Megger on big caps - UK Vintage Radio Repair and Restoration Discussion Forum

HoverJohn · 15th Nov 2021, 5:31 pm

Ok so I’m using a cheap megger on new caps I just bought to test them and now realise that it’s taking ages to charge the cap up anyone else had this - it’s a 60uf electrolytic at 250v
Although it’s a 470v one if I bung in 500v then drop it back it shows open circuit - much better….

Not ideal

Now I have to discharge it down via my multimeters 10k voltage resistors….

Bazz4CQJ · 15th Nov 2021, 6:12 pm

Meggers are really intended to test insulation, and track down insulation failures. A megger can be used to assess types of capacitors with low capacitance values and low leakage, such as the old paper caps and modern film caps. But for electrolytic caps, with high values of capacitance and higher leakage rates than the other types, a megger is not well-suited to this task.

B

high_vacuum_house · 15th Nov 2021, 6:27 pm

Be very careful with a Megger and a high value capacitor. You can easily charge it up and can store enough energy to give a bad shock or quite a crack and a flash if shorted.

The storage of energy in a capacitor is calculated using the equation Joules = 1/2CV squared where C is capacitance and V is voltage.

As said Meggers are best for testing low value wax paper and Hunts type capacitors of 0.1 uF and lower. The readings will me more meaningful as you are not using too much energy from the Megger to charge up the capacitor.

Christopher Capener

PJL · 15th Nov 2021, 6:33 pm

Not sure a Megger is suitable for any type of capacitor testing as it will be applying a voltage that is likely to be well over the capacitor rated maximum voltage which might cause the insulation to breakdown.

Bazz4CQJ · 15th Nov 2021, 9:12 pm

Quote:

Originally Posted by PJL

Not sure a Megger is suitable for any type of capacitor testing as it will be applying a voltage that is likely to be well over the capacitor rated maximum voltage which might cause the insulation to breakdown.

As the OP states we are talking about a "cheap megger", I think we are using the name in the generic sense. My Comark Insulation Tester ("megger") has ranges from 25, 50, 100, 250, 500 and 1000volts, so can be used for paper and film caps.

B

HoverJohn · 15th Nov 2021, 9:59 pm

Ok cheers guys it only has 250,500 & 1000v and for less than 20 quid off eBay I can’t fault it and was good at condemning the small paper caps. I just like testing the new ones going in on the HV side (new grid couplers etc) it’s been quite a good sense check on the volt meter as it reads 10 k resistance and the volt meter checks the megger to within a few volts of what it states.

As for fully charged brand new 60/100uf caps I’ve been pulling the leads off the megger and popping them on the volt meter and Leaving it to discharge slowly.

Diabolical Artificer · 16th Nov 2021, 7:19 am

To test electrolytics I charge them through an ammeter using either a current limited power supply or voltage doubler powered through my isolation tfmr and variac, again current limited, but the resistor to limit current has to be chosen carefully, no good trying to charge a 2200u cap through a 1M R. You'll be there all day.

After the cap has charged I should be linear to V, EG 2.5uA @ 250v, if it isn't then yon cap is suspect. Another way is to apply non regulated DC then measure ripple.

Andy.

stuarth · 16th Nov 2021, 11:05 am

You can use a low impedance voltmeter as both the limiting resistor and current/voltage monitor to avoid needing resistors “hanging in the wiring”.

For electrolytic reforming, an AVO 40/47 is 6mA FSD on its voltage ranges with a resistance of 20k or 80k on its 120V or 480V ranges respectively. You can use the same meter to gently but quickly discharge the capacitor after test. An AVO 7 could be used, but its 2mA FSD maybe too close to the leakage current of some electrolytics.

Stuart

Steve G4WCS · 16th Nov 2021, 12:06 pm

slightly off topic. our electrical training instructor had a big metal tin containing a 1uf 1000v capacitor. the lid was insulated from the base with a bit of tape, and the cap hidden inside was connected across the lid and tin. when we were off section, usually for health and safety lectures or similar, he would charge the cap with a megger, and upon our return, with would be thrown at one of the unsuspecting apprentices to catch. I don't suppose any of that carry on would be allowed nowadays

Station X · 17th Nov 2021, 8:29 am

Quote:

Originally Posted by Steve G4WCS

slightly off topic.

Back on topic please.

trobbins · 17th Nov 2021, 12:36 pm

The digital insulation resistance meter I have is by design current limited to 1.4mA, with typical 100, 250, 500 and 1kV ranges. I have used it for poly's up to 10uF.

But for e-caps imho it is best to use a variable dc supply through a 1k resistor and to measure the leakage current at rated working voltage, as that is the key performance parameter (which is not directly read from a megger, but can be interpreted).

A clip on voltmeter across the 1k shows the charging current, which settles to the nominal leakage current. 1k is often recommended by cap manufacturers for periodic maintenance reforming. The recharge/leakage current characteristic is a good indicator of cap performance in circuit, especially if you are using e-caps in series with balancing resistors to swamp out that leakage current, and if a datasheet is available then the leakage current parameter can be compared to measured.

