Tube tester HV power supply

[TheTrax]

Hi all,
I have a problem with a HV power supply for a tube tester
The circuit around the power mosfet don't work, no voltage at the output and all components are good !
See red circle)
J3 receive 12 VDC for the optocoupler circuit
J4 go to an ADC for measuring the output voltage
J5 dor to an ADC for measuring the current
J6 come from an DAC for setting the max current (0-5V)
J7 come from an DAC for setting the output voltage (0-5V)
I don't know what is wrong but no voltage at J2

maybe somebody have a typ
regards
Francis

 

[Les Jones]

Hi Francis,
What supplies J3 ? I would expect it to be a supply of a few volts so that the mosfet Q1 can have it's gate biased positive with respect to it's source to make it conduct . I would expect it to be a floating supply that probably only provides power to J3. Do you have a voltage between the pins of J3 ? ( I would guess that it would be between 6 and 12 volts.) Take care measuring this voltage because if the regulator starts working both pins of J3 will be a couple of hundreds volts positive of ground.

Les.

 

[dr pepper]

Yes J3 would need to be at least 12v or so for the fet to switch on effectively.
I dont know why theres an opto isolator when the i/p and o/p are connected together.
If the supply to J3 isnt isolated from circuit ground then that would certainly cause issues, ie short the o/p.
If you have around 1v or so across R4, then the o/p is shorted.

 

[Les Jones]

Hi dr pepper,
I disagree over the opto isolator not being required. The output side will be at about the output voltage of the high voltage regulator. The input side as at about ground potential.

Les.

 

[dr pepper]

Fair point.
A high voltage tranny could be easily used in place of Q3, but in hind sight that would require a high voltage amplifier circuit, the opto is effectively giving loads of gain froma few v to hundreds of v, so it earns its pay.
I dont nkow how it would affect the transient repsonse of the supply though, opto's tend to have an uneven freq response.

 

[TheTrax]

Hi Les
J3 receive 12 volts from an isolated power suppy

 

[TheTrax]

I gonna make al measurements on the components so maybe it's more clear for everybody
sorry for my bad english

 

[TheTrax]

I found the problem, the optocoupler supply (12) was reversed, now everyting is ok
many thanks for your help
Francis