Home-made bench power supply voltage drift problem question.

[RDL2004]

I built my own bench power supply last winter/spring. It is a variable voltage supply using a circuit from one of the National Semiconductor datasheets using an LM350T regulator.

It works pretty good, except when first turned on the voltage will not be what it was when it was turned off.

This is because the voltage drifts upward for a while after it's first turned on. The drift is not terribly huge (in the 100-200 mV range), but it is annoying. After being on for an hour or so, it no longer changes and is rock steady from then on.

I am assuming this is some kind of thermal drift. What could cause this and how could I fix it?

I only use this supply for low power work, it's very rare that it must supply over 750 mA, usually less than 500 mA, and its set up to put out 1.2 - 20 volts. 

The transformer is a toroidal type, 30 VA, 2X18 volt, the rectifiers are 1N5408 discretes in a full wave bridge. There are 2 electrolytic 3300uf 35 volt filter caps. The voltage adjust pot is a 10 turn wire-wound type.

It has the usual other capacitors and protection diodes. I think all the other caps except maybe one electolytic on the output are polypropylene types and all the resistors are all 25-turn trimmers.

Everything after the transformer is on one pc board (home-made), The LM350T has a heat sink, but it is not huge.

If more info would help, let me know.

[audioguru]

Hi Rdl,
The voltage of an LM350 decreases slightly as it warms up.
You have a wire-wound pot and its metal increases its resistance as it warms, doesn't it?
Maybe your circuit has a high current through the pot.
Maybe its other resistor drifts its value.
Maybe you used high resistor values that can be used by an LM150.

I would use a cermet pot that is naturally temp-stable. Its resolution would be so much better than a wire-wound pot that multi-turns wouldn't be necessary.
Please attach your schematic.

[Shahriar]

In TV reapiring, there are some problems that will beOkay a few minutes after the TV works. i.e. there is no Ruster,, {after few minutes, it appears.} it is alwasy beacuse of an Electrolit Cap. Bad Caps when get hot start working properly, but when they are cold, Some of their specifications Differs...

I should say that this happen mostly in those caps that get charge and discharge repeatedly

HTH - Shahriar