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ARR Series Three Phase, Six Pulse SCR Charger
Circuit Descriptions
RS-420, Rev. 7
6-8
The “Soft-Start” circuit consists of capacitors C11 & 14, diode D14, resistors R112, 114, 119, & 120, and
transistor Q14.
On turn-on, C14 is instantly charged to full voltage. This decreases the base voltage of Q13 which turns off and
increases the base voltage of Q11. Q11 then conducts full reset current to retard SCR firing.
Since C11 charges from the same source, the gradual build-up of its voltage turns on Q14. When Q14 conducts, it
reverse biases D14 which allows the base voltage of Q13 to be established at the level determined by feedback
thru R111.
One very important feature is provided by the use of R112 and C14 which are connected from the collector to the
base of Q13. This is sometimes called a snubber or anti-hunt circuit and is a useful method of eliminating
oscillation.
With this so called snubber it is possible to operate the charger as a battery eliminator without making any
modifications, either to the power or control circuit.