Do DC caps have an AC rating?
Existential Angst
Got some boards with bigazz 350 VDC caps.
Don't quite understand the DC rating, as caps don't pass DC, but can be used
to smooth rectified AC, etc.
Are DC caps really different from AC caps?
Can I use these, say, in an rpc?? If so, what would their AC voltage
rating be?
More specifrically, these are Sprague Powerlytic metal can caps, about 3"
dia, 5" high.
The numbers I see are 2500-350DC -- not 350VDC -- but is it safe to say
these caps are 2500 uF at 350VDC?
2500 uF seems perty g-d big.....
--
EA
dca...@krl.org
wrote:
> EA
Not good for RPC's. That type of capacitor is only good for DC and
have to have the polarity correct. Install them with the wrong
polarity and they are likely to get hot and explode. Putting AC on
them means the polarity is wrong half the time, so again likely to
explode. By likely I mean almost certain to explode.
Dan
whit3rd
wrote:
> Awl --
>
> Got some boards with bigazz 350 VDC caps.
> Don't quite understand the DC rating, as caps don't pass DC, but can be used
> to smooth rectified AC, etc.
> Are DC caps really different from AC caps?
Yes. DC allows a kind of chemistry (electrolytic capacitor) to
make a thin insulating layer, which gets destroyed by AC or
the 'wrong' polarity.
> Can I use these, say, in an rpc?? If so, what would their AC voltage
> rating be?
It takes two of 'em in series, with two steering diodes, to replace an
AC
capacitor. The capacitance of the series pair is HALF the
capacitance
of the individual capacitors.
> More specifrically, these are Sprague Powerlytic metal can caps, about 3"
> dia, 5" high.
> The numbers I see are 2500-350DC -- not 350VDC -- but is it safe to say
> these caps are 2500 uF at 350VDC?
> 2500 uF seems perty g-d big.....
Yep, that's the benefit of the electrolytic type; the insulator part
of the
capacitor is more compact than one can safely construct plastic or
paper
or ceramic equivalents. 350 VDC means two in series will hold off
about 250 VAC (because the peaks of 250 VAC are actually higher than
250V).
Using these in a rotary phase converter, they would be only connected
briefly, for starting; otherwise, you'd need to worry about the ripple-
current
rating as well as voltage and polarity. Capacitors can overheat and
blow up...
Tim Wescott
> On 2010-04-20, Tim Wescott <t...@seemywebsite.now> wrote:
>> rated" is spot-on.
>>
>> I hope that helps.
>>
>> Oh hell, I'll expand:
>>
>> All capacitors have a maximum voltage that they can withstand, from
>> which the ratings are derived. "AC" ratings are based on the maximum
>> expected instantaneous voltage that a cap is expected to see when the
>> line voltage is at the rated value. "DC" ratings are really
>> instantaneous ratings, and are more "real", if you will.
>>
>> Those are electrolytic capacitors. You can tell by the name, and by the
>> small size for the capacitance. Electrolytic capacitors are essentially
>> _really crappy_ diodes* with a lot of junction capacitance and with
>> electrolytic goo (salts, water, and some sort of gell) as one of the
>> terminals. So they have _two_ behaviors to instantaneous voltages: the
>> desired one, where the 'diode' is reverse biased and you're using it as
>> a cap, and the other one, where the 'diode' is conducting, the
>> electrolytic is boiling, and the cap is about to undergo a steam explosion.
>>
>> So you may not want to use them with AC, because within the first 1/60th
>> of a second they'll be fixing to explode -- and there's not much useful
>> work you can get out of an RPC in 1/60th of a second.
>
> Very educational.
>
> What if you connect two electrolytic capacitors in series, but with
> polarities reversed?
>
> +-...-+
>
> Then the pair would not be able to conduct current in either
> direction, but would be able to accept the charge?
AFAIK that's what's done to make "non-polar electrolytic" caps. But I'd
check before I did it.
Note, too, that electrolytic caps generally have problems with too much
AC current, and they are often not clearly rated for their current
handling ability, although things have gotten better in the last decade
or so.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Tim Wescott
Sorta kinda, but really no. And "AC" cap is a misnomer, although "AC
rated" is spot-on.
I hope that helps.
Oh hell, I'll expand:
All capacitors have a maximum voltage that they can withstand, from
which the ratings are derived. "AC" ratings are based on the maximum
expected instantaneous voltage that a cap is expected to see when the
line voltage is at the rated value. "DC" ratings are really
instantaneous ratings, and are more "real", if you will.
Those are electrolytic capacitors. You can tell by the name, and by the
small size for the capacitance. Electrolytic capacitors are essentially
_really crappy_ diodes* with a lot of junction capacitance and with
electrolytic goo (salts, water, and some sort of gell) as one of the
terminals. So they have _two_ behaviors to instantaneous voltages: the
desired one, where the 'diode' is reverse biased and you're using it as
a cap, and the other one, where the 'diode' is conducting, the
electrolytic is boiling, and the cap is about to undergo a steam explosion.
