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Re: Measuring Capacitance
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> From: Tesla List <tesla-at-poodle.pupman-dot-com>
> To: Tesla-list-subscribers-at-poodle.pupman-dot-com
> Subject: Re: Measuring Capacitance
> Date: Sunday, February 02, 1997 8:25 PM
>
> Subscriber: sfalco-at-worldnet.att-dot-net Sun Feb 2 17:39:56 1997
> Date: Sun, 02 Feb 1997 10:12:16 -0500
> From: Steve Falco <sfalco-at-worldnet.att-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: Measuring Capacitance
>
> > By the way, methods #4 and #5 can both be checked for error very
> > simply: After measuring the voltages across each component, add these
> > values together and compare them to the actual measured voltage across
> > BOTH components. If they don't add up within a few percent, then your
> > meter is loading down the circuit.
>
> I think this error check works for method #5 but not for method #4.
> When I measured a 550k resistor and a 4500 pF capacitor in series across
> 120v -at- 60Hz, I read roughly 0.8 volts across each component. I believe
> that this is due to the different phase shifts across the resistor and
> capacitor in circuit #4.
>
WHOA! .8 volts? This isn't a phase problem, this is a current problem.
The frequency is too low for this size capacitor! Your meter is loading
down the circuit because it is gobbling up all the current. The current
THROUGH the two elements must be MUCH greater than the current diverted to
the meter or you will get just such weird results! It is NOT a phase
problem but a current problem. What kind of meter are you using? Most DVMs
use a 10 Megohm input impedance. At 100 volts that corresponds to 10
microamps of diverted current. For example, if you were to measure the
voltage across two 10 meg resistors in series with 100 volts you would get
a reading of 33 volts instead of the actual 50 volts. Your very low
readings tell me you are NOT using a DVM. Your meter's impedance is
gobbling up all the available current and giving you really gross errors.
Always try to make the current flow THROUGH at least 10 times what you are
diverting. And if you are trying to measure the voltages with a D'arsonval
type meter, be aware that they usually have a rating of about 10K/volt. In
other words, it diverts about 100 microamps for a full scale deflection.
Now do you see why you get readings of .8 volts instead of 60 volts?
Use a DVM, and if the two don't add up to the total, then use a sine wave
generator and go for a higher frequency. Just make sure the meter is
designed for the frequency you use. And don't use a D'arsonval meter! That
sucker adds inductance into the circuit!
> As another example, the book I got this technique from had a 4k resistor
> and 0.7 uF capacitor in series across 115 volts, with Vr = 80 and Vc =
> 100. Thus the voltages may add up to less than the line voltage, or
> more than the line voltage, again because of the phase shifts.
>
The author made a boo boo. Trust me on this one. If this was a resistor,
coil and a capacitor circuit, then phase effects are definitely involved.
But NOT with just a resistor and a capacitor. Try using a high impedance
DVM and see what a difference you get. I bet the author was using a meter
that added inductance to the circuit. Thus when he made the measurements he
was actually changing the circuit from a RC circuit to an LCR circuit,
which is a different type of animal entirely.
> Steve Falco
> sfalco-at-worldnet.att-dot-net