My voltage multiplier (cockroft-walton) will not charge my capacitor bank past 150v. How can I fix this?

Question

I got 5 free 450v 4700uf capacitors that I put in parallel to form a 23500uf 450v capacitor bank (2.4kJ) in hopes of building a can crusher and later a railgun. I built a voltage tripler from four 350v 10uf capacitors and 4 400v 6A diodes. It outputs 500v (which is okay because the capacitors have a surge of 550v), but the capacitor bank still won't charge past 150v. I assume this is because the charger is not outputting enough current to compensate for the capacitors' leak rate. When I try to test how much current it is outputting with my multimeter, the first capacitor in the charger dies, so I don't have any way to measure the current (and my scope maxes out at 400v).

Is there a simple fix for this? Or am I better off using a different charger design? What other charger designs could I use? Thanks.

Answer 1

I'm going to do a little bit of copying and pasting from Wikipedia which as we all know is the fount of all knowledge:

In practice, the CW has a number of drawbacks. As the number of stages is increased, the voltages of the higher stages begin to "sag", primarily due to the electrical impedance of the capacitors in the lower stages. And, when supplying an output current, the voltage ripple rapidly increases as the number of stages is increased. For these reasons, CW multipliers with large number of stages are used only where relatively low output current is required.

So yes, you can expect it to not provide much current, and maybe the capacitors are trying to draw too much which is making the output sag too much (and the ripple become a real problem). The article goes on to say:

These effects can be partially compensated by increasing the capacitance in the lower stages, by increasing the frequency of the input power and by using an AC power source with a square or triangular shaped waveform.

Another thing it says is:

The biggest advantage of such circuits is that the voltage across each stage of the cascade is equal to only twice the peak input voltage in a half wave rectifier. In a full wave rectifier it is three times the input voltage.

So switching to a full-wave design instead of a half-wave design would reduce the number of stages for the same output voltage, and thus should also allow an increased current.

(Wikipedia, we worship you :) )

Answer 2

Since your high voltage source apparently outputs 500 V open circuit, but only charges your capacitors to 150 V, it's pretty clear your capacitors are leaking. This may be the reason they were available for free.

Try just charging up one of the capacitors and see how high it gets, then add the other capacitors accross it incrementally. Perhaps only one of them is leaking excessively.

Another issue is measuring such high voltage. What is the impedance of your voltmeter? If it's 10 MΩ, for example, then just the voltmeter alone will draw 50 µA at 500 V. How much can your high voltage source put out and still maintain 450 V?

If the voltmeter is the culprit, then allow the caps to charge without the meter connected. Wait a while to make sure the voltage has leveled off, then connect the meter and quickly look at the reading. In the short term it will show you what the caps got charged to, even if that voltage goes down as you keep the meter across the caps.

Another thing to do is to carefully clean all the high voltage caps. Dirt can cause significant leakage, especially at high voltage. I don't know what your caps are made of, but probably mildly soapy water won't hurt them. Get everything really clean, then rinse with distilled water and let it completely dry before powering up again. Let everything sit for 12 hours or more in a warm and dry place after all noticable moisture has already been removed.