# High Voltage Linear Regulator Output Not Staying Constant

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.

VOUT drift:

The circuit is stupid simple:

• Keep in mind that R6 will dissipate more power than your 24k load. Commented Apr 12, 2016 at 16:08
• "the output voltage is all over the place" doesn't really say how much drift you are seeing. Please give actual numbers. Otherwise it is impossible to assess whether what you are seeing is within the expected tolerances. Commented Apr 12, 2016 at 16:14
• @WhatRoughBeast Ah yes Commented Apr 12, 2016 at 16:21
• @user96037 I added a title to the plot in case it wasn't clear Commented Apr 12, 2016 at 16:22
• The device might be over-heating. Double the resistance of R4 and R6 to reduce current to LR8 and see what happens. To use a lower voltage regulator, you need to make sure the operating voltages are within limits even during power up or down. A zener diode from input to output could be a first step for that. Commented Apr 12, 2016 at 22:17