Simplistic MosFET HV Shunt Regs

[merlin el mago]

OK I will connect again with the old tx

 

[iko]

voltage across R4 284V
Puresound MM if you want I can remove & connect a dummy load of 15K
voltage across R9 0V
voltage across R5 0V

Yes, you should fix it first with dummy resistor load.

0V across R5 is completely wrong. The shunt mosfet, Q3 isn't working at all. It needs something like 2V-4V across R5 (Vgs) to start working. Take Q2 out and measure with the diode tester between B-C and B-E. One way there should be no reading. The other way you should get a value that is close to each other (B-C and B-E).

 

[rclourenco]

No. You should float two positives from double secondaries and bridges.

Sorry, one more question, would this be good for around 85V DC?
Thanks

 

[Salas]

With lower Vref resistors. Make them half.

 

[merlin el mago]

Old tx 115 Vout max.

 

[iko]

Roughly, this is how the circuit should measure if it worked correctly.

 

[merlin el mago]

Yes, you should fix it first with dummy resistor load.

0V across R5 is completely wrong. The shunt mosfet, Q3 isn't working at all. It needs something like 2V-4V across R5 (Vgs) to start working. Take Q2 out and measure with the diode tester between B-C and B-E. One way there should be no reading. The other way you should get a value that is close to each other (B-C and B-E).

MJE350 B-C & B-E close reading.

 

[merlin el mago]

Roughly, this is how the circuit should measure if it worked correctly.

Vin?

 

[Salas]

Old tx 115 Vout max.

How you had 260VDC out on 15k dummy yesterday? That was 17mA on the load only.

 

[merlin el mago]

Olx tx 115Vout is measured with Puresound MM instead the dummy load, it's the only change.

 

[Salas]

Oscillation then. Do you have scope?

 

[iko]

Oscillation then. Do you have scope?

+1

If you don't have a scope try listening to a radio on AM near the circuit. You should hear it in the radio if it's oscillating.

Vin for the circuit I showed does not matter, but it was about 275V DC.

 

[merlin el mago]

Yes I have a scope. I listened a radio on AM seems it's oscillating. IRF9610 cold IRF840 very very hot

 

[Salas]

Is there something extra on the preamp rail after the regulator? Some small capacitor for instance?

 

[merlin el mago]

No, reg goes direct to anode resistors.

Radio AM listened with 15K dummy load.

 

[Salas]

You still got same low voltage output situation with dummy?

 

[merlin el mago]

80Vout fix, can go down or high.

 

[Salas]

What changed from yesterday? Added some components? Do all semis show some voltages, like Vbe, Vgs?

 

[merlin el mago]

This morning I added the zeners 12V 0.5W

 

[Salas]

Maybe something wrong with them? Connection, whatever. Cut one end off on each string and see what happens.