switching high voltage supply

[JPI]

I work in the R&D department for an electrolytic capacitor company. I have been asked to develop a completely automated data logging system using an Agilent aquasition unit.
I have to measure 8 high voltage capacitors simultaneously, these capacitors will be measured for internal pressure via a pressure transducer, internal temperature vie a thermocouple and leakage current, they will be kept in an oven a 95 degrees for the duration of the test. The capacitors are supplied 500vdc using a GENH750w power supply, they will be under test conditions for weeks even months. The capacitors may reach 70psi at different rates therefore I need to turn off the supply to those particular capacitors individually.


So, I have 8 outputs from a labjack U3 that go high when then the capacitors reach 70 psi, the labjack is powered by a USB. The idea in my head is to use the output to some how switch a Transistor,MOSFET,N Channel,SPP02N60S5 to turn on and off the supply to each individual capacitor, possibly using an opt isolator. I am thinking that I need to lower the 500vdc to something that can safely supply the gate of the mosfet.

Please, please can someone offer some suggestions?

Thanks

John

 

[Nigel Goodwin]

As this is a slow (EXCEEDINGLY slow) application, I would suggest using relays might be a better soloution than transistors? - use a small transistor to feed the relay from your outputs.

 

[JPI]

The kind of relay that can handle 500vdc without fusing the contacts together costs a bomb, and I have a teeny weeny budget.

 

[Nigel Goodwin]

What fuses contacts is the high current when the contacts close, as long as you have suitable current limiting (which I imagine you have already?) it shouldn't be a concern. The other concern is sparking when the contacts open, but this is dependent on the voltage across the contacts, and as you're merely disconnecting an already charged capacitor this should be pretty low as well?.

Exactly the same concerns arise with using a transistor to switch them!.

 

[jpanhalt]

This sounds like a different view of the same issue you posted here on 02 May 2007. Did you ever get that circuit working? John

 

[JPI]

No I never tried the circuit. The trigger comes from my labjack, and I thought that both circuits needed to have a common negative, and I am sure it isn’t a good idea to link the negatives together. But since then I have been informed that so long as there is a potential difference on the gate it should work. I will give it a go.

 

[Optikon]

No I never tried the circuit. The trigger comes from my labjack, and I thought that both circuits needed to have a common negative, and I am sure it isn’t a good idea to link the negatives together. But since then I have been informed that so long as there is a potential difference on the gate it should work. I will give it a go.

x 2 on using relays.. you can deal with current limit no problem. You wont fuse contacts - this is a common application.