This is an old circuit which is still being used.

I am curious to understand that if 600 VAC is applied to AC side of the three phase rectifier which has diodes, and on the DC output side of the Bridge you have Capacitors connected so will these capacitors fight the Voltage rise on the AC input side of the Bridge as well ?? I am asking this because there is a coil connected to the AC side which when activated will remove 10 ohm 225 watt resistor by closing a contact. Surge-Suppression-Circuit


1 Answer 1


It's called inrush current limiting and works by allowing the capacitors to charge to close to their peak voltage value via the 10 ohm resistor. This limits inrush current when the capacitors are fully discharged. Once the output voltage is getting close to maximum, the relay contact shorts out the 10 ohm resistor. That relay coil will be slow acting but how slow will depend on the detail in the BoM and we don't have that data.

  • \$\begingroup\$ So the relay C1 must be slow acting type for proper functioning of the surge suppressor, if this is connected to the main AC bus will it protect the other equipment connected to the main bus from surges as well ? \$\endgroup\$
    – mxj262
    Commented May 2, 2018 at 11:26
  • \$\begingroup\$ It won't protect from surges unless the contact is open and it's only open briefly after power is applied. \$\endgroup\$
    – Andy aka
    Commented May 2, 2018 at 12:08

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