Electronic – Spanning high voltage between capacitors in series

capacitorsparkvoltage

first of all thanks for your time.

I've studied a lot before to make this post but theory and real world rarely make a good marriage.

I have 20kVAC of spike coming from an ignition coil (the one used for making the sparks in a car engine).

The spark is made through an oscillator and so I can modulate the frequency, the amplitude and the duty cycle.

At the output of the coil I put an HV diode bridge to make it DC and, after the bridge, I have a long series of electrolytic capacitors (50 units, SAMWUA 400WV HE 105°C (M), rated at 400VDC/82uF each) put in series.

I made a simulation with PSPICE and the software says that, at each spark, every capacitor is fully charged at each pulse (once put in series the capacitance of each cap is only 1,64uF).

Before to apply the 20kVDC to my capacitor's bank in the real world I would know if I can do it safely or I will risk a dangerous explosion or to damage the bank (I will make particular attention to the polarization, of course).

The second question is: is it true that – in the real world -, at the terminals of each capacitor, I'll have 400Vdc?

What about the first and the last capacitor of the series? Do they have to withstand 20kV or simply, like the others in the series, they will carry only 400V?

Best Answer

You should use balancing resistors across the capacitors, assuming it's a simple series connection of electrolytic capacitors.

The appropriate resistor to use across each capacitor is

R = \$ \frac {n V_M - V_b}{0.0015C V_b}\$

R is in M ohms

n is the number of capacitors

C in uF is the capacitance of each capacitor

\$V_M\$ is the voltage allowed on each capacitor

\$V_b\$ is the total voltage across the string

If we allow 400V across each cap and total voltage will not exceed 15kV then we have a balancing resistor value of 555K ohms.

Each one needs a power rating suitable for the highest possible voltage, so

\$P_R = \frac {V_M^2}{R} \$ = 0.29W for this example

The balancing resistors will suck 0.54mA in this example, which is 8.1W total, a significant power loss, so you may not be able to get full voltage from your coil.

Reference: Cornell-Dublier Aluminum Electrolytic Capacitor Application Guide

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