how to check 400v capacitor whith analog meter

how to check 400v capacitor whith analog meter is breakdown or no

 

If it has broken down and is now shorted or leaky, an ohmmeter will read a
low resistance value and that indicates a bad capacitor. If it is an
electrolytic type, hundreds of thousands of ohms would be normal and if it
is a non-polarized type, infinity ohms would be normal. Give the capacitor
time to charge up (the resistance will keep increasing with a good
capacitor).

 

Alert: Dont measure such a cap unless you are 100% sure the cap is
discharged. Otherwise, you run a high risk to damage your ohm-meter because
it doesn't endure any input voltage.

If you are using an analog ohm-meter, you should see the indicator pin rise
slowly until it stops at the top. That usualy means there is no leak in the
cap.

 

NOT TO 400V... make sure the cap is discharged before doing this
measurement. Set your analog meter on resistance (ohms) and measure across
the terminals. Note that an electrolytic cap will have a very low
resistance if you measure it the wrong way, so make sure the red lead is
on positive, and the black is on negative.

 

On Wed, 21 Dec 2005 12:00:48 -0600,

---
Like this: (view in Courier)


+--------------+
| +|-----[51K]---[51K]--->>--+--<<----+
| 0 - 400VDC | |+ |+
| VARIABLE | [CAP] [VOLTMETER]
| POWER SUPPLY | | |
| -|---[-MILLIAMMETER+]-->>--+--<<----+
+--------------+

For safety, just in case the cap fails shorted when you're testing
it, use two 51K 1 watt resistors.

With the supply cranked all the way down to zero volts, connect
everything up as shown and slowly start increasing the voltage out
of the supply. You'll notice that as you increase the voltage the
milliammeter will read some current which will then drop off to some
low value. Continue increasing the voltage until you get to 396
volts, then _carefully_ disconnect the voltmeter. At that point the
current indicated by the milliammeter will be the leakage current of
the cap at 400VDC, and if it's greater than specified in the
manufacturer's data sheet for that cap, it's bad.

If it's good, or if it's bad but hasn't blown up and is still
holding a charge, you now have a problem, which is what to do with
that charged cap to make it safe.

What I'd do would be to reconnect the voltmeter and disconnect the
milliammeter from the supply while the supply was hot, then turn off
the supply and discharge the cap across the two 51K resistors. If
it's a BFC it could take a while, so watch the voltmeter until the
voltage across the cap decays to the point where it's safe to
handle. Also, be aware that after you disconnect the cap from the
reistors the voltage across the discharged cap can rise because of
dielectric absorption, so watch out for that nasty surprise.

 

PS...

Since your voltmeter has an internal resistance, that means there
will always be current flowing in the circuit which won't be caused
by the capacitor's leakage current.

Assuming a meter with an input resistance of 10 megohms, that meter
current will be:

E METER VOLTS METER VOLTS
I = --- = ------------ = ----------------- = 10µA/100V
R METER R 10 000 000 ohms


So, if at any time during the charging process the current indicated
by the milliammeter fails to decrease to <= the sum of the meter
current and the capacitor leakage current allowed at that voltage,
then you've got a bad cap.

Also, I just noticed that this is going to seb, so be VERY, VERY,
CAREFUL and keep your wits about you when the supply voltage gets
above about five volts.

Remember 400V can kill you.