"Forming capacitors" - means what?


kd5kfl
 

I've been an electronics tech since 1964. Heard endless talk about
not plugging old gear in until you "form the capacitors". Never did
find out what that meant.

If anyone here knows, please clue me in.


Denton, Adam (Exchange)
 

It has to do with old capacitors that haven't been
powered up in a long time. They are still usable,
however, as in electrolytics, the dielectric is
formed chemically when voltage is applied. Apparently
when unused for a long time, the electrolyte either
separates or changes slightly so that it is sluggish
or otherwise unable to form the dielectric rapidly.

The effect is that when you throw the big red switch
(consider a filter cap in the power supply) the power
voltage comes up rapidly but the ancient cap takes
too long for the electolyte to form the dielectric.
As a result, the cap is basically a short circuit
and a HUGE inrush of current flows through the cap,
heating it up and probably destroying it.

To re-form, either power up the unit SLOWLY using
a variac (set at 20, 40, 60, etc. volts and let
each setting "cook" for an hour or two) until
you get to 120 (or 240). If you have no variac
you can insert a series incandescent light bulb
with the power lead to achieve the same effect.
Start with 25W for an hour, then 40, 75, 100 etc.
What you are doing is acknowledging that huge currents
will flow, but you are limiting them until the caps
come back to life.

They will, it is generally reversible. Web sites devoted
to antique radio restoration will probably have more
on this. You only have to do it the 1st time you power
up after say 5 or more years.

<<I've been an electronics tech since 1964. Heard endless talk about
not plugging old gear in until you "form the capacitors". Never did
find out what that meant.


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Bear Stearns is not responsible for any recommendation, solicitation,
offer or agreement or any information about any transaction, customer
account or account activity contained in this communication.
***********************************************************************


Robert Morein <morepub@...>
 

The internal insulating layer of an electrolytic capacitor is formed by chemical action between the electrolyte and the plate material, under the action of a DC current. In the case of an aluminum electrolytic, the electrolyte is potassium hydroxide.

When DC is not applied, the insulating layer gradually degrades. When DC is suddenly applied after a long period of non-use, it may be enough to arc between the anode and cathode.

If the capacitor is not installed, the insulating layer can be regrown by applying a low voltage through a current limiting resistor, gradually increasing the voltage over several hours. After 24 hours, the leakage current should decrease to spec.

If it's a piece of equipment, this is trickier. In some cases, such as audio amplifiers which typically do not have regulated supplies, a Variac can be used to avoid the most stressful part of the startup, when the unit is just turned on. I don't know that this would be of any use for modern equipment with switchmode supplies. For equipment with fractional horsepower motors, I suspect it would be harmful.

The Navy used to have a rule that all electronic equipment had to be powered up at least once a month.

----- Original Message -----
From: kd5kfl
To: TekScopes@yahoogroups.com
Sent: Friday, June 20, 2003 12:38 PM
Subject: [TekScopes] "Forming capacitors" - means what?


I've been an electronics tech since 1964. Heard endless talk about
not plugging old gear in until you "form the capacitors". Never did
find out what that meant.

If anyone here knows, please clue me in.


Don Black <jeans@...>
 

A capacitor consists of a pair of electrodes separated by a dielectric
(insulator). The amount of capacitance depends on several things; the area of the
plates, thickness of the dielectric and its dielectric constant. Different
materials have different constants, from 1 for a vacuum to several thousand for
some ceramics. that means the capacitance is multiplied by the dielectric
constant and the reason some ceramic capacitors have large values for small
capacitors (though they aren't very stable). To achieve very large values the
electrolytic capacitor was developed. They use aluminium electrodes separated by
an electrolyte based on borax (the propriety formulas are special "witches
brews). They are 'formed" in manufacture by passing al electric current through
the capacitor that produces a very thin oxide layer on the anode plate, very
similar to anodizing that's used to form a skin on aluminium products for
durability and decoration. This insulating layer is the dielectric for the
capacitor, the plates being the aluminium anode and the electrolyte. because it's
so thin the capacitance per unit area is very large and there are other tricks
like etching the anode to increase its surface area to pack more capacitance into
a given volume; compare a modern capacitor with older ones and see how they've
progressively shrunk in size.
Now, the question. When electrolytics are idle the insulating oxide layer tends
to disintegrate, allowing the electrolyte to directly contact the anode and
permitting a (large) current to leak through the capacitor. By passing a small
current through the capacitor it's possible to re-form the oxide coating and
repair the damage. The small current doesn't cause harm, while placing full
voltage across a leaky capacitor causes a heavy load on the supply (transformers
and rectifiers beware), heats the capacitor and may cause it to explode. That
wakes you up on a monday when one goes off in your face.
Manufacturers recommend reforming even new capacitors after a couple of years on
the shelf, though I've never had problems like this myself. However in old
equipment it can mean the difference between survival or destruction.
In practice some method of applying the full working voltage to the capacitor via
a current limiting network; simple resistors work OK limiting the current to 10
mA or so. There are formulas for the right current for different capacitors but
provided it's limited to a reasonable level they will reform if they're
serviceable. They eventually fail completely and then have to be replaced.
This site may give you some ideas
<http://www.angela.com/catalog/how-to/about_caps.html> and I'll email an article
from Mepco company on re-forming.
Don Black.