15th Nov 2021, 5:31 pm	#1
HoverJohn Pentode Join Date: Oct 2021 Location: Newbury, Berkshire, UK. Posts: 132	Megger on big caps Ok so I’m using a cheap megger on new caps I just bought to test them and now realise that it’s taking ages to charge the cap up anyone else had this - it’s a 60uf electrolytic at 250v Although it’s a 470v one if I bung in 500v then drop it back it shows open circuit - much better…. Not ideal Now I have to discharge it down via my multimeters 10k voltage resistors….

15th Nov 2021, 6:12 pm	#2
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: Megger on big caps Meggers are really intended to test insulation, and track down insulation failures. A megger can be used to assess types of capacitors with low capacitance values and low leakage, such as the old paper caps and modern film caps. But for electrolytic caps, with high values of capacitance and higher leakage rates than the other types, a megger is not well-suited to this task. B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

15th Nov 2021, 6:27 pm	#3
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,935	Re: Megger on big caps Be very careful with a Megger and a high value capacitor. You can easily charge it up and can store enough energy to give a bad shock or quite a crack and a flash if shorted. The storage of energy in a capacitor is calculated using the equation Joules = 1/2CV squared where C is capacitance and V is voltage. As said Meggers are best for testing low value wax paper and Hunts type capacitors of 0.1 uF and lower. The readings will me more meaningful as you are not using too much energy from the Megger to charge up the capacitor. Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television

15th Nov 2021, 6:33 pm	#4
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Megger on big caps Not sure a Megger is suitable for any type of capacitor testing as it will be applying a voltage that is likely to be well over the capacitor rated maximum voltage which might cause the insulation to breakdown.

15th Nov 2021, 9:59 pm	#6
HoverJohn Pentode Join Date: Oct 2021 Location: Newbury, Berkshire, UK. Posts: 132	Re: Megger on big caps Ok cheers guys it only has 250,500 & 1000v and for less than 20 quid off eBay I can’t fault it and was good at condemning the small paper caps. I just like testing the new ones going in on the HV side (new grid couplers etc) it’s been quite a good sense check on the volt meter as it reads 10 k resistance and the volt meter checks the megger to within a few volts of what it states. As for fully charged brand new 60/100uf caps I’ve been pulling the leads off the megger and popping them on the volt meter and Leaving it to discharge slowly.

16th Nov 2021, 7:19 am	#7
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	Re: Megger on big caps To test electrolytics I charge them through an ammeter using either a current limited power supply or voltage doubler powered through my isolation tfmr and variac, again current limited, but the resistor to limit current has to be chosen carefully, no good trying to charge a 2200u cap through a 1M R. You'll be there all day. After the cap has charged I should be linear to V, EG 2.5uA @ 250v, if it isn't then yon cap is suspect. Another way is to apply non regulated DC then measure ripple. Andy. __________________ Curiosity hasn't killed this cat...so far.

16th Nov 2021, 11:05 am	#8
stuarth Heptode Join Date: Feb 2010 Location: Heysham, Lancashire, UK. Posts: 669	Re: Megger on big caps You can use a low impedance voltmeter as both the limiting resistor and current/voltage monitor to avoid needing resistors “hanging in the wiring”. For electrolytic reforming, an AVO 40/47 is 6mA FSD on its voltage ranges with a resistance of 20k or 80k on its 120V or 480V ranges respectively. You can use the same meter to gently but quickly discharge the capacitor after test. An AVO 7 could be used, but its 2mA FSD maybe too close to the leakage current of some electrolytics. Stuart

16th Nov 2021, 12:06 pm	#9
Steve G4WCS Heptode Join Date: Sep 2004 Location: Preston, Lancashire, UK. Posts: 632	Re: Megger on big caps slightly off topic. our electrical training instructor had a big metal tin containing a 1uf 1000v capacitor. the lid was insulated from the base with a bit of tape, and the cap hidden inside was connected across the lid and tin. when we were off section, usually for health and safety lectures or similar, he would charge the cap with a megger, and upon our return, with would be thrown at one of the unsuspecting apprentices to catch. I don't suppose any of that carry on would be allowed nowadays

17th Nov 2021, 12:36 pm	#11
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Megger on big caps The digital insulation resistance meter I have is by design current limited to 1.4mA, with typical 100, 250, 500 and 1kV ranges. I have used it for poly's up to 10uF. But for e-caps imho it is best to use a variable dc supply through a 1k resistor and to measure the leakage current at rated working voltage, as that is the key performance parameter (which is not directly read from a megger, but can be interpreted). A clip on voltmeter across the 1k shows the charging current, which settles to the nominal leakage current. 1k is often recommended by cap manufacturers for periodic maintenance reforming. The recharge/leakage current characteristic is a good indicator of cap performance in circuit, especially if you are using e-caps in series with balancing resistors to swamp out that leakage current, and if a datasheet is available then the leakage current parameter can be compared to measured.