom is very small and regulation might get worse or don´t work properly. Don´´t know the LM338 too well though.
If you don´t need so much voltage swing set the resistors up for +-12VDC to see if it´s that.
But check the more obvious things first.

[jackinnj]

just a silly question and I cringe asking it, but did you insulate the LM338's from the heat sinks?

[drdagor]

Your system is probably oscillating. Because the capacitors are so large it doesn't really look like an oscillation, but that's probably what you have. With 4700 uf and almost no load you probably have a time constant of minutes.

Bypass capacitors is the normal fix for this.

In addition to the capacitors you have, it would be worthwhile to add a 10uf and a 0.1uf bypass capacitor to the regulator mounted as close to the regs as possible. The 10uf should be tantalum--the higher the voltage the better (since at the same uf value, the higher voltage ones have lower ESR).

Normally the 0.1 goes on the input side of the reg and the 10uf on the output side. But in your case, because the input capacitance is so huge on the input side, you may want to put a 10uf on both the input and output side.

Hope this helps.