kd5kfl wrote:

I've been an electronics tech since 1964. Heard endless talk about
not plugging old gear in until you "form the capacitors". Never did
find out what that meant.

If anyone here knows, please clue me in.




Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/


Don Black <jeans@...>
 

Re-forming with a variac or lamp in the power cord can be complicated by a tube
rectifier having to heat up and the load of the equipment as it starts to
operate. it's still a lot better than just applying full power but better if you
can use an external supply, preferably with the capacitors isolated.
Don Black.

"Denton, Adam (Exchange)" wrote:

It has to do with old capacitors that haven't been
powered up in a long time. They are still usable,
however, as in electrolytics, the dielectric is
formed chemically when voltage is applied. Apparently
when unused for a long time, the electrolyte either
separates or changes slightly so that it is sluggish
or otherwise unable to form the dielectric rapidly.

The effect is that when you throw the big red switch
(consider a filter cap in the power supply) the power
voltage comes up rapidly but the ancient cap takes
too long for the electolyte to form the dielectric.
As a result, the cap is basically a short circuit
and a HUGE inrush of current flows through the cap,
heating it up and probably destroying it.

To re-form, either power up the unit SLOWLY using
a variac (set at 20, 40, 60, etc. volts and let
each setting "cook" for an hour or two) until
you get to 120 (or 240). If you have no variac
you can insert a series incandescent light bulb
with the power lead to achieve the same effect.
Start with 25W for an hour, then 40, 75, 100 etc.
What you are doing is acknowledging that huge currents
will flow, but you are limiting them until the caps
come back to life.

They will, it is generally reversible. Web sites devoted
to antique radio restoration will probably have more
on this. You only have to do it the 1st time you power
up after say 5 or more years.

<<I've been an electronics tech since 1964. Heard endless talk about
not plugging old gear in until you "form the capacitors". Never did
find out what that meant.

***********************************************************************
Bear Stearns is not responsible for any recommendation, solicitation,
offer or agreement or any information about any transaction, customer
account or account activity contained in this communication.
***********************************************************************




Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/


aabasiry
 

Thanks Don for the great info. I agree totally for the case of Tube
power supplies that need heating in order to kick in and that puts
some limit on the minimum valtage that can be applied through a
Variac. I didn't understand, however, what you meant by using
an "External Power Supply". Do you mean disconnecting the capacitor
and applying a DC volatge (up to its rating of course!) with a DC
supply (through a resistor I would assume!)?
Please advise..
Thanks..

--- In TekScopes@yahoogroups.com, Don Black <jeans@n...> wrote:
Re-forming with a variac or lamp in the power cord can be
complicated by a tube
rectifier having to heat up and the load of the equipment as it
starts to
operate. it's still a lot better than just applying full power but
better if you
can use an external supply, preferably with the capacitors isolated.
Don Black.

"Denton, Adam (Exchange)" wrote:

It has to do with old capacitors that haven't been
powered up in a long time. They are still usable,
however, as in electrolytics, the dielectric is
formed chemically when voltage is applied. Apparently
when unused for a long time, the electrolyte either
separates or changes slightly so that it is sluggish
or otherwise unable to form the dielectric rapidly.

The effect is that when you throw the big red switch
(consider a filter cap in the power supply) the power
voltage comes up rapidly but the ancient cap takes
too long for the electolyte to form the dielectric.
As a result, the cap is basically a short circuit
and a HUGE inrush of current flows through the cap,
heating it up and probably destroying it.

