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Did you get my mail? (fwd)
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Chip Atkinson
http://bhs.broo.k12.wv.us/homepage/chip/info.htm
--- Tighten it 'till it strips and back off half a turn ---
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---------- Forwarded message ----------
Date: Thu, 30 Jan 1997 20:56:02 -0500
From: Steve Falco <sfalco-at-worldnet.att-dot-net>
To: chip-at-poodle.pupman-dot-com
Subject: Did you get my mail?
Hi - I sent the following a few days ago but did not see it on the list,
so I wonder if my mail is getting through? Is this the first time you
have gotten this or am I just being too impatient?
Steve
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> Does anyone have a circuit
> for a capacitor tester that can be built from parts?
I'm in a teaching mood I guess, so here is a rather lengthy answer to
the question. [ Chip - I hope I don't stray too far into EE theory. ]
There are a number of ways to measure capacitance. The last one I give
is probably the easiest, but read them all and see what appeals to you:
1) You can measure the RC time constant. I.e., charge your cap through
a resistor, and note how long it takes the voltage to build from 0 volts
to 63% of the battery voltage. Then T = R x C where T is the time in
seconds, R is resistance in ohms, and C is capacitance in Farads. A
circuit might look like this:
----------- Switch ------ Resistor --------------------
| | |
Battery capacitor voltmeter
| | |
-------------------------------------------------------
Start with the switch open, and the voltmeter showing 0 volts (short out
the cap for a few seconds to discharge it. Then close the switch, start
your stopwatch, and wait till the voltmeter hits battery voltage x 0.63.
Sadly, this works pretty well for large capacitors like electrolytics,
because you can do the timing with a watch. But for small capacitors
such as are used in tesla coils, you need to use a storage oscilloscope
or electronic timer/counter to measure the time 'cause it will be in
milliseconds or even microseconds.
And don't think you can get around this by making the resistor large,
like maybe a few hundred megohms, because then the leakage in your
voltmeter or o'scope will bias the readings. (The resistor must be no
more than say 10% of the impedence of your test equipment.)
2) You can also build a simple op-amp or transistor oscillator (sorry,
this exceeds my ascii art capabilities but you can find a circuit in a
hobby electronics book :), and measure the frequency of oscillation with
your home-made capacitor, then substitute known capacitors in the
circuit until you get about the same frequency. Then your capacitor and
the known capacitor are roughly the same value. (2% caps are pretty
cheap in the lower voltages and make good references.) This can be
pretty accurate, and works for the values used in Tesla coils, but you
either need a frequency counter or a good ear to estimate the
frequencies.
3) The "Guide to Electronic Measurements and Laboratory Practice"
(Stanley Wolf, Prentice-Hall, 1973) says the most accurate way is with a
bridge circuit:
--------------------------------
| | |
| R1 R2
| | |
| | |
AC generator |--- meter ---|
| | |
| | |
| R3 R?
| | |
| C3 C?
| | |
--------------------------------
The equations are not too bad:
R? = (R2 x R3) / R1
C? = C3 x (R1 / R2)
where R1, R2, R3, C3 are permanent parts of the bridge meter, and R? and
C? are your unknown capacitor. (That R? is confusing, but it represents
the loss or leakage in your capacitor. If you have a good capacitor, it
will be pretty close to 0.)
Anyway, we make R1 and R3 adjustable potentiometers, and play around
with them until the meter says we have balanced the bridge. Then we
disconnect the potentiometers and measure the values they were set to
with our ohm-meter, do a little math, and we know our capacitor value.
4) Easiest way: you can make a voltage divider:
|------------------------------
| |
| resistor
| |
120 volt AC (wall plug) |
| |
| capacitor
| |
|------------------------------
Measure the voltage across the resistor and the capacitor. Don't get
zapped - you are playing with line voltages here and there is not much
between you and eternity.
C = (Vr / R) / (2 x pi x f x Vc)
where Vr is the voltage across the resistor, R is the resistor in ohms,
pi is 3.141, f is 60 Hertz (or 50 in some countries - you know who you
are), and Vc is the voltage across the capacitor.
So there you are: measure two voltages, do a little math, and get a
result accurate to maybe 10%. You need a fairly high impedence meter to
read the voltages but use what you've got. Also, I'd pick a resistor
such that Vr is somewhere around 50 volts for reasonable accuracy. I.e
you don't want Vr to be 1 volt or 119 volts - get it somewhere in the
middle by chosing a different R.
Happy (and safe) measuring!
Steve Falco
sfalco-at-worldnet.att-dot-net