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Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
I've picked up several interesting little mil-spec HV charging inductors, and am interested in using them in small DC resonant-charged spark-gap TCs. One inductor is rated at 20H 50ma RMS at 900 PPS, and the other two are rated at 4.5H, 185ma RMS at 360 PPS. I plan on using the two 4.5H inductors in series. The coils will utilize variable-speed DC motor-driven RSGs, and I'd like to achieve a maximum break rate of at least 500 BPS.
About the only thorough analysis of DC resonant charging (in a Tesla coil application) that I have found is Richie's excellent writeup at:
However, I do have a few question about the design equations, and input from anyone skilled in the art would be appreciated. The coils will be designed around the inductors, so the inductors are the only "fixed" values at this point.
1. When calculating the resonant frequency of the charging circuit, are the charging inductor and the tank capacitor the only factors that need to be considered, or is there some influence from the DC reservoir capacitor?
2. Is the characteristic impedance of the charging circuit influenced by the DC reservoir capacitor?
3. Is there any special relationship between the frequency of the resonant charging circuit and the resonant frequency of the coil itself, or can the coil be "anything" as long as the break-rate is within the capability of the charging system to charge the tank capacitor?
4. What are the guidelines for sizing the value of the DC reservoir capacitor? Some other sources have recommended 10X the tank capacitor value, but these applications were for lower voltage DC-DC converters.
5. Is the pulse-per-second rating of the inductors typically based only on thermal considerations?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Provided you use a de-Qing diode, nothing is critical. The capacitor will charge to twice the DC bus voltage no matter what the break rate. The resonant frequency of the charging circuit just determines the transition from discontinuous current in the choke to continuous, but nothing noticeable happens at that transition. (Very heavy continuous current may make spark-gap quenching more difficult, though.)
The PPS rating of the inductors is presumably determined by I2R heating in the windings, and heating by core losses. The former depends on RMS current and the latter on, I guess, RMS voltage and frequency.
Registered Member #1738
Joined: Fri Oct 03 2008, 08:48AM
Location:
Posts: 5
First, the resistance R of the charging circuit influences the resonant frequency, but for small R it is negligible. Due to the R the voltage does not reach twice the supply voltage. Too large R will cause trouble. If the DC Reservoir is large enough and able to provide the necessary di/dit it appears like a constant voltage source and has no influence.
Second, see first
Third, the oscillation of the coil itself should have reached steady state, no energy in the circuit, when the charging curcuit starts a new cycle. But dont care
I planned to build such a system some time ago. After lots of math using differential equations I realised that I will never get a transformer providing the necessary current. Using a smoothing cap was no option for me. Due to this and lots other problems I never started this project. I did the maths in mathcad, the script still works. If you need support just let me know
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