To re-form, either power up the unit SLOWLY using
a variac (set at 20, 40, 60, etc. volts and let
each setting "cook" for an hour or two) until
you get to 120 (or 240). If you have no variac
you can insert a series incandescent light bulb
with the power lead to achieve the same effect.
Start with 25W for an hour, then 40, 75, 100 etc.
What you are doing is acknowledging that huge currents
will flow, but you are limiting them until the caps
come back to life.

They will, it is generally reversible. Web sites devoted
to antique radio restoration will probably have more
on this. You only have to do it the 1st time you power
up after say 5 or more years.

<<I've been an electronics tech since 1964. Heard endless talk
about
not plugging old gear in until you "form the capacitors". Never
did
find out what that meant.

**********************************************************************
*
Bear Stearns is not responsible for any recommendation,
solicitation,
offer or agreement or any information about any transaction,
customer
account or account activity contained in this communication.
**********************************************************************
*




Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/


Miroslav Pokorni
 

The effects are explained in appropriately scary way, but it is not accurate
and might lead to misunderstandings. For example, if capacitance measures up
it might be assumed that there is no harm in applying full power to the cap
or even worse, to apply DC power without current limiting (capacitance is
not important when dealing with DC bias, right?).

Yes, the capacitance might degrade over time, but killer is that leakage
current gets high. That is why reforming is necessary to decrease leakage
current and bring it to more or less nominal value for type of capacitor.

A scan of reforming procedure was circulated on this forum. The forum does
not allow to broadcast attachments, so if originator of question is still
interested, I will try to find that scan and send to him. The reforming
procedure is one recommended by Mepco/Electra and that is their view of
resurrection. There is no unified view what does it take to reform
capacitor, e.g. Mepco/Electra call for keeping capacitors at 85 C for 24
hour, while other manufacturers do not talk about heat treatment. However,
all manufacturers agree on applying rated voltage (some say 110% of rating)
for a period of time and with current limiting. Generally, all manufacturers
agree to rest capacitor for 24 hours after applying voltage before taking
leakage current measurement. Resting the cap for 24 hours is actually
universally accepted step when measuring leakage current, either in
production run or as a lot acceptance procedure.

Regards

Miroslav Pokorni

----- Original Message -----
From: "Denton, Adam (Exchange)" <adenton@bear.com>
To: <TekScopes@yahoogroups.com>
Sent: Friday, June 20, 2003 9:49 AM
Subject: RE: [TekScopes] "Forming capacitors" - means what?


It has to do with old capacitors that haven't been
powered up in a long time. They are still usable,
however, as in electrolytics, the dielectric is
formed chemically when voltage is applied. Apparently
when unused for a long time, the electrolyte either
separates or changes slightly so that it is sluggish
or otherwise unable to form the dielectric rapidly.

The effect is that when you throw the big red switch
(consider a filter cap in the power supply) the power
voltage comes up rapidly but the ancient cap takes
too long for the electolyte to form the dielectric.
As a result, the cap is basically a short circuit
and a HUGE inrush of current flows through the cap,
heating it up and probably destroying it.

To re-form, either power up the unit SLOWLY using
a variac (set at 20, 40, 60, etc. volts and let
each setting "cook" for an hour or two) until
you get to 120 (or 240). If you have no variac
you can insert a series incandescent light bulb
with the power lead to achieve the same effect.
Start with 25W for an hour, then 40, 75, 100 etc.
What you are doing is acknowledging that huge currents
will flow, but you are limiting them until the caps
come back to life.

They will, it is generally reversible. Web sites devoted
to antique radio restoration will probably have more
on this. You only have to do it the 1st time you power
up after say 5 or more years.

<<I've been an electronics tech since 1964. Heard endless talk about
not plugging old gear in until you "form the capacitors". Never did
find out what that meant.


