Capacitor charge rate
In a recent communication, you said a 12kv 120ma 60 cycle power supply will
effectively charge a .026mfd capacitor. I've been curious about this and was
doing some math and I came up with a different answer. Maybe others will
check this out and comment.
I calculate as follows: The available time to charge the capacitor is one
quarter of the 60 cycle waveform (peak voltage). This is .0042 seconds. To
achieve 97% charge on a capacitor we need 3 time constants. The thevenan
equivalent of a 12kv 120 ma transformer must look like an infinite current
12kv supply with a 100k ohm resister in series. We know we must achieve full
charge (97%) in .0042 sec and it takes 3 time constants to do this so we have
.0042 / 3 = .0014 sec for one time constant. We know one time constant = RC
and C = one time constant / R. Solving for C we have .0014 sec / 100k ohms =
.014mfd. Thus, a 12kv 120ma transformer will fully (97%) charge up to a .014
If this math is correct, I calculate that we need a 12kv 171ma transformer to
fully charge a .026mfd capacitor.
What happens when you have a power supply large enough to charge the tank
capacitor in say 1/8 of a cycle? Then we need a rotary spark gap to take
advantage of the extra power -- do I have this correct?