Quick question on Shunt regulator voltage check [resolved]

[Chris H]

Hi all,

I just wanted to check the voltage behaviour I am getting on the metal tab of each MJE5731A is correct.
When first plugged in, DC voltage rises quickly to around 340v before dropping back to just under 220V.
This all happens within 5 to 10 seconds of power on.
However, I note that there is a very slow continued drop of around 4V over the next 5 minutes.
The drop appears to be slowing so that by the time I switched off a further 0.1V drop was taking over 30 seconds, maybe more.
Is this normal and should I leave it on longer to check that it does stabilise (wasn't too sure about letting the tube get too hot on upside down chassis)?

Best,
Chris

[Paul Birkeland]

That wouldn't be particularly concerning.  Typically the kinds of problems we expect to see would be 275V on the tab or something far less than 220V, both of which indicate an issue with the build.

[Chris H]

Thanks for the quick response.
The voltage level was within tolerance and appeared to be stabilising at around 216V but I didn't wait long enough for it to stop dropping entirely.
Same result on both tabs (all LEDs lit).  I just wasn't sure about the initial rise to 340V and whether this is normal.
Is it worth leaving on longer to check the voltage does stop dropping?

[Chris H]

All good - stabilised at 217.5V and 215.9V.  I assume a small difference is within tolerance due to component tolerances?
Thanks again for the quick reply

[Paul Birkeland]

Yes, the tolerance of two resistors will move that voltage around a little bit.

[Chris H]

Great, thanks.