Power supply voltage is creeping

[veracohr]

I built a LM317/337 bipolar power supply for circuit experimenting a while back. I fried the 317 (positive) voltage regulators not long ago by accidentally connecting something wrong, and replaced it.

Now I've got the negative side creeping down. By that I mean that I have it set to 10V, and over the course of a couple hours the voltage slowly rises (in the negative direction). The first time it happened (Tuesday) I couldn't understand why my circuit was behaving the way it was, then discovered the negative side was around 18V. Yesterday it started at 10V, and I kept turning the knob down as it crept to keep it at 10V, and by the time I turned it off a couple hours later, the knob was at the 5V mark.

Think the negative voltage regulator is going bad? I've got a spare, but is there something that might have caused this?

[tomchr]

Measure the voltage difference between OUT and ADJ on the LM317/LM337. It should be rock solid at 1.25 V (-1.25 V for the LM337). If that's not the case, the regulator is either dead or not wired correctly. This assumes the input voltage to the regulator is within spec (i.e. min 2.5 V higher than the output voltage for the LM317; 2.5 V lower for the LM337).

~Tom

[DF96]

Is it getting hot? This will change the current at the adjustment pin. If your resistors/pot are too high in value then this may change the voltage.

[SY]

Other thing to verify is minimum current draw- if you don't have at least 10mA flowing in the divider string, you need to load the regulator. That's one that's bitten me before.

[AndrewT]

The common minimum for the 317 is 5mA, but for the 337 it is more usually 10mA.

To be safe use 100r as the upper setting resistor. This will draw ~12.5mA even when no load is connected.

[Speedskater]

Are you using a pair of resistors to set the ADJ point?

[veracohr]

Originally Posted by tomchr

Measure the voltage difference between OUT and ADJ on the LM317/LM337. It should be rock solid at 1.25 V (-1.25 V for the LM337). If that's not the case, the regulator is either dead or not wired correctly. This assumes the input voltage to the regulator is within spec (i.e. min 2.5 V higher than the output voltage for the LM317; 2.5 V lower for the LM337).

Voltage between the OUT and ADJ is about -1.27V. For comparison I also checked the positive side, and it was about 1.25V (give or take a few mV on both sides). Input voltage is WELL above output.

Unfortunately while checking the input voltage, I think I must have accidentally bridged the input and output pins with the probe, and released the magic smoke from the output capacitor. I guess this is a game of "see how many times I can burn up components on this before I just go buy a power supply."

Originally Posted by DF96

Is it getting hot? This will change the current at the adjustment pin. If your resistors/pot are too high in value then this may change the voltage.

I don't know, I haven't checked. Nothing I could notice just working next to it.

Originally Posted by Speedskater

Are you using a pair of resistors to set the ADJ point?

A 240 resistor and a 5k pot.

Just to be clear - I've been using it without this issue for a while. It just recently started.

[DF96]

What did you do to the circuit or whatever it feeds or is fed from, just before the problem started? You seem to be a bit accident-prone with probes! Did it originally work OK, or did you not notice the problem even though it may have been present? The 240 is a resistor, and not a thermistor?

Provided the voltages are low enough to be safe, to check the temperature of the chip just put your finger on it. If you can hold your finger on it comfortably then it is probably not overheating. Also worth noting whether it heats up quickly (within a few seconds) when power applied or more slowly.

What output current are you drawing from it?

[veracohr]

Well now here's something else that's interesting.

I just realized the input voltage I measured (before the magic smoke) doesn't make sense. It was +/- 45V, but I have a 56V CT transformer, so there's no way either side should be above 28V right? Even less after the rectifier and filtering.

Argh.

[DF96]

It would peak at about 39V with no current drawn - 1.414 x 28V. If your mains is a bit high then 45V is quite possible. This could make the regulator overheat if your heatsink is inadequate.