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G8MNY > TECH 31.03.06 13:21l 78 Lines 3287 Bytes #999 (0) @ WW
BID : 23934_GB7CIP
Read: DL1LCA GUEST
Subj: Reforming Caps
Path: DB0FHN<DB0MRW<DK0WUE<DB0RES<ON0AR<GB7CIP
Sent: 060331/0912Z @:GB7CIP.#32.GBR.EU #:23934 [Caterham] $:23934_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Nov 05)
(8 Bit ASCII Graphics use code page 437 or 850)
Electrolytic capacitors a 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_____
Electrolyte
ÍÍÍÍÍÑÍÍÍÍÍ Oxide
³ + Layer
This thin layer is the reason the capacitance value is so high compared to
other types & its thickness is proportional to the operating voltage.
NEED FOR REFORMING
The problem is that over time the formation layer shrinks & the capacitor will
need reforming. 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 .-" ³ /³
³ / ³ / ³ / ³
³/ ³ / ³ / ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>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 is too low much
higher currents will flow & heat up the capacitor. To reform the layer the
capacitor must be discharged. Hence on unused kit it is wise to power up
briefly for a few seconds, then let the HTs drop to zero & re-power.
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´>ÃÄÄÄÄÄÄÄo | oÄÄÄÄÄÄÄÄÄÄÄÄ¿
AC ³ ³ ³ ³
1Mê ³ + ÃÄÄÄÄÄÄÄ¿ ³
ÀÄÄÄÄÄÄ´ CAP __³__ _³_ (X) 2x 15W
_³_ 220uF ÄÄÂÄÄ / VOLT\ ³ 240V
/_\ 400V ³ \METER/ (X) LAMPS
NEUTRAL ³ ³ ~³~ ³
ÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÙ
The 1uF limits the charging current, 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 the charging current until there is very little rise in the voltage, then
discharge.
Volts Reached Formed to
³ rated V _ +10%
400v´ __ .ÄÄ¿ / ³
³ _..ÄÄ¿ .- " ³ / ³ / ³
³ ,,/ ³ / ³ / ³ / ³
³_,,/'' ³ / ³ / ³ / ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>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 about 10% higher than the rated
voltage is achieved. Anymore will risk insulation failure & or drop in the Cap
value.
Why Don't U send an Interesting Bul?
73 De John, G8MNY @ GB7CIP
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