Electronic – High Voltage Linear Regulator Output Not Staying Constant

voltage-regulator

I'm trying to use a 3-terminal linear regulator LR8 (Also tried TL783 and saw the same problem…) to regulate 180V from a fixed PSU down to 116V, to power some very light loads < 1mA.

But when I power this thing on, the output voltage is all over the place!

Currently, I have the rheostat adjusted to 0, so the resistance from Adj->GND is 16K. This means the nominal output voltage should be ~97V. I put a 24K load at Vout… depending on output voltage should set the output current to single-digit mA range. (EDIT: And similar current range through the set resistor R6).

It looks exponential, but the output cap is directly connected so obviously the time constant isn't minutes.

At 120V, the load dissipates ~0.6W, the set resistor dissipates 0.9W, and the LR8 dissipates 0.75W. The regulator is definitely hot to the touch, so I was thinking temperature effects? But with thermal resistance of 81C/W, this only brings the temperature up to maybe 100-105C, within the operating range. I'm not sure how to measure the actual temp.

Also, if you have ideas for a better regulation alternative to this circuit, please leave a comment! Thank you.

VOUT drift:

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The circuit is stupid simple:

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Best Answer

Looks like a possible temperature problem? The internal reference moves at temperature; since you have a gain of 80, you will see a volt on the output for every 12mV of drift on the reference, which usually will drift tens of mV. Check by cooling the part and watching the output.

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