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MEASURING Q OF CAPACITOR
Mc> Just a couple of wee questions about Tesla tank capacitors.
Mc> Mark Graalman and Richard Quick were giving very good advice
Mc> concerning measuring the Q of a capacitor:
RQ> This is one area where a scope with a frequency generator leaves
RQ> no doubt. The capacitor can be placed in the tank circuit and grid
RQ> peaked right at the operation frequency of the coil system. The
RQ> height of the grid peak can be directly compared with other cap-
RQ> acitors swapped in for just this purpose. This gives specific and
RQ> meaningful Q values if the setup is not varied between swapping
RQ> capacitors.
Mc> By grid peaked do you mean that the tap on the primary is adjusted
Mc> until the circuit is resonating and so your oscilloscope measures a
Mc> large voltage drop across the 1K resistor because there is a large
Mc> current thru it? I guess that when you change capacitors you retune
Mc> the primary until resonance is again reached and you compare the
Mc> voltage you measure now with the one previously gotten. The higher
Mc> Q capacitor will have the highest voltage drop across the 1k resistor?
OK, a grid peak is a little different than a grid dip. A grid dip is
used to determine the resonate frequency and Q of a secondary coil.
A grid peak is used to determine the resonate frequency and Q of a
tank circuit. I will try to cover both so people understand.
In a grid peak, you short the spark gaps out with a clip lead. One
side of the capacitor is grounded to the coax shield (scope and freq
generator cases grounded here also). The signal in, with 1K resistor
in the line back to the scope, is placed on the other cap terminal.
The tank circuit will show a nul voltage (flat or nearly flat line on
the scope) until the tank circuit resonate frequency is matched. When
the resonate frequency is matched, the tank circuit shows a voltage
rise (hence the term "grid peak") between the two capacitor plates.
The height of the voltage peak is commensurate with the circuit Q.
In a grid dip, the base wire of the secondary coil is connected to the
output of the freq. generator, with the 1K resistor in the line back to
the scope. The coil will show a voltage peak (significant voltage on
the scope) until the coil resonate frequency is matched. When the
resonate frequency is matched, the secondary coil shows a voltage drop
at the base (hence the term "grid dip"). The closer the scope reads
to a flat line (i.e. the greater the voltage drop) the higher the
measured Q of the coil.
These measurements provide pretty good frequency and Q data on both
the tank circuit and the secondary coil from the same basic setup
using a frequency generator, O'scope, and 1K resistor. I routinely
add an additional feedback line going to a frequency counter for
more precise data.
Hope this explanation helps!
Richard Quick
... If all else fails... Throw another megavolt across it!
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