Capacitor testing

[John]

Hello.
I have a question on capacitor testing (plenty on the web about caps but not
basic testing).
You have no access to a capacity meter, you only have a DVM, plenty of
bulbs and some batteries.
I know that using a moving coil meter you just select resistance, as the
capacitor charges the meter movement moves.
How can I test a cap with a DVM (no cap range) to indicate that it is
functional.
Thanks.

 

[CFoley1064]

Subject: Capacitor testing
From: "John" reeferjon@msn.com
Date: 4/1/2004 8:03 AM Central Standard Time
Message-id: <2jVac.6766$Id.3497@news-binary.blueyonder.co.uk

Hello.
I have a question on capacitor testing (plenty on the web about caps but not
basic testing).
You have no access to a capacity meter, you only have a DVM, plenty of
bulbs and some batteries.
I know that using a moving coil meter you just select resistance, as the
capacitor charges the meter movement moves.
How can I test a cap with a DVM (no cap range) to indicate that it is
functional.
Thanks.

Hi, John. DVMs measure resistance the same way the older Simpsons did -- put a
current across the resistor, then measure the voltage across it.

You can do a quick cap check for larger value capacitors the same way -- set it
to measure the highest resistance range, or use the range hold button to hold
it in a meg-ohm measurement mode, and just attach the leads to the cap (red to
cap +, black to cap -). You should see the reading on the DVM gradually go up
from zero to full scale. Especially for the cheapie DVMs, you might want to
make sure the cap is discharged before you test.

If your cap isn't tantalum, you also might want to try reversing the leads, and
seeing the reading go from negative overload up through zero and then to
positive overload.

If you don't have a range hold button but do have autorange, scrounge a
high-value resistor at the lowest end of your highest resistance range (say, if
your highest range is 20 Meg, get a 2.2 Meg resistor), and charge the cap up
with the resistor in series. That will keep the autorange from shifting down
(and also increasing the current across the DUT). This will also prevent the
meter from starting at zero, but you will still see the reading gradually rise.
You can't do the lead reversing thing here, though.

Note that this is only a very basic test to see if the cap is dissipating
current slower than the DVM is sourcing it -- I'd expect a little more from a
cap test. Also, for smaller value caps, it won't tell you if the cap is open
instead of shorted, which frequently happens. The voltage across the cap rises
too fast to distinguish between the two.

Good luck
Chris

 

[John]

Thanks
"CFoley1064" <cfoley1064@aol.com> wrote in message
news:20040401152444.29433.00000495@mb-m11.aol.com...

Subject: Capacitor testing
From: "John" reeferjon@msn.com
Date: 4/1/2004 8:03 AM Central Standard Time
Message-id: <2jVac.6766$Id.3497@news-binary.blueyonder.co.uk

Hello.
I have a question on capacitor testing (plenty on the web about caps but
not
basic testing).
You have no access to a capacity meter, you only have a DVM, plenty of
bulbs and some batteries.
I know that using a moving coil meter you just select resistance, as the
capacitor charges the meter movement moves.
How can I test a cap with a DVM (no cap range) to indicate that it is
functional.
Thanks.

Hi, John. DVMs measure resistance the same way the older Simpsons did --
put a
current across the resistor, then measure the voltage across it.

You can do a quick cap check for larger value capacitors the same way --
set it
to measure the highest resistance range, or use the range hold button to
hold
it in a meg-ohm measurement mode, and just attach the leads to the cap
(red to
cap +, black to cap -). You should see the reading on the DVM gradually
go up
from zero to full scale. Especially for the cheapie DVMs, you might want
to
make sure the cap is discharged before you test.

If your cap isn't tantalum, you also might want to try reversing the
leads, and
seeing the reading go from negative overload up through zero and then to
positive overload.

If you don't have a range hold button but do have autorange, scrounge a
high-value resistor at the lowest end of your highest resistance range
(say, if
your highest range is 20 Meg, get a 2.2 Meg resistor), and charge the cap
up
with the resistor in series. That will keep the autorange from shifting
down
(and also increasing the current across the DUT). This will also prevent
the
meter from starting at zero, but you will still see the reading gradually
rise.
You can't do the lead reversing thing here, though.

Note that this is only a very basic test to see if the cap is dissipating
current slower than the DVM is sourcing it -- I'd expect a little more
from a
cap test. Also, for smaller value caps, it won't tell you if the cap is
open
instead of shorted, which frequently happens. The voltage across the cap
rises
too fast to distinguish between the two.

Good luck
Chris

 

[Bob Masta]

On Thu, 1 Apr 2004 15:03:43 +0100, "John" <reeferjon@msn.com> wrote:

Hello.
I have a question on capacitor testing (plenty on the web about caps but not
basic testing).
You have no access to a capacity meter, you only have a DVM, plenty of
bulbs and some batteries.
I know that using a moving coil meter you just select resistance, as the
capacitor charges the meter movement moves.
How can I test a cap with a DVM (no cap range) to indicate that it is
functional.
Thanks.

Build a simple test circuit. If you build an
oscillator with the frequency controlled by the
cap, then you can measure frequency to
determine capacitance. How to do that
with a DMM? You need a frequency-to-voltage
converter. A simple F-V is just a one-shot
multivibrator (made from a logic gate) followed
by a simple RC filter to smooth the result.

Or, use the cap in a capacitive voltage
divider driven by an AC source (such as
the 60 Hz mains isolated by a low-voltage
transformer such as the common AC-out
wall-warts). The impedance of the cap
can thus be compared with a known cap.
If you have the standard cheapie DMM,
it has no sensitive AC ranges so you
will need to have a little AC-to-DC
converter ahead of a sensitive DC range.
You can make this with a couple of op-amps
and diodes.

If you do that, you can extend it even
futher to a Wheatstone bridge, where
you compare the output of the capacitive
divider with an adjustable voltage from
a potentiometer. When the voltage goes
to zero you read the pot and compute the
effective impedance of the cap.

But heck, if you are going to have to build
things like this, you may as well build a
real capacitance meter!


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com