Cockroft-Walton generator

Is your meter a 10 meg ohm input type ? also, 1.5 nf is small for 50 hz. you may want to try larger caps.. Also, you don't need 2kv caps at the voltage you're starting at.

Thanks for your reply.

yes, my meter is a 10MOhm multimeter. I was planning to add 4 stages (reaching 8 stages) and to switch to a 230V 50Hz supply, so 2kV should be barely enough. Do you think that I should try with 230V right now?

I also have 15nF @ 3kV caps, do you think the would be better suited?

I would try the larger caps first.

Also, check the forward voltage drop of your diodes. The 500 ohm resistance figure you give does not make sense. Maybe you mean 500 mV at 1 mA, but if you have high voltage diodes they may be several junctions in series, which would explain a low output with 12 VAC.

Paul

Thank you for your time.

I am using 1N-5408 diodes. Here's their specification for your ease:

In your opinion, how much should I raise the input voltage to get a noticeable effect?

A Cockroft-Walton ladder uses only the peak/peak input voltage stress across each capacitor, so

Two considerations may account for low output voltage: the rectifier capacitance (forms a capacitive divider with the ladder capacitors), and the voltmeter shunt resistance. I'd use an ammeter instead of a voltmeter, just calculate the output current of the CW rectifier into a short and see if it matches. If the current is low, that means it's the rectifier capacitance (and using larger ladder capacitors will solve the problem).

Again, I appreciate all of your feedback.

I connected the CW to a 230VAC 50Hz supply. My meter read 1080V (meter scale: 0-1kV), and I clearly heard the cracky sound of a sparkle when I closed the circuit, meaning that arcing happened, I suppose.

I will test it again with a professional voltmeter to see the actual output voltage, but it really seems to me that it works. If I need to run it at a lower voltage, I will increase the capacitance.

Thank you!

Claudio A.

Doing the calculations on the 15 nf you suggested yields ~212k Xc for each cap. That is a high value for what you're trying to do. If very low current is what you need then I guess it's fine how ever, it makes it very hard to measure using a conventional meter even at 10 Meg Z after you do a few stages. And remember that you'll get ~ 500--600 mv loss on each multiplier.

For 50 Hz, you really need large caps.! when I say large caps, I'm talking in the range of Uf's not nf's

P.s. We have vessel units at work that employ this technology that outputs

In the vessel is a photo multiplier tube so sensitive that is can detect a very small emission of photons which would indicate so arching taking place and thus shut down the system!... In our system, we half rings that form the capacitance's via the side walls of the vessel when the unit is closed up..

"cloudguitar"

The answer is in megohms.

...... Phil

I would use the CW to charge some other capacitors. Therefore, a low current output shouldn't be great a problem. Unfortunately, here in Italy components are difficult to find and quite high priced. In fact, the most interesting cap I can get here is a polystyrene 0.1uF rated for 3kV at 3.50=80 each (5$), way too expensive. And I cannot even get more capacitance. I would probably have to purchase from abroad, but I a moving from Milan to Boston in a couple months so it isn't probably worth it.

I'll try with 1uF if possible.

"cloudguitar" "Phil Allison"

I would use the CW to charge some other capacitors. Therefore, a low current output shouldn't be great a problem.

Each cap in the multiplier sees only a fraction of the output voltage - so you can use readily available caps.

..... Phil

The reactance of a 1.5nF cap a 50 Hz is 2.1 MOhms. Of course, the caps all charge in series so the impedance out at the end is very high. Reverse current in the diodes plus you meter's impedance loads the circuit down giving the output you observe. Also the forward voltage drop of the diodes adds and reduces the output by that amount.

You simply must use larger capacitors at 50 Hz or use a higher frequency. If you are starting with 12 Volts, you can use electrolytic caps of several microfarads. The thing you need to determine is what voltage are you trying to multiply and how much current do you want to deliver?

The reactance of the capacitors must be consistent with the max. current because of the voltage drop it creates.

Keep in mind that this type of voltage multiplier is usually used for very high voltages, I've used them at 200kV. With high voltage, a few volts across the caps is totally insignificant. At 12 volts it's a different story.