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G8MNY > TECH 07.04.04 08:27l 136 Lines 6060 Bytes #999 (0) @ WW
BID : 51829_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Simple SCR Regulated Charger
Path: DB0FHN<DB0FOR<DB0SIF<DB0MW<DB0ROF<DB0ERF<DB0FBB<DB0GOS<DB0EEO<PI8DAZ<
PI8HWB<PI8WFL<ON0AR<GB7CIP
Sent: 040406/2311Z @:GB7CIP.#32.GBR.EU #:51829 [Caterham] $:51829_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (updated Nov 03)
This is a simple self regulating SCR controlled charger. It was made to give
27.5V at 15A for charging a train carriage's 500 AH 24V battery system. It is
not suitable for use with electronic equipment that can't operate on hummy
supplies! Or due to RFI used near Radio Rx equipment!
Using a dual winding transformer or a single one with 2 other diodes so the
SCRs form part of a bridge... Or 1 SCR arm following a bridge rectifier.
The transformer must deliver an RMS voltage slightly greater than the battery
voltage plus diode & wiring losses. If too low triggering the SCRs can be
difficult, but a 2nd 2v winding put around a toroidal/leg of the transformer to
feed the trigger circuit can keep the unit working well on low mains (the train
PSU had to deal with mains of less than 200V AC at times).
This circuit is very efficient, only the transformer heats up with the high
pulse current, & SCR losses, no half power lost in dropper regulator!
For high current battery work fusing can just be "weak" wire links mounted in a
safe way not to cause fires etc, but anything to do with batteries needs some
fuse protection!
For recharging 12V batterys use 14.5V DC setting, for float (infinite charge
rate) use 13.8V, with sealed types use 13.5V MAX.
SCR1³\³
AC1ÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ ÚÄÄÄÄ¿ ³/³\. ³
ÀÄÄ´>ÃÄÄ´470 ÃÄÄÄÄÄÁÄij>³ÄÄ¿ ³
ÀÄÄÄÄÙ ³ ³
³ ³
SCR2³\³ ³ ³
AC2ÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ÃÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄ´
³ ÚÄÄÄÄ¿ ³/³\. ³ o
ÀÄÄ´>ÃÄÄ´470 ÃÄÄÄÄÄÁÄÄ´>ÃÄÄ´ / CHARGE
ÀÄÄÄÄÙ _³_ o FUSE
/_\' ÃÄÄÄÄÄÄo/oÄÄ LOADS
LOW ³ ZENER ³ LOAD
oÄÄÄÄÄÄÄ´ ³+ FUSE
_³_ __³__
SET VOLTS NORM \ / ===
ÄÂÄ ³
ÚÄÄÄÄ> oÄÄÄÄÄÄÄ´ ³ BATTERY
³ _³_ ³
³ BOOST \ / ³
AC returns ³ o ÄÂÄ ³
ÄÄÂÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄ
__³__
/////
GATE DRIVE POWER
The SCRs gates are actually pulse driven as they only see current for a short
time when the battery voltage is below the zener voltage, until the SCR fires
(turns on). Then the AC voltage input will be reduced to that of the battery
turning off the gate drive. This is important as SCR gate dissipation is quite
limited, even on large current SCRs. The gate Rs (470) should experimented with
to make sure there is good triggering for your SCRs, a general rule is to make
them 1/2 to 1/4 of the value that just triggers the gate OK.
FIRING PROBLEMS
In practice this circuit generally produces a sort of phase firing, as the
battery voltage ends up as a saw tooth dropping blow the zener voltage some
time in the next half cycle. But if the SCRs fire unevenly sometimes due to
inductance problems in the transformer, severe transformer heating can result
due to DC situation of the core resulting in a very low inductance across the
mains, so a resetting thermal trip is recommended on the transformer, just in
case it gets unduly hot.
SIMPLE VOLTAGE CONTROL
The output voltage can be adjusted in 0.6V steps by the use of diodes in series
with the zener.
VARIABLE VOLTAGE CONTROL
The Zener can be a virtual one for voltage adjustment...
³
ÚÄÄÁÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ¿
³ c \³ POT
³+ ³ÄÄÄÄÄÄ>10K
100uF === e /³ ³
³ _³_ ³
³ /_\' ³
ÄVE ret ÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄ
The Zener can be 5Ä10V (12v system). The transistors is an NPN. Set the voltage
pot so that the Zener action is 1V higher than the required Battery voltage.
CURRENT LIMITING
current limiting can be added to either zener circuit..
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
c \³ ³
ÃÄÄÂÄÄÄÄÄÄ¿ ³
e /³ ³10uF ³ ³
³ === ³ ZENER
ÃÄÄÄÄÙ 1K ³ or Virtual
³ ³ ³ Zener
ÄVE SUPPLY ÄÄÄÁÄÄÄÄ\/\/\ÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄ Batt return
SHUNT
The transistor is an NPN. Make the shunt resistor (thick wire link eg several
inches of coat hanger wire bolted in place as it will get hot) so at the
maximum current wanted it drops 0.6V, enough to make the NPN conduct, & hence
reduce the zener voltage & thereby the current. The 1K & 10uF filtering is
needed so that the average current is controlled not the high peak current.
If the output is shorted this current limit may not be very effective depending
on the trigger threshold of the SCR etc. but it will reduce the current!
Note, if the shunt is put on the supply side of zener, the voltage loss across
it will not affect the output voltage, but if it is put on the battery side it
is added.
METERING
The Meter can be almost anything from 100uA to 1A movement for current
monitoring, provided it needs less than 0.6V drop, using the same current
limiter shunt. If a sensitive meter is used a switch can be provided to measure
either voltage or current.
SUPPLY -ve ÄÄÂÄÄÄÄÄ\/\/\/\ÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄBattery -ve
³ SHUNT ³
³ I + oÙI V
ÀÄÄRCalÄÄMETERÄÄ>oÄÄÄÄÄRCalÄÄZenÄÄRÄÄ>Batt +ve
V
Leaving the I RCal preset in line on voltage measurement will produce a small
error, but simplifies wiring. Calibrate the V & I meter scales with external
meter, adjust suitable presets for full scale. In the voltage case an
additional fixed series R can stop accidents! Also a 10v zener in series can
give a more useful offset scale eg 10-15V.
/QSL
73 De John, G8MNY @ GB7CIP
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