Power SuppliesRelays Circuit

220 V Mains Power ON Delay Circuit Diagram

Purpose of power ON delay circuit:

The power ON delay circuit provides a simple means to implement soft-start on heavy mains loads. Basically, this is achieved with the aid of high-power series resistors and a relay. The circuit also enables the load to be switched on with a small (light-duty) switch. Two relays are used to connect or bypass a series of power resistors inserted between the mains ‘live’ wire and the load. The ‘soft start’ is achieved by first connecting a resistor between the mains and the load, and then short-circuiting the resistor again so that the full mains voltage is applied to the load (equipment). The purpose of ‘soft’ switching is to prevent fuses blowing as a result of very high rush-in currents.

Description of power ON delay circuit diagram:

The relay coils are connected in series, and powered by the mains voltage. The reactance of series capacitors C1 and C2 keeps the coil current at about 27 mA, while the total direct voltage across the coils is 48 V. When the delay is switched on with S1, relay Re-2 comes on after Re-1 because its coil is shunted by two large electrolytic capacitors, which take some time to be charged from the current source formed by the mains and the series capacitors. When Re-2 is energized, its contact short-circuits the power resistors. The simplicity of the circuit gives rise to a minor disadvantage some time must be allowed between switching OFF and switching ON again. This is necessary to enable C4 and C5 to discharge.

power ON delay

Depending on the length of the switch-on delay required, the values of C4 and C5 may be increased a little. It should be noted, however, that the rush-in current is limited to about 5 A (at a mains voltage of 240 V) by R3-R6, and that the resulting dissipation equals about 1200 W in four 5-watt resistors! Therefore, when a longer switch-on delay is wanted, higher-wattage resistors (the PCB can accommodate fairly large types, which may be fitted vertically) must be used.

The current drain of the circuit is practically that of the two relay coils in series, that is, about 27 mA. The current through the neon lamp is negligible. The specified relays have contacts rated at 16 A. If there is reason to fear that the PCB mounted terminal blocks and their solder connections are not up to the load current, parallel-connected spade terminals (‘fast-on’ type) should be used instead.

Power ON delay Circuit Specifications:

Finally, the values of some components depend on the mains voltage and frequency as follows:

220V, 50 Hz: C1=330 nF; C2=220 nF

230 V, 50 Hz: C1 = 470 nF; C2 = not fitted

240 V, 50 Hz: C1 = 470 nF; C2 = not fitted

(Capacitor working voltage: 630 V d.c. /250 V a.c.)


110 V. 60 Hz: C1 = 680 nF; C2 = 470 nF: also Ri = 120 O.

(Capacitor working voltage: 400 Vd.c. /130 V a.c.)


This circuit is connected directly to the mains, and carries potentially lethal voltages. Never work on the circuit, or adjust it, while it is connected to the mains. Observe all precautions relevant to working with equipment or components connected to the mains.

Power ON delay circuit Parts list:


  • R1 = 270 Ω
  • R2 —1M Ω
  • R3-R6 = 10 Ω, 5W


  • C1 = see text
  • C2 = see text
  • C3 = 22 µF, 40 V radial
  • C4 = 1000 µF, 40 V radial
  • C5 = 470 µF, 40 V radial


 B1 = I3380C1500 (380 V piv, 1.5 A)


K1; K2; K3 = 2-way PCB terminal block, pitch 7.5 mm.

S1 = on/off; mains-rated 1 A.

Re-1 ;Re-2 = V23056-A0 I 05-A1 0 1 (Siemens) .

La1 = chassis-mount neon lamp with integral resistor.

4 spade (fast-on) screw-on type plug for PCB mounting.


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