Oct 30, 02:04 PM
Cockroft-Walton and Villard Cascade High Voltage Multiplier Tips
If you want to build a high voltage low current supply in the 40kV-150kV range then here are some tips:
1) Don’t use a neon sign transformer or the 120V from your power outlet. The reason is that these are too low in frequency, and the latter is too low in voltage. The 60 Hz frequency means every second there will be 60 push-pull cycles on the multiplier, but the typical HV capacitors you can buy tend to lose their charge faster than that. This means the output will be pathetic and have deep ripples as the voltage can never maintain high levels consistently. Metaphorically it’s like an old car or lawnmower sputtering while trying to start up. Now, if you use large enough capacitors then yes you can get by with neon sign transformers, but cost and size of the capacitors become issues.
2) Use a flyback transformer instead. These run at 8kHz to 30Khz, way better than the 60Hz. That’s fast enough that the capacitors in the multiplier will stay charged up and the output remains relatively smooth and high in voltage. Then you can use 330pf, 1000pf, or 10nF high voltage capacitors and do just fine.
3) Make sure the flyback output is AC, not rectified DC. Some flybacks have diodes and smoothing capacitors built in. The problem then is that only the ripple component of that will offer a push-pull into the multiplier, and hence you will barely get anything out since the ripple is low. Note that if you place the ground wire near the output wire of a DC flyback it will generate a nice spark, but that doesnt mean it’s AC.
4) Use a ZVS driver if you want to efficiently power your flyback. These can be had for 15 bucks off eBay or aliexpress. You wind a center tapped winding on the flyback core with some insulated wire, anywhere from 5-center-5 turns (10 turns total) to 15-center-15 turns (30 turns total). That means three wires come out from this primary that you wind, which you insert into the corresponding ZVS screw terminals. The higher the number of turns, the lower the frequency and lower the voltage output of the flyback. This may be necessary to avoid blowing your diodes and capacitors.
5) Voltage output from the flyback can be gauged by measuring the spark length beween bare wire tips. It will be approximately 1kV per millimeter. So 1 centimeter = 10mm = 10kV. This is a very rough estimate. So if you wind 20 turns total and find that the voltage is around 12kV but you need 8kV, then reduce the output by winding a proportionately higher number of turns: take 20 times 12 divided by 8 = 30 turns (15 and 15).
6) Capacitors must be rated a little higher than the flyback voltage. So if the flyback outputs 15kV, then the capacitors should be rated 20kV or higher.
7) Diodes must be rated twice the flyback voltage. Thus if all you can find are 20kV diodes, the input must be 10kV or less. Or, you can put two diodes in series to bring that up to 40kV rating, twice a 20kV input. So for a 15kV input from the flyback, you can use 20kV capacitors and then two 20kV diodes in series. To double the current rating, put two diodes in parallel. Thus to double voltage and current rating, you should have two pairs of diodes (two in series, and then two of that in parallel = 4 diodes) instead of just one diode.
8) Use an output resistor to protect your diodes from too much current. If you short circuit your multiplier, or allow it to produce nice big sparks, then without a resistor you will get some very high current surges that will likely burn out one or more of your diodes. Get a high voltage resistor, 100Mohm to 500Mohms, 10 watts or higher. These can be bought off eBay or aliexpress. They look like long red sticks.
9) Stack your capacitors in parallel in the earlier stages. For the first 1/4 of your multiplier, it’s helpful to have higher capacitances. This helps build up more charge to power the rest of the multiplier more smothly. It allows for better load handling (less voltage reduction and ripple once a load is connected). So the very first two capacitors can be 3 or 4 in parallel, next stage 2 or 3, and next stage 2, and the rest 1. That is, if you can afford buying extra capacitor for that purpose. You can get by without doing this, but if you have caps to spare then it’s a good idea to parallel them up.
10) Use corn / canola / vegetable oil as an insulator instead of mineral oil. The properties of such oils is equal or better than mineral oil and it’s non-toxic, and potentially cheaper. That is, put your multiplier circuit in a tub or well-sealed (at the bottom at least) PVC or clear acrylic tube and fill it with oil. This prevents arcing that normally occurs in air, since oil has 3x the dielectric constant of air. Some people use paraffin wax with a few hot glue sticks dissolved in there to prevent shrinkage upon cooling, but a solid potting compound like this is a pain to remove if one of your diodes blows. Also, if an arc does form in oil, the oil is fluid and heals itself whereas a solid dielectric will have a little hole blown through where the arc went, encouraging further arcing.
11) Make sure your output resistor and beginning of the output cable are immersed in the oil too. The voltage prior to hitting the resistor is pretty high and has a tendency to arc over to ground if exposed to the air. So that part need to be in the oil. The upper half of the resistor doesn’t have to be, but if it fits under the oil then that’s better.
12) Use silicone as a sealant, not hot glue. Oil will dissolve hot glue. Silicone, the kind used on bathtubs and window sills, is a decent dielectric insulator and resists oil and breakdown from ozone and UV. Get the low odor formula if available.
13) If you need a low-ripple output, dual polarity (not just one polarity and ground, but -kV, +kV, and ground) and good load handling, then use a Villard Cascade instead of Cockroft Walton. It’s basically just two half-the-number-of-stages Cockroft-Walton multipliers that meet at their bases. So the flyback voltage feeds into the center of the circuit rather than the beginning. The advantage is that with half the number of stages per polarity, you get a better smoother output. The thing about Cockroft-Walton is that ripple and sag (from bad load handling) increases dramatically with the number of stages. Use half that number, and problems go away. So instead of 0 to 100kV, you get +50kV and -50kV but with a stronger smoother output voltage.