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G8MNY > TECH 09.03.13 19:28l 99 Lines 4606 Bytes #999 (0) @ WW
BID : 14573_GB7CIP
Read: GUEST
Subj: Reforming Caps
Path: IZ3LSV<IK6ZDE<VE3UIL<GB7COW<GB7CIP
Sent: 130309/0835Z @:GB7CIP.#32.GBR.EU #:14573 [Caterham Surrey GBR] $:14573_GB
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Jul 07)
(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 -
_____³_____
Electrolyte
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 in a porous
tissue used to initially keep the foils apart. If the electrolyte dries out the
capacitance value often decreases by >100 times.
NEED FOR REFORMING
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. This 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 quicker.
Volts Reforming_ flash _flash . flash
³ . - ~ ³ 1 .-~ ³ 2 / ³ 3
³ ./' ³ / ³ / ³
³./ ³ ./ ³ ./ ³
ÀÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄ>Time
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.
POWERING UP REFORMING
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.
REFORMING CAPS
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.
240V 1uF 400V Charge Discharge
MAINS ÄÂÄÄ´ÃÄÄÂÄÄÄ´>ÃÄÄÄÄÄÄÄÄo | oÄÄÄÄÄÄÄÄÄÄÄÄ¿
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
ÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÙ
The 1uF limits the charging current to a max of 80mA, but the peak voltage can
be +680V, 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.
REFORMING
Apply charging current until there is very little rise in the voltage (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
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄ>Time
C a p v a l u e d e c r e a s e s ->
Repeat the charge & something like a 10% voltage increase will be seen for each
charge discharge cycle. 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.
NON POLAR TYPES
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, but using them
on AC solves this. Typicaly 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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