Electrolytic caps in series

mentioned to answer the OPs original question/

therein lies the rub. Sometimes thats not so easy to do. And a reduced margin may mean cheaper caps.

You've probably already noticed that good cap manufacturers also have a surge voltage rating. very handy, especially with regenerative loads.

bloody good.

as the caps get bigger, leakage also increases - ISTR proportional to C.

it also doesnt help that the data sheet values can be wildly pessimistic. I know with Hitachi that the quoted lifetime figure is actually the process -3 sigma point, and is about half the average lifetime.

Cheers Terry

Reply to
Terry Given
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One day Phil Allison got dressed and committed to text

Yup, thats the one. The coupling caps were all grey jobs (this is an asian 'knockoff') but as I recall the fenders were like that. Funny how it kept going since all the caps were leaky and you could hear the DC on the pots.

-- Regards ..... Rheilly Phoull

Reply to
Rheilly Phoull

"Glenn Baddeley = top posting bullshit artist

** Correct - then you can expect to get close matching.
** For a cap to do as you suggest, it must be a faulty cap - ie one that is not able to meet maker's specs for max voltage or leakage at rated voltage.

Ballast resistors are no help if a cap is faulty.

** Wrong - WW resistors are only readily available in values up to about 10 kohms - far too low a value for use in parallel with high voltage electros.

Sustained high DC voltages make carbon or metal film resistors go high in value or open circuit - even if used well below their power rating . Composition resistors also go high value with time, heat and the application of high DC voltages.

The point is simple - you are better off without ballast resistors across electros unless they are 100 % reliable.

........... Phil

Reply to
Phil Allison

and the more caps in parallel, the better the matching (variations tend to cancel out)

LHS of bell curve. not terribly likely, but if you use 1,000,000 caps per year, you *will* see some.

bingo. lots of people seem to like throwing money at equipment so that "in the event of a failure, such-and-such happens." In the event of a failure, the costs associated with returning the equipment for repair are often far in excess of the actual replacement parts cost, so why bother.

depends on your definition of "readily available". From a hobbyist perspective, yes. For a real company, no.

whats the mechanism?

yep.

Reply to
Terry Given

** What an asinine and pointless comment - faulty caps get removed from gear at the factory.
** What an asinine and pointless comment - faulty caps get removed from gear at the factory.
** What an asinine and pointless comment - faulty caps get removed from gear at the factory.
** Same one everyone else uses - f*****ad.

From a hobbyist

** What an asinine and pointless comment..
** Just trolling - or you need to know ??

** What a pointless comment.

............ Phil

Reply to
Phil Allison

Hi Phil,

Maybe, but there is a low % of failure rate that will be encountered eventually, especially after 20+ years. I would rather "insure" for that possibility that risk a catastrophic failure and expensive repair.

If one cap has 1mA leakage and another cap has 2mA leakage, the additional voltage across one of the caps may lead to its premature failure.

It depends on the type of fault. If it is an excessive DC leakage current problem, the ballast resistors actually help by reducing the degree of voltage variation across each cap due to variation in leakage.

eg. Design for 450 V across each cap, using 10W resistors.

R = E^2 / P = 450 x 450 / 7.5 = 27 KOhm. These are available in 10W, or use two 12K or 15K 5W in series. I = E / R = 17mA approx. This would protect an excess leakage fault of 5 - 10 mA.

Sorry, I don't agree. Every text book and article I have ever seen uses resistors. eg. The ARRL handbook represents decades of HAM experience, and their HV power supplies for valve transmitters always use them.

Cheers, Glenn. (If you pers>

Reply to
Glenn Baddeley

"Glenn Baddeley ** STOP TOP POSTING !!!!!!!

** What does "maybe " mean ???

Do you see the point or not ??

I'll post it again: " If neither elector cap is faulty - then nothing bad can happen when wired in series and used at around 70% of rated voltage. "

So far - you have simply not addressed this at all.

You have yet to post any sort of case.

** Totally begs the question.
** That is a whole bunch of "ifs" - there.

The caps in question have tested at less than 10 uA of leakage at 70 % of rated voltage.

You are IGNORING facts and just making up numbers that suit you.

** I carefully described the type of fault.

Go read my post again !!!!!!

** Shame how the 47 uF caps in question only have 2 to 10 uA of leakage.

Shame how 17 mA of extra current is not acceptable in a valve amp or many other situations where such electros are used.

Shame how the 7.5 watts of heat from the resistors would damage the electros if placed near them.

Shame how high value WW resistors are notorious for failing open when subjected to continuous, high DC voltages.

Shame what a load of crapology you post.

** Huh - so it is OK by you if one resistor fails open and the other then forces an electro overvoltage ???

Do you realise you just totally contradicted yourself ???

** Ok - so you rely on rote learning and the superstitions of others.

How intelligent - NOT !!

** You are posting mindless tripe - Glenn.

Worse, you INSIST on top posting - the method only used by fools and wankers.

** Confusing the matter with irrational drivel is not my idea of "objective".

........... Phil

Reply to
Phil Allison

Hi Phil,

OK, you've tempted me to post again. You are quite entitled to your personal opinion, and I to mine. Most of your points are quite valid and arguable, but I do see my point of view as still plausible. Would anyone else care to throw in their personal opinion or experiences?

Glenn.

