Simplistic MosFET HV Shunt Regs

[merlin el mago]

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[merlin el mago]

Salas I only get max. 30mA CCS with 150VAC tx, what's the possible issue? I use CCS 30mA as load, also I use 100R 100uF to split B+ for both channels. R9 & R10 both are 56K in place of both 68K. The SSHV2 CCS works well to max. 30mA, I tried to lower the CCS load to 20mA but the same problem SSHV2 max. CCS 30mA.

 

[Salas]

Maybe the reg's CCS doesn't work and it shows just what is pulled at its output. Try test it with simple resistor as a load too.

 

[merlin el mago]

With resistive load of 3K9 SSHV2 CCS measure 50mA.

 

[merlin el mago]

I used 4K7 because is 10W (3K9 only 2W), max. CCS 45.3mA & fixed Vout 171V.

 

[merlin el mago]

Wow

 

[merlin el mago]

Replaced with a new resistor but the problem of regulating Vout persist.

 

[Salas]

Is the transistor next to it ok?

 

[merlin el mago]

From your trouble shooting "The JFET must be producing low and trimmer controlled current. About 2.2mA for 300V." How to measure?

 

[merlin el mago]

Is the transistor next to it ok?

BJTs KSA1381
Q4 seems OK Vbe 0.6V
Q5 seems gone Vbe 0.05V

There is a way to measure the JFET Q6 & the mosfet Q3 to know if are OK or not?

 

[Salas]

JFET's current can be calculated by mV drop on the resistor and trimmer paralleled between it's gate and source i.e. across R12. Voltage / Ohms of those.

 

[merlin el mago]

JFET is not drawing none current, voltage accross R12 470R is 0.034V with 170Vout. Can be Q5 isn't damaged?

 

[merlin el mago]

Replaced with a new JFET Q3 0.035V across R12 (still not OK)
Q5 Vbe 0.048V (can be due to JFET isn't drawing enough current)
I will check R11 1K trimmer.

 

[merlin el mago]

Was Q5. Now can set the SSHV2 CCS to 50mA & adjust Vout but I have a problem CCS load can reach 15mA each channel, only one channel 13.5mA max. and the other 11.1mA. CCS load tested without SSHV2 & reach both channels 15mA each, attached CCS schematic used.

 

[Salas]

Try 100 Ohm resistor in series with reg outputs and CCS loads to buffer possible difficult interactions.

 

[merlin el mago]

Used 100R in series & 100uF in parallel between de SSHV2 & the CCS load as you suggested to split the B+ for both channels. Have I to get off the parallel capacitors?

 

[Salas]

Caps not absolutely necessary, try with just resistors

 

[PKI]

what do you think about using something like this to get raw 400V dc for the regulator: 3A Rectifier Filter Power Supply Board Audio Amplifier Single 18V AC-DC Convert | eBay Of course changing all component for proper voltages? Space is tight, so looking for something small...

 

[Salas]

Given that you find proper sized HV cap and high PIV bridge that fit well I am afraid of narrow space between adjacent copper lanes at places. Although the bottom side pictures are blown out and I can't say for sure. HV needs safe distances not to arc over.

 

[merlin el mago]

Caps not absolutely necessary, try with just resistors

The issue continue without 100uF caps.