Electrolytic caps in series

[Phil Allison]

"The Real Andy"
Phil Allison

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

** 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

 

[Terry Given]

"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

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

 

[Phil Allison]

"Terry Given"

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

** 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 about electromigration?

** What - without the right Visa ??

what about VR37s, they are designed for very high voltage pulses.

** Values start at 1M ohm - useless.





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

 

[Terry Given]

"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'.

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.

** What - without the right Visa ??

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

http://www.theo-phys.uni-essen.de/tp/forsch/krug.html

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.

** Values start at 1M ohm - useless.

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

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

Cheers
Terry

 

[Phil Allison]

"Terry Given"
Phil Allison wrote:

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

** My sources

1. Farnell in One cat.

2. Philips General cat 1978.

1998 PA08B databook:

** 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

 

[Terry Given]

"Terry Given"
Phil Allison wrote:







** My sources

1. Farnell in One cat.

2. Philips General cat 1978.

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

** 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

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

 

[David L. Jones]

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.

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