Phil Allis>

Reply to
Glenn Baddeley

"Glenn Baddeley" wrote in news:d9t3p4$pb7 $ snipped-for-privacy@mws-stat-syd.cdn.telstra.com.au:

Been involved in designs that used both approaches. Went away from the no resistor approach because techs used to get a shock from picking up the electronics module, needed to bleed away that charge (700V). Never saw any failures of caps in either situation, with a 20 year service life of the product. Never paid much attention to this 'problem' before this.

Reply to
Geoff C

"Geoff C"

** The possible need for a bleed R is a separate issue to the ballast resistor one.

The OP wanted to replace a single 500 volt cap with two in series - there was no bleed R in his case.

** Ditto.

............ Phil

Reply to
Phil Allison

( snip drivel)

( snip more crazy stuff)

** So they don't just do weird things to sheep in NZ ........

............. Phil

Reply to
Phil Allison

"Glenn Baddeley" = still bloody top posting !!!

** STOP !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

** So how do you know it really is a good point ??

.......... Phil

Reply to
Phil Allison

in this case a resistor is good for a bleeder :) thats about all i ever use them for

Reply to
The Real Andy

"The Real Andy" Phil Allison

** What type do you use that can stand 250 volt DC indefinitely ??

High value resistors, of all types, seem to have a very high failure ( failing open) rate after a few years when subjected to continuous DC above about 100 volts.

.......... Phil

Reply to
Phil Allison

any ideas as to the mechanism? come across any decent app notes?

I presume that as high value Rs the peak power dissipation would still be pretty low - I've seen lots of low-value resistors fail after being abused in this way, eg 0603 10R 12V gatedrive resistor.

what about electromigration? wouldnt that tend to bridge out the serpentine cuts, thus reducing R

what about VR37s, they are designed for very high voltage pulses. we used pairs of 470k VR37 resistors in series for DC bus and line voltage sensing, +/-300V - 500V about earth so 150V-250V per resistor. never had a single failure over perhaps 10 years and 40,000 products.

We also used 100k PR02's across each half of the DC bus cap bank, so about 0.9W continuous at 600V/2 and 1.6W at 800V/2(short term during regen). dont know about failures there though, as power electronics failures tend to trash the entire area, so its quite possible. the dissipation is a bit high at 800V, but luckily the nearby electrolytics will provide some cooling :)

At one stage I tried damping an EMI filter with 1R PR02s in series with the 1uF caps. If turned on at an appropriately high line voltage (which one phase always is in 3-phase systems) the PR02 emitted a bright flash of light and went open circuit. probably because (400V*1.4)^2/1R = 320kW peak pulse power. We found that by observing the flash...wtf?! A carbon composition resistor ate the peak pulse though.

Cheers Terry

Reply to
Terry Given

** The resistors that fail are operating within published specs.

Ergo - the "mechanism" is bad manufacture.

Not much mention of that in any "app notes'.

** What - without the right Visa ??

** Values start at 1M ohm - useless.

........... Phil

Reply to
Phil Allison

I too suspect this is largely the reason.

I once had a batch of 0.1% resistors from vishay that were off by up to

25%. it was a ups DC balancing circuit, and it didnt like it at all. Only 20-30 units were affected.

vishay were great, wrote me a nice report. basically contaminated material wrecked part of a batch, but the effect was triggered by high(ish) temperatures so they all measured ok at the factory, and through our genrad pcb tester, but once the unit heated up to 50C or so, they drifted like crazy.

in general my experience is that nice cheap components are quite often unreliable shit, and the only way to tell is to wait a year or two. e.g. every company I've ever worked for has had a cheap ceramic bypass capacitor horror story to tell.

lol. other than its existence, I dont know much about electromigration, but it appears these guys do:

formatting link

whaddya know, its kind of analogous to crack propagation, so certainly could cause open circuits. AIUI its an issue at IC level because the small dimensions result in high E field strengths. The same would occur in a serpentine resistor at high voltage.

I have 100pcs 470k sitting in front of me.

1998 PA08B databook:

VR25 100k - 15M 1,5,10% E12/E24 1,600Vdc peak VR37 100k - 33M 1,5% E24/E96 3,500Vdc peak VR68 100k - 68M 1,5% E24/E96 10,000Vdc peak PR02 0R33 - 1M 1,5% E24/E96 500Vdc peak

Cheers Terry

Reply to
Terry Given

** My sources

  1. Farnell in One cat.

  2. Philips General cat 1978.

** I believe you - but that is not the simple answer.

The simple one is to use a few cheap 0.5 or 0.75 watt MF resistors in series - so each cops less than 100 volts and runs cool.

........... Phil

Reply to
Phil Allison

I "inherited" a full set of philips 2000 databooks (many are 1998). I regret throwing out so many databooks in the past :(

absolutely, and with smt its trivial to put down as many as required (although per-part power dissipation also drops). The overall footprint is not dissimilar, but the inductance is a whole lot lower, as is the capacitance. designing to cope with single component failures is a lot easier (eg 6 aint much more than 5, but 2 is a lot more than 1) too.

Cheers Terry

Reply to
Terry Given

Did the same thing with a 500V DC distributed supply system. Was much easier to simply use a few SMD resistors in series as the ballast resistors, rather than having to source high voltage ones. Too bad there wasn't the same choice on the caps, volume realestate was at a premium (more so than PCB realestate) so we had to run tight margins and had little selection available when it came to the cap vs voltage vs volume vs height tradeoff.

Dave :)

Reply to
David L. Jones

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