Delay circuit for toroids


Connection delay of the mains power


    This unit is a delay unit that can be connected directly to the mains power supply. Itīs not obligatory to use one but it is a good idea, specially if you have a big toroidal transformer larger than 300 VA. This unit has a delay circuit and for the delayed time the mains power is supplied through power resistors minimizing in this way the big inrush current due to big capacitors and big toroidal transformers in the power supply. When everything is stable it shorts the power resistors and supplies the mains power directly.

    In the following schematic you can see the circuit. Itīs very simple and needs no voltage supply. 

    The inrush current is suppressed down to 5.5A through the resistors R4-R7 right after power up. After the delay time that is determined from capacitors C2 and C3 the relay shorts the resistors and all the current is supplied to the transformer. The relay in this circuit can work with toroidals till 2000 VA. The voltage for the supply of this circuit comes directly from the mains power supply through C1, R3 and B1 so it is very dangerous to touch this part of the circuit.

    In the pic below you can see what the PCB looks like. You can order these from Elektor magazine. The code is written on them.


List of parts

  • R1, R2                                   470K 0.25W 1% mf

  • R3                                         220R 0.25W 1% mf

  • R4, R5, R6, R7                      10R 5W

  • C1                                         330n 250V (for direct mains connection)

  • C2, C3                                   470uF 40V electr.

  • B1                                         B250C1500

  • Re1                                       24V (250V-8A contact)

  • F1                                          Depends on the amp