Re: Measuring Q of capacitors
Subject: Re: Measuring Q of capacitors
From: mconway-at-deepthnk.kiwi.gen.nz (Mark Conway)
Date: 26 Jan 1995 02:49:38 GMT
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Just a couple of wee questions about Tesla tank capacitors.
Mark Graalman and Richard Quick were giving very good advice concerning
measuring the Q of a capacitor:
MG>provide a least a 1v p-p output, and a VTVM or other hi-z meter with a RF
probe, or even a scope. You can get an idea about the cap by checking for any
low frequency series resonant points say less than 1 Mhz. Take the generator
and hook the capacitor to its output using short leads and a 1k ohm resistor
in series with the capacitor. Hook your meter or scope across the 1k ohm
resistor, now slowly sweep the generator from approximately 50Khz to about 3
or 4 Mhz. looking for any peaks in the voltage across the 1k ohm resistor.
All capacitors have a series resonant point due to internal inductance, and
operating the capacitor at or close to this point can set up heavy parasitic
currents within the cap.
Why do you need to go up to 1 Mhz? Most coils I have seen or heard about seem
to resonate about 120 kHz or so. Will there be significant currents in a
Tesla tank circuit up to about 1 MHz if the tank circuit is resonating at say
120 kHz? What I'm trying to say is that if say I make a cap and it has a
significant resonant peak say at 800 kHz does it mean that this cap would be
useless in a tank circuit that resonates at say 100kHz? Is the 1 MHz limit a
rule of thumb about how close the resonant peak can be before it adversely
affects coil performance?
RQ>This is one area where a scope with a frequency generator leaves no doubt.
The capacitor can be placed in the tank circuit and grid peaked right at the
operation frequency of the coil system. The height of the grid peak can be
directly compared with other capacitors swapped in for just this purpose.
This gives specific and meaningful Q values if the setup is not varied
between swapping capacitors.
By grid peaked do you mean that the tap on the primary is adjusted until the
circuit is resonating and so your oscilloscope measures a large voltage drop
across the 1K resistor because there is a large current thru it? I guess that
when you change capacitors you retune the primary until resonance is again
reached and you compare the voltage you measure now with the one previously
gotten. The higher Q capacitor will have the highest voltage drop across the
_/_/_/ _/_/_/_/ Mark Conway
_/ _/ _/ Deep Thought BBS, Auckland, New Zealand
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_/_/_/ _/ Internet: mconway-at-deepthnk.kiwi.gen.nz