Variac power supply simulation in PSUD2

[NixieFreak]

Hi everyone,

I am currently designing a very simple power supply for firing up some gas discharge. It should be capable of supplying 0-500V DC at 50mA max. Here you can see the schematic: http://www.jb-electronics.de/tmp/netzteil_3.png

I made a simulation in PSU Designer II [see the attachement] and I have some questions:

1) I measured the primary and secondary resistance of my 230V->400V transformer and used the program to calculate the overal impedance. Is 700 Ohms a reasonable value?

2) What resistance do I use for the caps? It is three 200uF 400V caps in series. I used the impedanc formula Z=1/(2Pi*f*C) and got the 43 Ohms. Does that make sense?

Thanks a lot,
Jens

[AndrewT]

The resistance that PSUDII uses for capacitors is the esr. Expect those HV caps be be <<1ohm.

I don't understand "my 230V->400V transformer".
What is the voltage specification of your transformer?

Why do you need three series connected 400Vdc capacitors for a 500Vdc supply?

[NixieFreak]

Hello Andrew,

I see, so the resistance required is really a DC value, OK. Unfortunately, for the ESR there is no description in the datasheet. But I guess if it is 1 Ohm or .1 Ohm is not really important.

The transformer I use is an isolating transformer used to isolate 400V AC (primary) from 230V AC (secondary). I connect it backwards to create 400V AC. The transformer has 20VA.

The three series capacitors are too much, hm? OK, I was just trying to make it as safe as possible, but I guess two caps will do just as well.

Thanks,
Jens

[AndrewT]

But, your back to front 400:230Vac transformer does not become a 230:400Vac transformer.
The turns ratio of the transformer is not 230:400. You need to measure the open circuit voltages to find the turns ratio. That open circuit voltage will be (1+regulation) higher than 400Vac. When you turn it back to front the output voltage will be ~ (1+2timesregulation) lower than the 400Vac expected. Fortunately since you only need 500Vdc the lower output voltage means you have to shed less excess voltage so saving a lot of heat in the inductors and resistors required to get down to 500Vdc.

[NixieFreak]

Andrew, thanks for the hint. I test-wired the AC part (so basically, just the variac and the isolation transformer) and you re right, of course, it only gets to 360V AC. However, the isolation transformer has three taps: 280V, 400V, 420V. When I use the 420V tap I get 380V.

Rectifying this will yield 537V which is still OK for my purpose.

Any ideas on limiting the current to 50mA? The 10k resistor is only in there to have something, but I would like a circuitry that limits to 50mA independent on the actual voltage.

Thanks
Jens

[AndrewT]

Try a cascoded CCS that is set to just over 50mA.

[NixieFreak]

Hi Andrew,

There is a low side shunt in my circuit anyway, because I will include an amperemeter with a microcontroller. So I just added a small comparator stage onto that shunt as seen in the attachment. That's why I figured I can build my own current sense circuit:

If 50mA is exceeded by a few mA, this will create a voltage larger than the 265mV reference provided by R10 and R11. The output of the comparator will go high and can alert an interrupt pin of my microcontroller which in turn will then open a relais or something else.

Does that look like something that will work?

Jens