Capacitor complacency

[farmerjack61]

Hello there I am a Newb but have a valuable tip for all experienced electro-junkies.

Dont let your standards slip, because it could cost you a new ESR or Capacitance Meter.

I purchased some Rubycon 10mF/63 Caps from China for a dollar each, they arrived today, and I was just bustin' to test them out.

I recall building my ESR meter a few years back and they repeatedly warned me about discharging caps before testing - Sage-like advice I would say. To that end I made a jigger for discharging them slowly - an alligator lead cut in half with a 150r/5w resistor soldered into the middle.

I begrudgingly dragged it out of the draw today, and wow! what a surprise I got as the sparks flew and a little crack went off just like a mini-bolt of lightning. I had accidentaly shorted the terminals with my discharge lead.

ASSUME NOTHING AND PREPARE TO BE SURPRISED.

I have no idea how they became charged as they have been in transit for about two weeks.

They were packaged quite well in plastic bag with little blocks of styrene corking the sharp terminals.

I suspect it had something to do with the plastic bag and the styrene rubbing together.

Styrene foam has some interesting properties and does react to magnetic fields in some circumstances, for example; I reached across my bench recently and as my hand hovered over a little block of styrene, it began to spin. The phenomena blew me and a mate away and we spent the afternoon pretending we were Uncle Martin. (Uncle Martin was an Alien in an old TV series from the seventies. Uncle Martin could mysteriously move objects by simply pointing at them - great show). Further to this we worked out that if we pointed at the foam particle it would spin one way for the 'pointer' and another for the thumb.

You work it out, but I have told a hundred people about this, and not one believes me, to add to my frustration I have not been able to repeat the experiment with any success. BUT IT DID HAPPEN.

Krikeys, I sound like one of those people that just saw something strange in the sky - but that's another story. Watch this space!

BTW. All caps measured within 5% of rated capacity and were very low ESR of approximately 0.02 ohms, so I would suggest you get onto the auction site if you need any caps and are prepared to use a part that is in the lower capacity region - but all parts were above 9.5mF, so that's good enough for a power tank.

Bye for now, Phil E.

[Mooly]

I think what you are describing is an effect called dielectric absorbtion.

In my college days I noted that electroylitic caps that had been in use and then discharged fully seemed to recover some charge back over time. My lecturer at the time dismissed it and looking back seemed unaware of this effect.

It's very real though and particularly caps that have been in use for an extended period of time exhibit this. Reservoir caps in switch mode supplies can "charge" back to many tens of volts just sat on the shelf.

I must admit with new caps I have never encountered this at all and so your experience in a way sets little alarm bells off.

Are you sure the caps are genuine ?

[farmerjack61]

Originally Posted by Mooly

Are you sure the caps are genuine ?

Hi there Mool's thankyou for that info. It just goes to show that you really have to be carefull, and I know what you mean about those blasted smps caps. Ouch

I am almost 100% certain that they are rejects from the factory. I am hoping they were rejected because they were all a little bit on the low side.

Wanna play Rhoulette?

L8ER Dude!

[davidsrsb]

Not so bad at 63V, but you have to be really careful with valve ht caps. Sometimes the amplifier designer forgets the discharge safety resistor

[farmerjack61]

Originally Posted by davidsrsb

Not so bad at 63V, but you have to be really careful with valve ht caps. Sometimes the amplifier designer forgets the discharge safety resistor

Hello Dave, It's nice to meet another enthusiast from over this side of the globe - hope it's reasonable weather over there today.

My brother Mark, loves the old valve amps and specialises in those used for Guitar players. It is an interesting hobby isn't it. I am going to look up the discharge resistor, as I haven't seen it used - maybe as I get a bit more experience, the opportunity to open up a valve amp will arise.

Have a good day and enjoy the warmth in Kuala Lumpur. I envy you because we have had 25-50mm of rain per day for almost a week down here. I know you get some big rain up there on the peninsula too, but here in Tasmania it is quite cold (freezing) when it rains. I think I may be part-reptile because I am always craving a warmer environment.

Cheers and bye for now, Phil E.

[AndrewT]

I have found that all electrolytics recharge themselves after a full or patrtial discharge.

I usually discharge for 10seconds or so and go through the small collection. Then start again for a further 5seconds and repeat a third time for a further 5seconds.

I have found that three short discharges does a far better job than a longer discharge of the same total time.

But, there is some chemistry going on. The electros will still recharge to some extent.

Why did they arrive with a part charge?
There could be at least two reasons.

1.) the manufacturer may have "tested" a few samples to ensure compliance with specification. That "testing" requires the DUT to be reformed before Test and they have a time limit after which the reform must be done again.

2.) the manufacturer may reform all caps to a minimum proportion of the guaranteed voltage rating to minimise warranty returns for the few that "blow up" during their first fast charge.

[john curl]

This is NORMAL for a higher voltage cap that is electrolytic. It CAN be dangerous IF you make too good a connection with something flammable. There is nothing wrong with the caps, however. All electrolytic caps do it, and that is why very high voltage caps, much greater than 200V should come with a shorting wire when new.
The mechanism should be easy to understand. First, the cap has to be FORMED. This means that a voltage greater than the rated voltage has to be on the cap for a certain amount of time, then it is discharged, probably through a resistor, to keep the peak current to a minimum, then sent out to the outside world. The residual charge that remains in the dielectric develops a voltage over time across the output terminals, usually only a few volts. Accidently shorting it can give sparks, and even weld the screwdriver at the contact point. If in doubt, FIRST measure the voltage across the new cap with a voltmeter, and if something significant is there, find an appropriate value resistor, like an 8 ohm dummy load for a loudspeaker to discharge it with.

[farmerjack61]

Hello John,

So it's not some strange kind of Voodo. BTW We have a suburb named after you in Sydney - Curl Curl

Cheers and thanks-Phil

[Mooly]

Originally Posted by john curl

This is NORMAL for a higher voltage cap that is electrolytic..............

Strange to say in all my years in the repair trade I never encountered a new cap (and we are talking values up to 470uF/450 volt used as mains reservoir caps) that had noticeable charge on it. And hundreds of such caps and many thousands of "lesser" electrolytics caps have passed through my hands as I both ordered them, and unpacked them, and restocked them, and worked with them. No colleagues had ever remarked on this either.

However take a cap out of a piece of equipment that had seen service for weeks/months/years and discharge and remove and put that cap on the shelf and yes, it could recover to maybe 60/70/100 volts even, and then bite the unwary. Here, catch

New caps from reputable suppliers... never in my experience.

Edit... just a thought. Do all new caps have to be formed at the manufacturing stage ? or can they be sent out unformed accepting that maybe an odd one may suffer when voltage is first applied. Are you paying a premium for caps that have been "fully formed" and tested in other words.

[CBS240]

I found out through a bit of pain that when the chasis circuit is removed for repair, CRTs will pick up anode charge if the plug is not grounded.(zapp!)

I suppose you could consider a CRT a 'capacitor' as well.