Query Electrolytic Cap values
 

Hello, knowledgeble colleagues. I am replacing PS filter caps on an old tube
CW transmitter that I believe are (all) shot and responsible for the harsh AC
hash-n-hum. I'm not really trying to restore it; I just want to clean up the
signal to use it, so it's not important to me to try to conceal multiple caps
into the original paper holding combo can above the chassis.
There are some nonstandard values, however. Now, I know I can add the
capacitance sums by wiring in parallel to achieve my targeted uF requirements,
but can I use caps designated for higher V without consequence?
Can I substitute, e.g. a 450/30 and a 450/20 in parallel to replace a 400/50?
A 450/100 to replace a 400/100?
How 'bout a 450/8 to replace a 450/4? (Instead of bothering to put 2 in
series.)
As a general rule then, as long as I exceed Voltage and Capacitance values
called for in the schematic, am I ok?
Thanx in advance for your advice.

In general, electrolytic capacitors should be operated not terribly much
below their ratings. In the cases you cited, the difference is too small to
worry about, and the 450 V jobs will do fine as subs for the 400 V ones.

Also, there is no concern over ESR or leakage or even capacitance tolerance.

73, Bob

The higher voltage values are no problem. Do not stray to far from the
original Mfd values. Remember you can also series for example 2 200 volt
400Mfd caps to get a 400 Volt 200 Mfd cap.

Good luck in your endevors and if I can help you further, let me know.

Fred W4JLE


"Twelve VDC" wrote in message
...
Hello, knowledgeble colleagues. I am replacing PS filter caps on an old
tube
CW transmitter that I believe are (all) shot and responsible for the
harsh AC
hash-n-hum. I'm not really trying to restore it; I just want to clean up
the
signal to use it, so it's not important to me to try to conceal multiple
caps
into the original paper holding combo can above the chassis.
There are some nonstandard values, however. Now, I know I can add the
capacitance sums by wiring in parallel to achieve my targeted uF
requirements,
but can I use caps designated for higher V without consequence?
Can I substitute, e.g. a 450/30 and a 450/20 in parallel to replace a
400/50?
A 450/100 to replace a 400/100?
How 'bout a 450/8 to replace a 450/4? (Instead of bothering to put 2 in
series.)
As a general rule then, as long as I exceed Voltage and Capacitance
values
called for in the schematic, am I ok?
Thanx in advance for your advice.

Bob wrote:

In general, electrolytic capacitors should be operated not terribly much
below their ratings.

I am curious: where did you hear this and what is the rationale? I've
never heard anything similar before, although I suppose it is possible.

73 ... WA7AA


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Zoran Brlecic wrote:


Bob wrote:

In general, electrolytic capacitors should be operated not terribly much
below their ratings.

I am curious: where did you hear this and what is the rationale? I've
never heard anything similar before, although I suppose it is possible.

I tripped across the concept many years ago. Seems that electrolytic
caps are not created equal, said devices being dependent upon the plate
area and the reciprocal of the oxide layer thickness on the aluminum
electrode to supply the proper capacitance and on the oxide layer
thickness for the DC Working Volts rating. This oxide layer depends
upon the polarizing voltage for its maintenance, and the electrolytic
electrode is formulated to maintain the proper oxide thickness at the
capacitor's normal voltage. Insufficient voltage results in a thinner
than expected oxide layer, resulting in increased capacitance (not
normally a Bad Thing) and decreased breakdown voltage (which is a Bad
Thing). I don't know if it is possible for a derated cap to lose enough
oxide to drop its breakdown voltage below its normal working voltage, but
it would result in a dramatic failure if it ever did happen.

YMMV, IIRC, IANAL, and any other disclaimers one wishes to insert.
Perhaps there's a component engineer reading this group who could
speak to what I remember, or believe I remember, on this matter.

de kg7yy

Zoran Brlecic wrote:
Bob wrote:

In general, electrolytic capacitors should be operated not terribly much
below their ratings.


I am curious: where did you hear this and what is the rationale? I've
never heard anything similar before, although I suppose it is possible.

73 ... WA7AA


Hi,
In capacitor size/working voltage rating. I won't put lower
WV one, if in the case of filter cap. also I won't put lower value.
Anyhow, this type caps have wide tolerance in general.
73,
Tony, VE6CGX

Tony Hwang wrote:

In general, electrolytic capacitors should be operated not terribly much
below their ratings.

I am curious: where did you hear this and what is the rationale? I've
never heard anything similar before, although I suppose it is possible.

In capacitor size/working voltage rating. I won't put lower
WV one, if in the case of filter cap. also I won't put lower value.
Anyhow, this type caps have wide tolerance in general.

Tony, the issue is using an electrolytic cap not much *below* its
voltage rating, not using it above its maximum voltage. For example, a
cap rated 200V should, according to the poster, be used in the 150-200V
range, and not in the low voltage applications, like 12V or 24V.
This is what I asked about, since I'd never heard it before. Of course,
in practice, this is really a non-issue due to the physical size of the
caps, so no sane person will replace a 15V cap with a 450V one, at least
not on a permanent basis.

73 ... WA7AA




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Zoran Brlecic wrote:
Tony Hwang wrote:

In general, electrolytic capacitors should be operated not terribly
much
below their ratings.


I am curious: where did you hear this and what is the rationale? I've
never heard anything similar before, although I suppose it is possible.


In capacitor size/working voltage rating. I won't put lower
WV one, if in the case of filter cap. also I won't put lower value.
Anyhow, this type caps have wide tolerance in general.


Tony, the issue is using an electrolytic cap not much *below* its
voltage rating, not using it above its maximum voltage. For example, a
cap rated 200V should, according to the poster, be used in the 150-200V
range, and not in the low voltage applications, like 12V or 24V.
This is what I asked about, since I'd never heard it before. Of course,
in practice, this is really a non-issue due to the physical size of the
caps, so no sane person will replace a 15V cap with a 450V one, at least
not on a permanent basis.

73 ... WA7AA




Hi,
There is a reason for that cosidering how this type caps work.
Putting in a 200WV cap in a 15WV application is not a good idea.
Tony

Tony Hwang wrote:

There is a reason for that cosidering how this type caps work.
Putting in a 200WV cap in a 15WV application is not a good idea.

Because...




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On Mon, 16 Feb 2004 01:38:56 GMT Zoran Brlecic
wrote:

Bob wrote:

In general, electrolytic capacitors should be operated not terribly much
below their ratings.

I am curious: where did you hear this and what is the rationale? I've
never heard anything similar before, although I suppose it is possible.

I hear it a lot, but never with any real justification. The only
reason that I can see why this would be a problem is that you've
wasted some money.

I really don't think a 50V electrolyic capacitor suffers a bit if you
only put 10V across it.

I've also never seen any mention of this in any manufacturer's data
sheets or catalogs.

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Jim Adney
Madison, WI 53711 USA
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