Make a soft starter for AC loads

Published by MKDas on

For large inductive AC loads, a soft starter is a very important circuit. A soft starter reduces the initial power supply to the load and potentially reduces electrical and mechanical shock to the device. Specially motors. The soft starter also reduces the spike in the initial current to any kind of AC load. The power is slowly provided to the load which reduces stress to the load. In this article, we’ll learn to make a soft starter for an AC load.

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Use of soft starters:

When a large load is connected to the supply, a surge current flows to the load. This surge current is much higher than the normal load current. Specially for inductive loads. When an inductor is cold or not energized, it works like a short circuit. Imagine an inductor like this:

If AC supply is supplied to this inductor what will happen? An inrush current which is known as transient current too will flow through this inductor. Why? When the inductor was not energized, it was cold. In a cold state, the resistance of the inductor is very low. That is why, when the supply is turned on, an inrush current flows due to this low resistance path. But when the coil is energized due to magnetic flux, it draws only the steady current which is pretty low. Read mode about soft starter from Wiki.

This inrush current can damage the system as well as the connecting cables too. That is why for large inductive load or in some other types of large loads a soft starter helps a lot to reduce the inrush current.

For inductors, the soft starter performs the best.

Types of soft starter:

Soft starter can be different types. Some old system use variable transformers and gear motor mechanism to make a soft starter. Some system use TRIACs, some system use resistor and relays. Based on configuration, soft starter can be divided into:

  • Electro-mechanical soft starter
  • Thyristor based soft starter
  • RLC delay switch-based soft starter

Electro-mechanical soft starter:

This technique is very old and used in old systems when electronics were not so familiar. In this type of system, they used variable transformers. Which worked like a voltage-controlled soft starter.

Variable transformer

Nowadays, we know this as a variac. Think about this large bulky device which was used in old-time as a soft starter. Nowadays after electronics Thyristors are the most popular device for this purpose.

Thyristor based soft starter:

As you learned to make an AC dimmer circuit in my previous article so you know how an AC dimmer based on thyristor works. If you missed, I’ll request to read that again from here: “AC dimmer circuit with TRIAC“.

Now you know how a Thyristor works for dimming purposes. This AC dimmer technology can be used as a soft starter circuit. In soft starter, the load is driven from maximum firing angle to provide the lowest switch state, and then gradually angle is reduced to zero to full switch.

Firing angle control of thyristor to drive load

This is a very simple technique to make a soft starter. Modern soft starters are designed based on this technique. Larger the capacity, some other features are added like short circuit prevention, phase failure, earth fault etc.

A modern soft starter based on Thyristor firing angle control is:

Although 3 phase induction motors Star-Delta (✩-△) method is used some AC loads have to be used electronic soft starter especially large loads.

RLC delay switch based soft starter:

This is the easiest one as a soft starter. A resistive (R), inductive (L) or capacitive(C) device is used in this type of soft starter circuits. For small loads, this type of soft starter is very popular. Based on load nature, R or L or C is used. I’m telling something like this one:

Resistive soft starter switch

Or this:

Inductive soft starter switch

Or even this:

Capacitive soft starter switch

The last one is not so popular for soft starter, and using an inductor across the switch is not good either. Because a large inductor have to be used as the soft starter. But it is used in high current cases specially in SMPS based circuits. It works like a mechanical spring absorbing vibrations.

The most popular one in this type is using Resistor across the switch and a delay circuit. Using the resistor makes the soft starter small and cost-effective too.

Now a delay switch will make the resistive soft starter complete.

Circuit diagram for resistive soft starter:

It is a very small delay switch circuit with a resistive soft starter. The C1 capacitor and R1 charging resistor form a RC timing circuit which is creating a delay time. This delay time can be calculated by

Delay = 1.1RC. But the best suggestion is to use a variable resistor in the position of R1. Then tune as your required time. The R3 resistor works as the discharging resistor for Capacitor C1. When the power switch is turned off, this discharging resistor discharges the capacitor C1. There is another resistor(R4) across capacitor C2. It is also used to discharge the capacitor C2.

So why we need to use discharge resistors across the capacitor? Imagine, when power is off and just within seconds power is back what will happen? Our delay switch will not work. It will be turned on directly because the capacitors are already charged which will turn the transistor on. But if we discharge the capacitors, this will not happen. Each time power is cut, our delay circuit will work.

Yes, if power returns in seconds it will take less time than the first time but the delay will be sufficient to form our soft starting circuit.

Resistive soft starter test:

Here in simulation in proteus, you can see how the delay works.

This one is a very simple one but it helps a lot in various inductive loads to turn on softly. Now, we can make it in a more controlled way using our dimmer circuit mechanism. If you missed that article please visit: “Make an AC dimmer with PIC12F675 and TRIAC“.

Soft starter controlling firing angle of TRIAC:

As we know that, we can control the output of a TRIAC controlling its firing angle and deliver the required output (a % of input) to the load. If we use this firing angle in a way that delivers the lowest power to the load in the beginning and then gradually increases the power to full by a certain time (around 2/3 seconds or 5s max) then it will work as a controlled soft starter.

So why it is called a controlled soft starter? Because we can control each step of it. If we want to control our time to full power, we can. If we want to deliver a certain power, we can. Even if we want to integrate a current or voltage sensing, we can. That is why it is called a controlled soft starter.

Circuit diagram of controlled soft starter:

Here, the transistor Q1, resistors R4 & R5 are used for the zero-crossing detector circuit. The signal is feed to the INT pin of the micro-controller. The micro-controller is generating a timing pulse for TRIAC which is delivered to the Gate of TRIAC through an Opto-coupler MOC3021. The delay time between the zero-crossing signal and the TRIAC firing signal is slowly reduced. Thus a soft starting is done.

Practical test:

Note: If your load is heavy and if you do not pressurize your TRIAC, you can use a relay across it. When a full switch is done, just turn on the relay. Then after a while turns off the gate signal of TRIAC. in this way, TRIAC will be long-lasting and will get a rest when the load is fully on.


In this article, we have seen how a soft starter is a work and we have made two types of soft starters. For small loads and where no controlled start is required, you can use the resistive delay switch-based soft starter which is very small and easy to make. And when you need a controlled start, you can use the TRIAC based controlled soft starter.

I hope this project was helpful to you. If you make one for yourself, it will be a great pleasure for me. Anywhere you need help, let me know. Please share this project and subscribe to my blog. Thank you.


Mithun K. Das; B.Sc. in EEE from KUET. Blog:


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