***********************************************************************
Bear Stearns is not responsible for any recommendation, solicitation,
offer or agreement or any information about any transaction, customer
account or account activity contained in this communication.
***********************************************************************


KB6NAX
 

I don't find anything to quibble with in this thread. I've
reformed 'lytics in numerous old radios and audio amplifiers and
monitoring capacitor temperature is the best indicator for
controlling the reforming process. I recently reformed the 'lytics
in a McIntosh MC75 amplifier. The power supply uses silicon diode
rectifiers in a voltage doubler arrangement which if full line
voltage is applied will blow leaky capacitors to kingdom come. I
controlled capacitor temperature by adjusting the variac to keep the
capacitors from becoming uncomfortably warm. A capacitor that
exhibits high leakage current will of course act like a resistor and
heat up. As the capacitor warms up the leakage current increases.
The increased current will increase the temperature and if reforming
does not act to limit the temperature rise the capacitor will go into
thermal runaway and blow its juices out (and in some cases the
capacitor may explode). The McIntosh caps ran warm for about 16
hours before they reformed sufficiently to cool down. To be sure, I
kept the process going for 48 hours going to full line voltage after
about 24 hours (don't be in a rush to increase voltage!). The friend
who I restored the amp for is happily listening to it daily. How do
I measure capacitor temperature? With my hand (only one hand though,
I follow the one hand rule when working on high voltage stuff and so
far it has saved me to write this - hi!). I remove the tubes in
vacuum tube gear to keep the capacitors from being heated by the
tubes. Use of a metered variac helps as you can see if power supply
current is increasing, a sure indicator that trouble is ahead. Not
all old capacitors will reform, they will continue to exhibit high
leakage current and as someone mentioned they will eventually short.


cinerama84106
 

I use an EICO 1030 variable high voltage bench supply to re-form
caps. This not only has adjustable DC voltage output of up to 400
volts but it also has a ma meter on the front panel as well for
monitoring leakage. This unit has been valualable to reform
capacitors,and I agree on the temperature idea..... A warm capacitor
is definately a leaking capacitor. It does take about 48 hours to
properly reform a cap!! I start out at 100 volts(for a 450 volt cap),
50 volts(for a 350 volt cap) then dial it up by about 50 volts every
4 hours or so. When its at the max cap voltage I let it run another
24 hours checking the caps temperature frequently t be sure its not
too hot. Sometimes they will get a tad bit warm, but eventually the
caps temp goes back down to room temperature. Never had one leak or
explode on me this way.
Mark


Miroslav Pokorni
 

The gradual voltage increase seems just to extend time of the process and
does not seem to buy anything effective. Checking temperature is probably
good thing, but you have to be checking very often and be a damn fast gun to
react to any increase.

It seems to me to be much more effective to have a current source that would
limit current to, say 10 mA, and ram full rated voltage to the cap. If cap
shorts, 10 mA at 450V is below 5W, not enough to destroy anything, and if
cap reforms current will drop. That is how caps are made, in first place,
then reforming can be done in same way.

Current source is quite a simple thing, a power FET on a moderate heat sink,
an opamp and a voltage reference of any kind would do the job.

Regards

Miroslav Pokorni

----- Original Message -----
From: "Mark Gulbrandsen" <cinerama84106@yahoo.com>
To: <TekScopes@yahoogroups.com>
Sent: Sunday, June 22, 2003 8:30 AM
Subject: [TekScopes] Re: "Forming capacitors" - means what?


I use an EICO 1030 variable high voltage bench supply to re-form
caps. This not only has adjustable DC voltage output of up to 400
volts but it also has a ma meter on the front panel as well for
monitoring leakage. This unit has been valualable to reform
capacitors,and I agree on the temperature idea..... A warm capacitor
is definately a leaking capacitor. It does take about 48 hours to
properly reform a cap!! I start out at 100 volts(for a 450 volt cap),
50 volts(for a 350 volt cap) then dial it up by about 50 volts every
4 hours or so. When its at the max cap voltage I let it run another
24 hours checking the caps temperature frequently t be sure its not
too hot. Sometimes they will get a tad bit warm, but eventually the
caps temp goes back down to room temperature. Never had one leak or
explode on me this way.
Mark


KB6NAX
 

Yes, using a current source is the best way to go. You set its
compliance to the rated voltage of the capacitor and its current to a
value that will not produce excessive dissipation in the cap at full
voltage. Problem is, most reforming goes on with caps still in the
equipment and any one of several or more caps can be leakers that
need reforming time. The nice thing about power supplies with vacuum
tube rectifiers is that when the rectifier's heater is running at
substantially lower voltage the cathode is emission limited and the
plate resistance is high, providing you with a nice current limiting
resistor. Still, a variac is the simplest way to effectively control
the reforming process. Never run the voltage up to fast if you don't
want to ruin a potentially good cap and make yourself more work.

Arden