So you may not want to use them with AC, because within the first 1/60th
of a second they'll be fixing to explode -- and there's not much useful
work you can get out of an RPC in 1/60th of a second.
* In fact, you can take the same metals and chemical goo that you use to
make electrolytics, and make electrolytic diodes instead. They were
popular in the early 1900's before vacuum and mercury rectifiers came
down enough in cost to be feasible.
Ignoramus3512
Very educational.
What if you connect two electrolytic capacitors in series, but with
polarities reversed?
+-...-+
Then the pair would not be able to conduct current in either
direction, but would be able to accept the charge?
> * In fact, you can take the same metals and chemical goo that you use to
sta...@prolynx.com
wrote:
DC rating means exactly that. Yes, they'll pass AC, but they're
electrolytics built for DC PS filtering use. Probably are 2500 uF +-
50% or so. The more amps out of the power supply, the more
capacitance is needed. If you hook them up to AC, you'll get a big
boom as the electrolyte turns to steam. The dielectric on the
"plates" of the capacitor is the oxide film formed from the action of
the electrolyte on the aluminum foil. If you put AC on them, this
gets stripped and things get hot.
If they're more than a few years old, they're probably useless without
reforming. I use a Sprague cap test meter for this, runs up to 450v
and also reads leakage current. I usually start at about 1/4 of the
rated voltage and increase as the leakage current goes down, usually
takes 3-4 hours per step. If the current never goes down as time goes
on, it's a dud, time to go to the next one. Electrolytics are
fungible items, need to be changed out every so often as ESR and
leakage current increase. NOS aluminum caps may well be junk, they
don't age well even just sitting there.
AC caps either have oil-impregnated paper or solid insulator materials
for dielectric. If you need AC caps, buy AC-rated caps, DC
electrolytics aren't going to do it for you.
Stan
Dave__67
wrote:
DC rating indicates that measuring from anode to cathode the voltage
can be 0 to +(rated volts), but that the voltage anode to cathode can
not be allowed to go negative.
In other words, DC indicates the allowable polarity/bias.
Using it as an AC cap will result in a short life, I believe, how
short depending on the actual type (electrolytic, etc).
The capacitance may not even be the same biased in reverse.
Reverse biasing an electrolytic can lead to near-instant dramatic
failure as an electrochemical reaction (in some) destroys the internal
plate-plate insulator.
Dave
Jim Wilkins
wrote:
> ...
> What if you connect two electrolytic capacitors in series, but with
> polarities reversed?
>
> +-...-+
>
> Then the pair would not be able to conduct current in either
> direction, but would be able to accept the charge?
>
> > * In fact, you can take the same metals and chemical goo that you use to
> > make electrolytics, and make electrolytic diodes instead. They were
> > popular in the early 1900's before vacuum and mercury rectifiers came
Aluminum and tantalum electrolytics inadvertently installed backwards
may take a while to vent or explode. I've encountered a batch of
dipped tantalums that were marked backwards and ALL installed wrong.
The applied voltage was considerably lower than the rating and they
failed fairly quietly, blowing a chip out of the side with a muffled
POP. Electrolytics can also grenade and fill the air with caustic-
covered aluminum confetti.
Two connected back to back for AC should be bypassed with diodes, or
better just buy AC-rated ones.
DC caps have an AC ripple current related to permissible heating from
their Equivalent Series Resistance (ESR). Caps meant for linear power
supplies are cheaper and have a lower ripple current rating than ones
designed for switchers.
jsw
James Waldby
> On Apr 20, 10:43 am, "Existential Angst" ... wrote:
>> Got some boards with bigazz 350 VDC caps. Don't quite understand the DC
>> rating, as caps don't pass DC, but can be used to smooth rectified AC,
>> etc. Are DC caps really different from AC caps?
>
> Yes. DC allows a kind of chemistry (electrolytic capacitor) to make a
> thin insulating layer, which gets destroyed by AC or the 'wrong'
> polarity.
>
>> Can I use these, say, in an rpc?? If so, what would their AC voltage
>> rating be?
>
> It takes two of 'em in series, with two steering diodes, to replace
> an AC capacitor. The capacitance of the series pair is HALF
> the capacitance of the individual capacitors.
If the steering diodes are correctly installed, the circuit
consists, during up-cycles, of one capacitor and one diode;
during down-cycles, the other capacitor and other diode. So
the harmonic-mean series-capacitance formula, C = 1/(1/C1 + 1/C2),
doesn't apply. That is, if capacitance of each capacitor
is C, the effective capacitance is C, not C/2.
[snip rest]
--
jiw
Joe Pfeiffer
Non-polarized capacitors also have a max VDC rating, which means the max
voltage can go from -(rated volts) to +(rated volts). Of course, the
capacitors he's describing would be polarized.
--
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be glad of an opportunity to serve others by any invention of ours;
and this we should do freely and generously. (Benjamin Franklin)