On May 29, 7:21 am, Rob wrote:
I have several DMMs, including a Fluke 77. Let's say I'm testing a
high voltage, possibly charged cap such as a motor run capacitor. Is
it safe to use the DC voltage measurement function to see if the cap
has been safely discharged? What about on cheaper DMMs, is this
typically possible? Does it make sense to measure the resistance
across the leads of a cheap DMM with another DMM when it is in DC
Voltage mode to see if this would be the case?

Have you tried Google?

On May 29, 1:48 pm, Malissa Baldwin
wrote:
On May 29, 7:21 am, Rob wrote:

I have several DMMs, including a Fluke 77. Let's say I'm testing a
high voltage, possibly charged cap such as a motor run capacitor. Is
it safe to use the DC voltage measurement function to see if the cap
has been safely discharged? What about on cheaper DMMs, is this
typically possible? Does it make sense to measure the resistance
across the leads of a cheap DMM with another DMM when it is in DC
Voltage mode to see if this would be the case?

Have you tried Google?

I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

On May 29, 12:25 pm, Rob wrote:
On May 29, 1:48 pm, Malissa Baldwin
wrote:

On May 29, 7:21 am, Rob wrote:

I have several DMMs, including a Fluke 77. Let's say I'm testing a
high voltage, possibly charged cap such as a motor run capacitor. Is
it safe to use the DC voltage measurement function to see if the cap
has been safely discharged? What about on cheaper DMMs, is this
typically possible? Does it make sense to measure the resistance
across the leads of a cheap DMM with another DMM when it is in DC
Voltage mode to see if this would be the case?

Have you tried Google?

I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

O.K. so if you can find your answer using Google then why post your
question here?

If you do it again I will send a complant to .

On May 29, 3:25 pm, Rob wrote:

Have you tried Google?

I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

The answer you were "looking for", or the actual answer? And since
when is impedance to be taken for resistance?

Luckily, the Flukes should have internal fuses and should *just* be
able to accept a charge from a small electrolytic cap without
permanent damage. Capacitors will deliver a near-infinite amount of
*current* for a very brief period... the bigger the cap, the ever-
closer-to-infinite-current it will deliver. Can you say OUCH!!

I am not quite sure what your point is after all. Are you attempting
to test the quality of the cap? The voltage in the circuit? Both?
Neither? If you are attempting to test a cap, you need a cap tester.
Preferably one that will test the cap at full operating voltage. After
that, an ESR meter. Short of this, a VOM will only just barely test a
cap, and only after full discharge and only then on the Ohms setting
for internal resistance, and only then to an _extremely_ limited
degree with results that are only just better than none at all.

I keep a very nice Fluke with an internal capacitance checker... this
measures capacity only with leakage (on electrolytics) tending to show
up as excess capacitance. In the field, it is better than nothing, and
it is very useful testing new caps when precision values are required.
But when I am testing electrolytics, a meter that will test at full
voltage is the only way to fly.

Peter Wieck
Wyncote, PA

Rob wrote:
On May 29, 1:48 pm, Malissa Baldwin
wrote:

On May 29, 7:21 am, Rob wrote:


I have several DMMs, including a Fluke 77. Let's say I'm testing a
high voltage, possibly charged cap such as a motor run capacitor. Is
it safe to use the DC voltage measurement function to see if the cap
has been safely discharged? What about on cheaper DMMs, is this
typically possible? Does it make sense to measure the resistance
across the leads of a cheap DMM with another DMM when it is in DC
Voltage mode to see if this would be the case?

Have you tried Google?


I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

by standard, they have 10 Meg ohms ..


--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Jamie t wrote in
:

Rob wrote:
On May 29, 1:48 pm, Malissa Baldwin
wrote:

On May 29, 7:21 am, Rob wrote:


I have several DMMs, including a Fluke 77. Let's say I'm testing a
high voltage, possibly charged cap such as a motor run capacitor. Is
it safe to use the DC voltage measurement function to see if the cap
has been safely discharged? What about on cheaper DMMs, is this
typically possible? Does it make sense to measure the resistance
across the leads of a cheap DMM with another DMM when it is in DC
Voltage mode to see if this would be the case?

Have you tried Google?


I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

by standard, they have 10 Meg ohms ..



Not necessarily;the cheapo Harbor Freight DMM I have is only 1 MegR,and
I've seen a 22 MegR DMM.

but even 1 MegR would not discharge a cap very fast.

--
Jim Yanik
jyanik
at
kua.net