Simplistic Mosfet HV Shunt Regs

[albertli]

Originally Posted by regal

try replacing the mpsa92

This is what I have but without D1

[Salas]

Was MJE350 getting particularly hot for long periods before the excess drift appeared recently? Does it have some sort of sinking?
Is the 170 in hot environment? Provide it with some heat relief. A little foil or special TO-92 sink. If you will blow over the 170 while watching Vout you will know.

[albertli]

Originally Posted by Salas

Was MJE350 getting particularly hot for long periods before the excess drift appeared recently? Does it have some sort of sinking?
Is the 170 in hot environment? Provide it with some heat relief. A little foil or special TO-92 sink. If you will blow over the 170 while watching Vout you will know.

My mistake not checking the temperature on the 350, but now they're all new. I didn't put any sink onto them, I'll try doing this if it's necessary. No sink for 170 either.

[AndrewT]

The k170 has just 0.6V across it. It dissipates ~4mW. It never gets warm. It does not need a sink.

[regal]

Originally Posted by albertli

This is what I have but without D1

I blew up an MJE that gave symptoms similiar to yours

[Salas]

Originally Posted by AndrewT

The k170 has just 0.6V across it. It dissipates ~4mW. It never gets warm. It does not need a sink.

Not for that. To increase its thermal mass and keep it steadier to ambient. If its drifting so much due to K170 and ambient temp reasons in that build, of course.

[AndrewT]

If the temperature inside the case varies, then adding a sink to the jFET will drag the jFET Tc & Tj along with the varying Ta. It will make tempco worse not better.

However, the temperature inside the case should be reasonably stable. This will allow the jFET to hold it's own temperature ~ 1Cdegree above Ta.

[Salas]

Don't know, if the temp rise is the case maybe a BF245A lowest IDSS possible from a bunch or 2SK117GR could be steadier. But it has to be checked that the JFET is the one so much affected in that build.

[albertli]

Since I have the 840 & 350 replaced, does it mean it's a good idea to replace the 170 so to check ?

[Salas]

Yes.