Maker Pro
Maker Pro

capacitor testing for novice please

poorcol1964

Oct 9, 2018
8
Joined
Oct 9, 2018
Messages
8
hi , i have 3 capacitors i need to test 0.5uf 350-450v ( see photo )
please can someone explain in simple terms as i am new at this kinda thing , thanksP_20181023_224912.jpg
 

Bluejets

Oct 5, 2014
6,087
Joined
Oct 5, 2014
Messages
6,087
There is no capacitor test range on that meter.
Could be wrong but I'd say offhand, testing the paper one would not be necessary..
It's so old it a wonder it's not unravelling.
Edit:- focusing in on your photo (rather difficult) I'd say that goes for all of them.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
Joined
Jan 21, 2010
Messages
25,507
The yellow one is dead. It has high leakage.

Just guessing, but if anyone is offering odds, I'm sure they favour that position.
 

kellys_eye

Jun 25, 2010
5,336
Joined
Jun 25, 2010
Messages
5,336
You need what is called an ESR meter. The Chinese sell a multi-function semi-conductor tester (ostensibly) that also has an ESR function and can be purchased for around £10 - delivered.

But if you are testing for the purpose of re-using then unless you have a specific requirement to re-use those exact parts then we'd recommend you change them for the modern equivalents.

If this is a case of vintage restoration then consider 'stuffing' the old cases with the modern parts.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
Joined
Jan 21, 2010
Messages
25,507
An ESR meter won't measure leakage, and leakage is likely to be the major issue for old high voltage capacitors.
 

duke37

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
You will need a 250V DC power supply (old radio?) and measure the voltage with the capacitor in series with the meter. You should get a reading of zero.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
4,781
Joined
Jun 21, 2012
Messages
4,781
hi , i have 3 capacitors i need to test 0.5uf 350-450v ( see photo )
please can someone explain in simple terms as i am new at this kinda thing , thanksView attachment 43527
A capacitor is constructed by winding into a cylinder two parallel foil sheets, usually aluminum, insulated from each other by a layer of plastic. Conductive leads are then welded to the two aluminum foils and the whole assembly is then potted in plastic. A somewhat different technique is used for mica-insulated and polarized electrolytic capacitors, but the principle is the same: two parallel plates, insulated from each other, form the capacitor.

This means that, ideally, a capacitor presents as an open circuit when DC is applied to the two terminals, because the insulator between the two plates prevents conduction of DC. If a DC voltage is applied and a sustained current occurs, this current is called leakage current. Measure this FIRST before wasting any more time trying to evaluate the capacitor. A leaky capacitor should be destroyed (cutting it in two halves works pretty good) lest it wind up being used by mistake.

Testing a high-voltage rated capacitor for leakage can be risky business, especially for someone not experienced in doing so. Firstly, a good capacitor will store energy (as a charge separation between its plates) for a LOOONG time. Good capacitors with large voltage ratings and large capacitance ratings can be LETHAL when energized! Secondly, a good capacitor doesn't present much leakage current when connected to its working voltage... a few nanoamperes is typical and this tiny current can be difficult to measure. It also occurs after the initial charging current, which can be several amperes, has decayed to nearly zero, so some sort of current range-selection is necessary to avoid damage to the meter measuring the leakage current.

An alternative to measuring leakage current is to infer it by energizing the capacitor with a power supply voltage, removing the voltage, then measuring the voltage across the capacitor after a ten second (or longer) interval. This is not an easy measurement to make accurately because the voltmeter used to measure the capacitor voltage represents a "leakage path" in parallel with the actual leakage path and this will cause the capacitor to immediately begin discharging through the voltmeter when the power supply voltage is removed. You can leave the voltmeter connected to the capacitor terminals and monitor the rate of voltage decay when the power supply voltage is removed, but this may occur too quickly for a small-valued capacitor and/or capacitors energized with small working voltages. Alternatively, you can connect the voltmeter a few seconds after the power supply voltage is removed and quickly note how much the initial energizing voltage has decreased. Again, it can be tricky to obtain accurate readings, but if you practice the technique on a few "known good" capacitors you will learn to distinguish certifiably "bad" capacitors from "probably good" capacitors.

There are other physical parameters that you can measure (besides actual capacitance and leakage resistance), but most of the time a simple verification of the capacitance value and an estimate of the leakage resistance at the working voltage is all that is required to successfully use a capacitor in a real circuit.

All capacitors exhibit an effective resistance in series with their leads, Some of this is caused by the wire used to make connections to the plates and some has a chemical nature, especially when electrolytic capacitors are considered. Effective series resistance, or ESR, can be measured fairly easily with special ohmmeters, but it is generally only a relevant parameter when the capacitor must pass large alternating currents, Transmitter capacitors, used to conduct radio frequency currents, while blocking direct current, need low ESR values to avoid having to dissipate too much RF energy as heat inside the capacitor. High-current DC power supplies subject to a lot of ripple current also benefit from capacitors with low ESR values for the same reason. For most practical circuits you will encounter, ESR is simply a number you don't need to know.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
Joined
Jan 21, 2010
Messages
25,507
Whilst it is often quoted that a high voltage is required for testing leakage, at least one person (with a lifetime collection of faulty capacitors!) has determined that leakage can be measured at low voltage.

Google "Mister Carlson's lab capacitor leakage". The videos and the circuit may still be restricted to his patreon followers, but there are discussions on other sites.

His (tested) conclusion is that leakage is like a high value parallel resistance rather than a lower breakdown voltage.
 

caps

Jul 29, 2018
183
Joined
Jul 29, 2018
Messages
183
Capacitor leakage chart for standard aluminum elektrolytic capacitors.
 

Attachments

  • Leakage.jpg
    Leakage.jpg
    62 KB · Views: 7

hevans1944

Hop - AC8NS
Jun 21, 2012
4,781
Joined
Jun 21, 2012
Messages
4,781
...leakage is like a high value parallel resistance...
Agreed. This conclusion is also supported by the table posted by @caps in his post #11. Nevertheless, I believe it is most useful to test for leakage at the working voltage direct current (WVDC) rating of the capacitor. The one time I rely on a 10 volt leakage test for a 450 WVDC electrolytic capacitor is when Murphy will step forward to make the damn thing blow up in my face when I apply its rated voltage.:mad:
 
Top