ARR Series Three Phase, Six Pulse SCR Charger
Circuit Descriptions
RS-420, Rev. 7
6-1
6.
CIRCUIT DESCRIPTIONS
6.1
BASIC CONTROL
The secondary winding of the auxiliary voltage transformer (AXF) supplies the AC voltage to a full-wave
rectifier composed of diodes D21, D22, D23 and D24 in the control circuit. This DC voltage is then filtered by
capacitors C23 and C24, and regulated by resistors R23 and R24 and zeners Z21 and Z22.
The secondary winding of the AXF also supplies the AC voltage to the phase control of the firing circuit. This
phase control consists of the saturable reactors L1 and L2; the diodes D25, D26, D27, D28; the resistor R19, and
the transistor Q11.
The zeners Z23, Z24, Z25, and Z26 provide a square wave input for the phase control circuit. This allows the
phase control to provide a large angle of retardation which is essential for operation close or down to short circuit
of the DC output; this also makes the phase control circuit completely independent of AC line changes.
A pulse of current is produced in the phase control network every half cycle at the moment when either L1 or L2
saturates. It passes through R22 and either D27 or D28, depending on which reactor saturates. The voltage drop
that this current produces across R22 makes the junction of R25 and R214 more negative. This turns transistor
Q21 off.
When transistor Q21 turns off it causes transistor Q22 to turn on. This allows capacitor C21 to discharge
producing a fast rising pulse in the primary of the pulse transformer applied to the gates of the silicon controlled
rectifiers (SCR). The SCR that is forward biased is turned on by this pulse.
When one saturable reactor is conducting, the other is being reset. The reset current path consists of diode D25 or
D26, the control transistor Q11 and the secondary winding of the AXF. The amount of reset current determines
when the reactor being set will saturate in the next half-cycle. More reset current will result in later saturation in
the next half-cycle. This means the SCR will also turn on later, which means reduced charger output. Less reset
current thus produces more charger output.
The amount of reset current is determined by the signal fed into the base of Q11. This signal is derived from a
network which senses any changes in the output voltage of charger. This network is composed of a voltage
divider, a reference zener, and a differential amplifier.
The voltage divider consists of resistor R12 and the float-equalize adjustment potentiometers. This divider is
directly across the output of the charger.
The differential amplifier consists of transistors Q12, Q13; resistors R13, R14, R111, R115 and R116; and zener
Z11.
The voltage across R12 is proportional to the output voltage and is set by adjustment of the float potentiometer to
equal the zener voltage of Z11. This balances the voltages at the bases of Q12 and Q13 so that the current in each
of the collectors is approximately equal. When the charger output voltage decreases the voltage across R12 drops.
Q13 is then turned on more. This causes Q12 to turn off so that the sum of the currents thru R115 remains
essentially what it was when the amplifier was balanced.
When Q13 turns on the potential at its collector is lowered. This turns off Q11 and results in less reset current in
the reactors. Hence the SCR’s are fired sooner which allows more current output from the charger. This, of
course, tends to maintain constant charger voltage output.
