# Re: Resonant cap?

```Original poster: "Steve Ward" <steve.ward@xxxxxxxxx>

Hi JT,

Some assumptions must be made, firstly that the transformer acts as a
constant current source.  Current = charge per time (i=Q/t).  Also,
charge = capacitance times voltage  (Q=CV).  With these 2 equations
you can solve for how long it will take to charge a given capacitor to
whatever your desired voltage is (usually a little less than
VRMS*sqrt(2)).  Given 3kvAC, thats roughly 4.2kV, so lets say you want
to charge to 4kVDC.  Lets say for example you want to charge whatever
capacitor 120 times a second (so in 8.333mS).  So we find the charge
possible given the supply current and the time alloted.
.01A*.00833s=.0000833C.  Now we find the capacitance: .0000833C/4000V
= 20.8nF.  So that is the value i would aim for personally, but you
should be able to extrapolate from the information ive given if you
desire some other break-rate than 120bps.

To keep the units clear:

Current (i) in amps (A)
Charge (Q) in coulombs (C)
Time (t) seconds (s)
Voltage (V) in volts (V)

Steve Ward

On 1/29/07, Tesla list <tesla@xxxxxxxxxx> wrote:
```
```Original poster: "JT Bowles" <jasotb@xxxxxxxxxxx>

I am just thinking of rectifying the output to DC now. It will make
things much simpler. One problem arises though; how the heck does one
determine the appropriate cap size, with a HV DC source?

>From: "Tesla list" <tesla@xxxxxxxxxx>
>To: tesla@xxxxxxxxxx
>Subject: Re: Resonant cap?
>Date: Sun, 28 Jan 2007 14:32:26 -0700
>
>Original poster: Mike <megavolts61@xxxxxxxxx>
>
>That's where the 26.5pF  came from, Gerry.   JT,  does it show on
>the ferrite any numbers?   There are a few vendors of ferrite cores
>that you can look up the properties of ferrite materials.  Perhaps
>it's possible to build a driver circuit that is a bit lower
>frequency...say 5 - 10 kHz which would allow you a larger resonant
>cap size.  The only other suggestion is to rectify the output and
>use a voltage multiplying circuit to raise the output
>voltage.  Then, you could fire your coil at whatever frequency you wish.
>Mike
>
>
>
>Original poster: "JT Bowles" <jasotb@xxxxxxxxxxx>
>
>Hi JT,
>
>For your 30KHz transformer, the formulas that you want are these:
>
>Zout = Vs_oc_rms/Is_sc_rms
>
>Cres (nf) = 10^9 / (2*pi*freq*Zout)
>
>use 30KHz for your frequency.  The assumption here is that the
>winding resistance is insignificant compared to the current limiting
>leakage reactance.  If you can measure the primary and secondary
>winding resistance, we can verify the assumption.
>
>Gerry R.
>
>
```
>------------------------------------------------------------------- ----------
```>
>
>For the...thrid time now, online calculators only work for
>50/60Hz.......
>
>My Transformer runs at 30kHz, 2KV AC, 10ma.
>
>Again, you cannot use an online calculator in this case. Go back to
>deep fried neon. Look above where you put in the voltage and current.
>It has a selection for"50 Hz" and "60Hz". NO 30KHZ.
>
>So far, I have only received one real response; 26.5pF. I calculated
>by hand and received a similar answer. I need some more opinions
>though.
>
>
>We won't tell. Get more on
><http://us.rd.yahoo.com/evt=49980/*http://tv.yahoo.com/collections/265>shows
>you hate to love
>(and love to hate):
```
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```>TV's Guilty Pleasures list.
>

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