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Re: High Voltage Transformer

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 10:29 AM 3/3/2005, you wrote:
Original poster: NuclearFirestorm@xxxxxxx
The cap I'm charging is a 225 uF 15kV cap. It will have a 5 MOhm energy dissipation resistor, 500W 5 kOhm current limiting resistor, and a string of balanced diodes to perform half-wave rectification. The input power will come from a 110 input 0 to 130 Volt out variable transformer. The variable transformer is to be raised at a certain rate to keep the power from being initially too high. The circuit has been designed and simulated in pSpice. I believe the charge time was 20 or 30 seconds, I forgot the exact number. The only missing part was a transformer.

Thanks.

Hmmm..
25 kJ in about 20 seconds. That's 1 kW. Your current limiting resistor's gonna get pretty hot. Basic rule of thumb when charging a capacitor from a "stiff" voltage source (i.e. constant voltage) through a resistor is that the same energy as stored in the capacitor will be dissipated in the resistor. (You can fairly easily calculate it by integrating the instantaneous power in the resistor)

The 5 Meg energy dissipation resistor is for what? Bleeder? You know that it will be dissipating 45 Watts (with 15kV across it). One RC timeconstant is 1125 seconds (about half an hour...). Your capacitor is probably not designed to have that much energy stored in it for that long a time (unless it's a huge DC filter capacitor.. but that's a huge stored energy for a filter).


A few suggestions.

Get a high power resistor of a few hundred ohms to put in series between the charging rectifier and the capacitor. That way, if you inadvertently have a flashover, the reverse current won't destroy your rectifier. Choose the resistor properly so that it can absorb twice the energy stored in the capacitor. If you hook up the circuit like this:


transformer
rectifier
shorting switch (across the output of the rectifier... it would put r and C in series)
series resistor
capacitor
load

If you close the shorting switch, you'll discharge the capacitor into the resistor.

Choose a value so that the current is no more than the tens of amps region. 500 ohms is 30 amps, and will have a time constant of about .11 seconds.. it will discharge to a safe voltage (<50V) in about a second. That series resistor will also help limit the charging current (unless you've got something else)


For charging capacitors, some sort of voltage multiplier is really nice. First, you don't need a big giant transformer. Second, it produces a gradually rising output voltage as the current decreases, so the loss in your charging resistor is less.