10 pole single throw...

[TaDa]

Hi!

I want to safely discharge 10 high voltage capacitors with the flick of a switch.
The caps will, in normal use, be wired in series but I wanto to discharge them in parallel because it'd be quicker - and, hopefully safer when I'm poking around inside.

I was thinking of a 10 pole single throw switch that would switch in 10 chunky discharge resistors one each across each of the caps .

But, i cannot find any 10 pole single throw switches

I don't really want 10 (spst), 5(dpst) or 4(tpst) or 3(qpst) separate switches if i can avoid it.

Any suggestions would be appreciated.

Thanks

 

[colin55]

Use a rotary switch. They come in 5 pole 2 or 3 position and use two banks.

 

[bychon]

Mouser part # 690-C4D2005S

 

[dougy83]

The caps will, in normal use, be wired in series but I wanto to discharge them in parallel because it'd be quicker - and, hopefully safer when I'm poking around inside.

If they;re in series already, the easiest way is to connect a resistor across them (keeping them in series); requires only a spst switch and a big resistor. Series/parallel connection doesn't affect the discharge time.

 

[mneary]

It's a good thing to discharge each one, since discharging a bank in series will leave a charge on any capacitor that is less than or more than the nominal value of the others.

 

[dougy83]

It's a good thing to discharge each one, since discharging a bank in series will leave a charge on any capacitor that is less than or more than the nominal value of the others.

Good point; the leakage resistance of the caps (but not the capacity) may change the charge distribution. This will potentially charge the leaky caps up backwards when the string is discharged - there's a simple fix of putting a diode in parallel with each capacitor; the diode is reverse-biased when caps are charged and doesn't allow any cap to charge to >~1V in the reverse direction when the string is discharged. Then a single SPST switch and resistor may be used.

 

[TaDa]

I like the last trick with the diode and the SPST switch.
Nice one!

Thanks

 

[gabeNC]

Mouser part # 690-C4D2005S

$400+ ????

 

[bychon]

Makes you think about using smarter methods, doesn't it!

 

[TaDa]

Ok, if each capacitor is in parallel with a suitable diode (conveniently like a voltage multiplier circuit is) what is a sensible rating for the resistor if...

The final voltage is 2kV and the capacitor bank is 100uF - I understand this gives me 200Joules
Asserting that 1Watt = 1Joule per second then a 1 second discharge time would require a 200W resistor
Since W=V*I, this yields I=0.1Amp
With V=I*R I get R = 20kOhm

This sounds a bit high to still need a 200W resistor what sort of resistor and discharge period should I be aiming for?

 

[colin55]

You are talking about a value of 200watt-seconds.
In other words you have a 200watt globe and will turn it on for 1 second.
How hot do you think it will be?
It will feel hot because the globe has very little mass and you will feel the heat.
Now get a 200 watt radiator bar and turn it on for 1 second. It will not feel very hot because the bar has mass and takes time to heat up.
That's the same with a 10 watt resistor. Put a 10 ohm resistor across a car battery for 1 second and it will heat up very little.
Put a 10 watt 1 ohm resistor across a car battery for 1 second and it will heat up a fair amount but it will not glow red hot.
It will just be "very hot."
That's the same with your problem. Even though you are dissipating 200 watts, it is for a very short period of time and a 10 watt resistor (or two 10 watts resistors) will be suitable.
Use a 470R 10 watt resistor.

 

[TaDa]

Thanks colin55 - that makes a lot of sense - time to go rummaging in the 'chunky resistor' drawer