High Voltage Bias for DIY Electrostatics

[oublie]

Originally Posted by kavermei

It's an old thread, but allow me to present my solution to you all.

I bought a 'cold cathode fluorescent lamp', or CCFL, the type of colored tubular lamp that kids put inside their computers to show off. They are dirt cheap, maybe $8. Those lamps need a high AC voltage, ...
Kenneth

What level of ac voltage do these little things put out?

[kavermei]

Mine give about 1100V RMS. They have a little series capacitor at the output, as a rudimentary current limiter I suppose; if I remove that, I get even higher voltages. Exactly how much, I can't measure anymore without a HV probe.

Because the frequency is so high, one could easily make a multiplier ladder with many stages, and keep the losses down. Using more than ~10 stages at 100/120Hz requires huge capacitors to reduce the losses!

[kavermei]

Here's some photos. I said earlier I used 4.7nF caps, I was wrong, they are 3.3nF

[oublie]

Thanks kavermei ,

I have a few of those in a box somewhere, ill have a look, i suppose i could test them with a standard multi by hooking one up to a standard step down trafo of a known step and test the voltage that way.

might be just the thing i need rather than building a complete bias from scratch.

[Ulf Eliasson]

Why do you insist to use a trafo? You could design a safe cascade multiplier supplied directly from the mains outlet.

[kavermei]

Originally Posted by Ulf Eliasson

Why do you insist to use a trafo? You could design a safe cascade multiplier supplied directly from the mains outlet.

I wouldn't call that safe. How can you possibly say that?? If you're not careful, or the socket is miswired, that'll put live mains on both your stators (through the audio trans secondaries).

You would pretty much need to make your speakers a Safety Class II device, to avoid blatantly violating all possible safety specs for electric equipment.

Finally, it'll put DC on the power grid, which is not very polite towards other equipment.

Am I missing something here??

Kenneth

[beun]

Let me comment from a electrical engineering background:

1) Running transformers backwards. It has been mentioned before but when you do that at line frequencies (50/60Hz) you will saturate the core thereby creating a short circuit and burning up the transformer: the smoke signaling

It can be done but the frequency of the input signal has to go up by the same factor as the voltage ratio of the transformer.

2) There is no problem creating a high bias voltage using a cheap 110/220V in and 220V out small (5W or so) isolation transformer and a 8-10 stage multiplying stack using 1N4007 diodes and 100nF 630V capacitors. Since each diode and capacitor only has 1 times the rectified 220V across it there is no need for fancy multi-kilovolt diodes or capacitors.

[geraldfryjr]

the one major importance for using a transformer (that nobody has ever mentioned) is that it limits the amount of current able to be drawn from the line, should any thing short out.if you short out the line there is a potenial 10,000 amps of current draw and once that arc gets started your not going to be able to readily stop it.not so true with only 1 amp or less. jer

[oublie]

Personally each way as fine as long as your careful. I modded a stax electrostatic power unit to use a higher bias but that only dealing with a jump from 240 to 580 v dc. when dealing with the big boys i is vital that safety is the number one priority.

Of course self biasing from the amp speaker volage and using a large number of stages up rectifiy and up the dc voltage has been done before but is it as good?

[Ulf Eliasson]

This is made by commercial designs also so I hope I'm not overlooking something, but please comment...
Two factors make it safe:
1. The audio trafo gives the DC isolation needed to protect your power amplifier.
2. Limit the highest possible current with resistors both to the membrane and to the middle point of the audio trafo’s speaker side. The latter to eliminate the risk of electric shock Kenneth warns about.

If you use a power trafo anyway make sure to pick one without any connection between primary and secondary coil.