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Re: Need help with mystery
Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
12kV, 64 ohms... What is the power in the resistors when the gap conducts
multiplied by the duty cycle?
(12000 / 64)^2 * 0.01 = 350 watts ;-))
You have a 50mA current with 190 amps spikes maybe lasting 100uS (120 times
a second). Those spikes are like hitting the resistors with lightning!
Those numbers are a guess at what you have there, but when the gap conducts
it is dead shorting the 64 ohm resistor with 12000 volts. The inductance
of the resistors and the resistances of the caps and wiring are probably
limiting the power somewhat from the above. If you go to 1000 ohm 50 watt
(12000 / 1000)^2 * 0.01 = 1.4 watts
+ 0.05^2 x 1000 = 2.5 watts
When you raise the resistance, the 6uF cap will stay charged to a higher
voltage too since it is being somewhat discharged by the 64 ohm resistor.
All that heat can be converted to streamer ;-)
At 09:44 PM 7/21/2001 -0600, you wrote:
>This is more puzzlement regarding a problem I mentioned in earlier posts. I
>have a DC supply of about 12 KV which has a reservoir cap of about 6 mfd.
>It connects to a RSG which alternately charges the tank cap from the DC
>supply, then discharges it into the primary (SPDT switch action).
>If I directly connect the output of the reservoir cap to the RSG, the
>charging gap spark is quite bright and noisy (same as the discharging gap
>spark), and tends to erode the electrodes. The charging gap sparks can be
>toned down by putting a power resistor between the reservoir cap and the
>RSG. This lets the tank cap charge a bit more slowly and doesn't burn up
>the gap electrodes as much. Charging gap spark is reduced to a much
>smaller, dimmer spark, and TC performance is still good.
>Here is the puzzle. My most recent power resistor is 6 400 ohm 10 watt
>power resistors in parallel for 67 ohms at 60 watts. Current going the
>resistors is high amperage pulses, but the average current is only 50
>milliamps. Current is measured with an ordinary moving coil ma meter.
>Power dissapation for this series resistor is I-squared R = .05 x .05 x 67
>which is about 0.17 watt. So the resistors will stay cold, right? Wrong!!
>When the TC runs for a minute, the resistors get literally smoking hot! I
>am guessing the power dissapated is well over 100 watts!! This means the
>resistor impedance is at least 40K??
>Another clue is that the voltage pulse across this power resistor is on the
>order of 5 KV as it will jump across a 1/4 inch gap. If the power resistor
>is a pure resistor, this would indicate current pulses of about 75 amps
>flowing into the 19 nF tank cap.
>What in the world is going on here?? My guess is the wire wound power
>resistors are also inductors - thus the higher impedance. I suppose I need
>to make a big bank of carbon resistors (non-inductive) to see if the same
>heating effect occurs. (another alternative of internal arcing between
>turns of the power resistor doesn't solve the mystery. This would not
>generate any more heat than pure resistive heating).
>So, list, why do my power resistors get smoking hot?
>I suppose the same effect should happen with a normal AC powered TC. How
>about if some of you add a power resistor in series with your spark gap and
>see what happens? If you have a 60 ma NST, for example, the RMS current
>through the resistor should be 60 ma or so. If it was 100 ma through a 50
>ohm power resistor, it should only dissapate a half a watt. A big resistor
>would barely get warm after a few minutes. What do yours do?
>Thanks for any light you can shed on this mystery.