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Soft Start Circuit 2

In Soft Start Circuit Board, there are many different parts. For HIFI fans, this means bad sound ... ...

Less is more ... I will show you a simple, inexpensive Soft Start Circuit. The one and only one drawback is ... ... you have to control it manually each time you switch on the equipment.

For the 220V a.c. supply at home, the rated overload current of the MCB is 10A, 20A or 32A. Therefore, we have to keep the starting current of the equipment, e.g. transformer, within 10A.

Moreover, smaller current can prolong the life of the switch, give you a safer circuit, use cheaper parts ... ...

I = V / R

  • I - current
  • V-voltage
  • R-resistance

R = V / I
modified to R must be larger than V / I, R > V / I

R > 220V / 10A

R > 22 ohm

In other words, if a 22 ohm resistor connect to the equipment in series, the current will be no more than 10A and the MCB will not trip.

Of course, we can use a larger resistor because we can further reduce the current and use a High Resistance, Low Wattage resistor.

Soft Start Circuit schematic diagram Schematic Diagram


50W 8 ohm high power resistor For this example, I use 8 ohm 50W resistor x 4. So, R1 + R2 becomes 8 ohm x 4 = 32 ohm.

I = V / R

I = 220 / 32

I = 6.3 A ... this is acceptable for the MCB rated as 10A

As mentioned above, there is one drawback, you have to switch on the Transformer ( equipment ) following the procedures:

  1. Close S1

  2. Close S2

  3. Open S1

Open S2 for switching off the equipment

If you accidentally Close the S2 first, the MCB will trip because the resistance of the Transformer at the beginning is nearly ZERO, this forms a "short-circuit", very very large current.

Although there is a drawback, inconvenience, there is one very good reason to use this Soft Start Circuit rather than the one I introduced to you before.


  • Cheap, you can lower the cost by using High Resistance, Low Wattage resistor ... about US$ 1 for Qty. 100, rather than 50W 8 ohm resistor ... about US$ 3 for Qty. 1.

  • Better electrical contact, you can use HIFI grade switch for S2, rather than the Relay

1/6 W resistor, US$ 0.3 each

resistance from 1 ohm to 2.2M ohm

2.2 M ohm = 2,200,000 ohm