Switching HV Negative Ion Generator Output

[pnielsen]

I would like to build a HV low current source that can be connected across a large area plate capacitor and switched at very low frequency (1-10Hz). The purpose is experimentation with dielectric films.

The first thing that came to mind was a domestic negative ion generator. Some have an acceleration grid that is connected to “ground”, thus providing access to a complete circuit. Within the frequency range mentioned above, I would expect the capacitor to draw negligible current.

Is there any practical way to switch the ion generator’s output? In other words, between the negatively charged pins and ground with the cap wired in series across them. As I understand, the nominal voltage is around 2-5KV at whatever small current the HV bridge can supply.

With regard to cycling the cap, I am thinking the capacitance may be low enough so it does not retain charge. However, if this is not the case, would be the best way to provide a discharge path that is does not overload the ion generator and will tolerate the voltage applied?

 

[dougy83]

If I understand you correctly, you can just use a HV relay. If you use a SPDT, you can connect the capacitor plates to the HV generator output (when relay enabled), or to ground (when relay disabled) (preferably through a resistor).

 

[pnielsen]

There are suitable negative ion modules listed on eBay. However, most eBay output 6-10KV. https://www.ebay.com.au/sch/i.html?...gative+ion+generator+module&_sacat=0&_fosrp=1

The highest voltage solid state relay I can find is this one at 1.5KV. https://www.mouser.com/datasheet/2/315/semi_eng_he1a_aqv25-1299736.pdf
Given that the ELF current through the cap will be very small, could I get away with that?

Alternatively, I have looked at vacuum relays but they are expensive. There does not seem to be anything in between. Can anyone suggest a cost-effective solution?

 

[dougy83]

Yes, HV relays are very expensive.

Another option would be to put a discharge resistor across the output capacitor, and switch the input voltage to the ion generator. I imagine the discharge resistor will have to be very large, as the output resistance of the ion generator will possibly be 20M.

Or you can stick with a mechanical switch, and have a motor driving a rotating conductive disk with slots that touch a discharge wire. The switching frequency is set by the number of slots/teeth in the disk, and the speed of the motor.

 

[pnielsen]

Yes, HV relays are very expensive.

Another option would be to put a discharge resistor across the output capacitor, and switch the input voltage to the ion generator. I imagine the discharge resistor will have to be very large, as the output resistance of the ion generator will possibly be 20M.

I like the simplicity of this approach. I assume the 20M resistor would discharge the Cockcroft-Walton bridge each time the ion generator's power supply is switched off. This could work well for for the relatively slow switching.

But how can I verify on the bench that the concept is working as intended, and charge is not being retained across the cap between cycles?

 

[dougy83]

Yes, it will have to discharge the cockcroft walton multiplier each time, which means your charge and discharge curves will be ramps.

If you use the mechanical switch shorting out the output/plates, you will have a much sharper drop off.

You can verify the plate voltage by using high voltage probe to an oscilloscope, or possibly by connecting a neon indicator across the plates in darkness. Another option is to have a piece of string on one plate, which will be bent towards the other plate when there's potential, and bent less when there's none.

 

[pnielsen]

Yes, but connecting a neon will draw far more current than the plate cap on its own and affect the reading. I suppose I could make up some kind of capacitive probe for the scope. I also thought of wiring a step down transformer in series, but the frequency might be too low. Will have to try that one hands-on. Thanks for the suggestions.