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G8MNY  > TECH     20.01.22 10:24z 104 Lines 4879 Bytes #26 (0) @ WW
BID : 58772_GB7CIP
Subj: Reforming Caps
Sent: 220120/1020Z @:GB7CIP.#32.GBR.EURO #:58772 [Caterham Surrey GBR]

By G8MNY                                   (Updated May 15)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Electrolytic capacitors are formed by repeatedly apply a formation voltage that
corrodes the anode of the pure aluminium foil, to make a very thin insulating
oxide layer.
               ³ -
          ÍÍÍÍÍÑÍÍÍÍÍ Oxide
               ³ +    Layer

This thin layer is the reason the capacitance value is so high (1000x) compared
to other types of the same size. The oxide layer's thickness is proportional to
the operating voltage. The electrolyte is a WET salt solution (Borax) in a
porous tissue used to initially keep the foils apart. If the electrolyte dries
out the capacitance value often decreases by >100 times & the ESR (Equivalent
Series Resistance) goes very high too. Failed capacitors often look pregnant
after high pressure has occurred.

The problem is that over time if the capacitor is not exercised to the
formation voltage, the layer shrinks in thickness & the capacitor will need
reforming to safely work at its rated voltage. Failure to do refoming means DC
current continues to pass & the capacitor heats up & may explode!

Noticeable reforming occurs in some applications like a photographic flashgun
every time the capacitor is 1st used after a few days, & it takes much longer
to charge up as formation current is flowing. After each flash the recharge is

Volts  Reforming_ flash     _flash      . flash
    ³     . - ~  ³ 1     .-~ ³ 2       / ³ 3
    ³  ./'       ³      /    ³        /  ³
    ³./          ³    ./     ³      ./   ³

But for larger power capacitors in PSUs, the formation voltage will be some 10%
greater than the rated voltage, so when the formation voltage (voltage the
oxide layer will withstand) is too low, much higher currents will flow & heat
up the capacitor. To reform the layer the capacitor must be charged &
discharged sometimes repeatedly. Hence on unused kit it is wise to power up
briefly for a few seconds, then let the HTs drop to zero & re-power.

If kit has not been used for many years, then some reforming can be done by
briefly repeatedly powering on (until HT appears) for a few seconds, then
powering off for a few minutes. Do this a couple of times for each year of non
uses & you will avoid the capacitor or valve rectifier blowing up.

For new caps that may have been stored for a while the same applies. Here is a
reforming circuit I have used for high voltage electrolytics.

230V    1uF 400V        Charge   Discharge
 AC    ³      ³  1N4007        |              ³
       ÀÄ1MêÄÄ´              + ÃÄÄÄÄÄÄÄ¿      ³
              ³        CAP   __³__    _³_    (X) 2x 15W
       1N4007_³_       220uF ÄÄÂÄÄ  / VOLT\   ³  240V
             /_\       400V    ³    \METER/  (X) LAMPS
NEUTRAL       ³                ³      ~³~     ³
The 1uF limits the charging current to a max of 75mA, but the peak voltage can
be +650V, so an eye on the meter is needed! THIS IS A HIGH VOLTAGE MAINS
CIRCUIT! Take safety precautions!

For lower voltage high value Caps, use a variable voltage bench PSU to do the
same thing.

Apply charging current until voltage rise nearly stops (e.g. current stays
flowing), then discharge the capacitor.

Volts                                          Reached        Formed to
    ³     Charge Discharge Cycles               rated V          __ +10%
400V´                             ___           .ÄÄÄ¿         .ú'  ³
    ³            ___         _.-"~   ³       .-'    ³       ú'     ³
    ³      _.,-'~   ³      -'        ³     -'       ³     ú' Rated ³
    ³_.,-'~         ³   .-'          ³   -'         ³   ú' C value ³
             C a p   v a l u e   d e c r e a s e s  ->

Repeat the charge discharge cycles & something like a 10% voltage increase will
be seen for each. Repeat the cycles until the voltage is about 10% higher than
the rated value is achieved. Anymore than that will risk insulation failure &
or a drop in the rated capacitance value.

These are really just 2 electrolytic capacitors in series in one can. So a
middle foil is needed in manufacture to make the 2 caps, but not wired to the
outside world. Because of this no direct reforming can be done again, but using
them on AC solves this, as long as the do not get hot! Typically used in a
rotator controller, for low voltage motor phase split capacitors that must have
a high C value, as the voltage is low.

Why Don't U send an Interesting Bul?

73 De John, G8MNY @ GB7CIP

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