single phase soft starter circuit diagram and working

Some electrical component comes with inrush current problems (like motor, capacitor, buck/boost converter etc.). It basically means that these devices draw very large current during starting. Let’s say you have connected these devices with some safety circuit and power for power it up. But because of its inrush current as you power it, power supply cut the power due to over current. In that case we need to limit its inrush current. For that we need a circuit called soft starter. In this project I have designed a single phase soft starter circuit diagram. This means this circuit will work only for single phase appliances.

Inrush current graph

Limit the inrush current

For limit the current we can use resistor in series. It will limit the inrush current but it will create power loss. We need such resistor who decreases its resistance over time after powering up. Such resistor does exist and called thermistor.

It is a type of resistor whose value decrease or increase depending on temperature. If we connect it in series with device which cause the inrush current, it will limit the inrush current. But now when current is started flowing, it will heat up and decrease its resistance. So very less power will be lost.

But there is still one problem. Let’s say we have powered the device with thermistor previously and turned it off. Since thermistor heated up during previous power up, it will take some time to cool down. If we want to turn the device on again immediately, there will not any inrush current limiting because thermistor is still hot. If you are working with such a device which doesn’t need to power off and on frequently, you can go with the thermistor.

Single phase soft starter functional block diagram

So, we have to use the resistor with some additional logic. We need to design such circuit which let firstly current pass through resistor but after some time it should bypass the current flow.

Soft starter logic

Figure shown above is functional diagram of a single phase soft starter circuit. For switch we will obviously choose the relay by which we will short the resistor after 1 second. For turning relay ON we will use the MOSFET as switch. For 1 second delay we need a RC (resistor and capacitor) circuit with 1 second time constant.

Simple RC circuit

In this figure a RC circuit is shown.

RC time constant (T) = RxC

RC timing graph

As you can see from graph one RC time constant is the time at which voltage across capacitor is 63% of its input voltage. In above case voltage across capacitor after one time constant will be 5×0.63 = 3.15 volt. This time constant depends upon value of capacitor and resistor.

Single phase soft starter circuit

So, let’s build previously described circuit.

Soft starter circuit 1

We have created the circuit we discussed earlier. Here, to bypass current from resistor we have used the relay and to turn ON relay we have used the MOSFET as switch. To turn switch ON after 1 second we have used the RC circuit with 1 second time constant. Diode D1 is connected in parallel with relay’s coil to prevent the high voltage spikes at source side of MOSFET.

But there is still have a problem in this circuit which related discharging of capacitor. We need to discharge the capacitor every time circuit is turned off. Otherwise next time when we power the device relay will activate initially because capacitor holds it charge for long period of time. So, we need to add another resistor which will pull the capacitor to ground.

For build complete soft starter circuit we need following components –

  1. A relay
  2. Power resistor (10Ω)
  3. Diode 1N4007 or 1N4002
  4. MOSFET (IRFZ44N)
  5. 10 mF capacitor
  6. 2 Resistors (100kΩ)
Soft starter circuit

In this circuit as we have connected the resistor R3, it will create voltage divider with resistor R2. So, voltage across capacitor will reach up to 6 volt. Which is enough to activate the MOSFET.

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