„Soft start“ transformer

This circuit is used to eliminate current surges that have inductive loads such as motors and transformers after switching on. Soft start can be used, for example, in an amplifier to switch the supply transformer. If you hit the peak amplitude, even a 500VA transformer can reliably equip a 16A circuit breaker.

Circuit design:

soft start transformer

The mains voltage is fed to the rectifier bridge M1, which rectifies the voltage and charges capacitor C2. In series with a rectified bridge, there is a capacitor C1, where capacitive reactance is used to limit the alternating current. Resistor R1 is used to limit the current surge at start-up, because the jumper is almost short-circuited at start-up. With the components listed in the diagram, the switch-on delay is approx. 300 ms; we will change it by modifying components C1 and C2. Zener diode DZ1 is used to limit the voltage on capacitor C2, according to which the relay coil is also selected.

During the charging of capacitor C2, the current passes through the power wirewound resistors R2 and R3 and the total value of 30R. The total resistance is suitable for 25-100R, at higher values ​​the resistors could be damaged. The value of the resistors can be adjusted according to the connected load, the total resistance of 30R I used to switch a 1000VA toroidal transformer.

An input voltage is applied to terminals K1, and an output device is connected from terminal K2. It is good to tin the force paths to increase the „cross-section“ of the conductor.

Placement plan:

soft start current

Printed circuit board:

softstart pcb

The installation plan and the PCB are 1:1 600dpi (for information on how to print the PCB, see the article How to print the PCB). The actual PCB size is 73.3 x 37.4 mm.

Part list:
R2, R310R/10W
C1470nF/275VAC (275VAC minimum)
C2220uF/35V electrolytic
M1DB107 (DIP package)
RE1G5L 24V
K1, K2PCB screw terminal, 2 pins, RM 5mm

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