What size capacitor do I need for my CO2 laser power supply rectifier?

[zuulmusic]

So I'm building a CO2 laser for fun, basically.
I have a tube that requires 11-18KV DC to fire.
Right now I have a 20A variac, a 15KV transformer (30ma out) and then I need to build a rectifier.
I have a bag of "1A, 1000V" diodes.
So far, I built a full-wave bridge rectifier out of them. I used 6 of these diodes for each "diode" in the bridge, so 24 total. This was to handle the peak voltage.
So I have the variac to the 15kv transformer to the rectifier to the tube.
I can't get the full-wave bridge rectifier to put out enough voltage.
When I use a string of diodes in a half-wave rectifier setup, It works, but I get a TON of AC in the output (tested with multimeter at low voltage levels, about 100vcd and 400vac).

Also interested in using a capacitor to level things out, but don't know what size to get. I figure 20kv peak, but what about the farads?
So any help would be appreciated!
Thanks!
Bill

 

[waht]

I'd suggest you look around for high voltage diodes and capacitors on ebay, they have pretty good deals sometimes.

If you connect 15 diodes (1000 V) in series that will give you max voltage rating of 15 KV. That would count as 1 diode.

So for a half-wave rectifier you need 30 diodes and for a full wave you 60 diodes.

 

[zuulmusic]

So I've got to use more diodes... i will try it and see how that works.
thanks

 

[berkeman]

I'm more used to seeing a diode-cap ladder arrangement used as the voltage multiplier output of a moderate-voltage transformer. That distributes the high voltage drops across multiple diodes and caps. Check out some of the circuit ideas at this page:

http://www.discovercircuits.com/P/pwr-hivolt.htm

You could also just use the flyback circuit from a TV set. Black and white sets are in the 15kV range or so, and color sets are in the 25-35kV range. But for heaven's sake, be careful zuulmusic. Working with high voltages like those can get you seriously hurt or killed. Especially using a flyback transformer from a TV set, since the output is DC referenced to ground unless you use an isolation transformer at the TV circuit input. You *DO* know what an isolation transformer is, right?

 

[dlgoff]

"But for heaven's sake, be careful zuulmusic. Working with high voltages like those can get you seriously hurt or killed."

If you get the thing working, I would say...Be careful of the lasers UV output. You should check into the proper eye protection.

 

[zuulmusic]

I'd suggest you look around for high voltage diodes and capacitors on ebay, they have pretty good deals sometimes.

If you connect 15 diodes (1000 V) in series that will give you max voltage rating of 15 KV. That would count as 1 diode.

So for a half-wave rectifier you need 30 diodes and for a full wave you 60 diodes.

Are you sure about this? In this rectifier configuration, current is always flowing through two "diodes" with the load between then essentially. So wouldn't I need more like 32 diodes (8 diodes for each of the four "diodes" in the rectifier). That way its going through 16 diodes each half cycle.

Since this is for an educational display, I wanted to keep this as a fairly simple full wave bridge rectifier design. Also I already have everything except a filtering capacitor. Any ideas on the values for that?
Thanks!