Doorknob Capacitor
Doorknob Capacitor
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Wattage rating of Doorknob capacitor, in the measured by voltage?
Building a high power linear amplifier (solid state), I need to know if there is a formula out there that can tell me what voltage rating I need on the final output loading capacitor. The amplifier will be producing 12,000 watts peak @ 15 volts DC. I need either an air variable capacitor that can handle the amount of wattage, however I noticed that loading capacitors are rated in Kilovolts not wattage specs. Any idea how I can measure this current with the correct voltage rating without toasting the output capacitor?
Assuming your load is 50 Ohms (a normal impedance) then the final output loading capacitor will see the full voltage of 12000 Watts at 50 Ohms. Using P = E^2 / R, the RMS Voltage of 12kW at 50 Ohms is 775 Volts. The peak voltage is 1.41 times that or 1,095 Volts. The same calculations for a 75 Ohm load yield 949 VRMS and 1,341 Volts peak. Some allowance should be made for varying load (generally the load of an antenna varies with frequency and other factors) so the capacitor should withstand at least 2,500V and much higher would be better. If you want to use a fixed load cap, the TDK FD, FHV, or UHV style (doorknob type) ultra-high voltage may be a good choice. The current through the capacitor is dependant on the frequency and capacitance. Or you could use air variables for both the tune and load caps, giving you extra control of the output. Air variables are generally not current limited (they are very low loss) but do have voltage ratings. I'm not sure how practical a 12kW amp running off 15 VDC will be and whether a matching network can even be built. Even today, vacuum tubes (running at plate voltages around 1 kV + ) may be a better choice for a high power amp.