High Voltage RC

[zappedagain]

I am using a 3.3K resistor to charge and discharge a 33pF cap to 210V at 4000 cycles per second. While the dynamic power dissipation (p=0.5*V^2*f*C) is low (4mW), this pushes me just past the working voltage threshold for most surface mount parts (0805 resistors typically have a 150V max, above that they can start to arc). I see a few manufacturers have higher voltage parts available, but they don't seem to be readily available (i.e. stocked at distributors).

I'm thinking about using two 1.6K 0805 resistors in series. This reduces the working voltage across each of them to 105V, well within the working range.

At first I was concerned that parasitic capacitance would put too much voltage across the resistors (0.5pF gets a lot more voltage then 33pF when they are in series). Luckily my rise time is fairly slow (7 uS), so the parasitics have negligible effects.

Do you see any other reasons this approach won't work?

Thanks,

Z

 

[itsmoked]

Nope!
It is a standard approach, if you don't want to up the size.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zappedagain]

Hmm, it looks like I might have been getting overly complicated. Now that I take the rise time into account (i=C*dV/dt = 250uA), my maximum voltage across the resistor is only 0.8V. With that fact I can stay with a single 0805 resistor. Even at the drivers limit of 600V/uS I'm still under 83V.

This still seems too easy though...

Z

 

[itsmoked]

That's not what your original question suggests. You say you are using 210V..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

he said applied voltage is 210v but I'm sure you'll agree that at any given time part of the 210v drops accross resistor and part drops accross capacitor




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[zappedagain]

Right, while charging and discharging with the ramp, the voltage drop across the resistor won't get over 83V; the rest of the voltage drop will be across the capacitor.

Z