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30A AC Soft Starter (Inrush Current Limiter)

by: Feb 02,2023 3787 Views 0 Comments Posted in Technology

By Hesam Moshiri, Anson Bao

Copyright: Attribution-NonCommercial-NoDerivs (CC-BY-NC-ND)

Are you tired of dealing with the damaging effects of inrush currents on your industrial devices? Look no further than an AC inrush current limiter (soft starter). Inrush current, also known as surge current, is the large amount of current that flows into a load at start-up. This can cause damage to equipment, reduce its lifespan, and lead to costly downtime. But with an AC inrush current limiter, you can eliminate these problems. Simply, a soft starter works by limiting the initial current flow, ensuring a smooth and efficient start-up, while protecting your equipment from damage.

So I decided to design this AC soft starter that is equipped with a fail-safe mechanism. During start-up, the inrush current passes through a power resistor, and after a delay (adjustable between 1ms to 1s), a 30A power Relay shorts the resistor and applies the full power to the load. If this Relay fails for whatever reason, the power resistor won’t melt everything; the logic circuit activates the fail-safe Relay that turns OFF the load to prevent any damage. 3 LEDs indicate the Supply, Normal, and Fault conditions. I selected the cheap ATTiny13 MCU as a controller.

To design the schematic and PCB, I used Altium Designer 23. The fast component search engine (Octopart) allowed me to quickly consider components’ information and also generate the BOM. To get high-quality fabricated boards, I sent the Gerber files to PCBWay. I used the Arduino IDE to write the MCU code, so it is pretty easy to follow and understand. 

Let’s get started :-)

Download the Gerber, Order the PCB or a Fully Assembled Board

Circuit Analysis

Figure 1 shows the schematic diagram of the device. As it is clear, the heart of the circuit is the ATTiny13 microcontroller [1].

Figure 1

Schematic diagram of the 30A AC Soft Starter (Inrush current limiter)

I start explaining the schematic from the top. AC-IN is the connector for the AC Mains input. R2 is the 10D5651K [2] varistor to eliminate voltage spikes at the input. U1 is the HLK-PM12 [3] AC to DC module that handles 12V at the output. This voltage is suitable to drive the Relays. R1 is a 5K potentiometer to adjust the delay. You can adjust the delay to something between 1ms and 1s. ISP is a male pin header for the AVR ISP programmer. IC1A and IC1B are parts of the ATTiny13 microcontroller. R3 is a pullup resistor for the RESET pin. C2 and C3 are decoupling capacitors for the VCC pin.

R4 and C4 build an RC filter at the input of the REG1. REG1 is the 78L05 SOT-89 5V regulator [4] to build a stable 5V power for the microcontroller. D1 is a 3mm yellow through-hole LED to indicate a proper supply voltage and R5 limits the current to D1. C5 and C6 are output capacitors to stabilize the regulator output voltage and reduce the noise. NTC is a 2 Pins XH connector to wire a 10K NTC to the board. R6 and NTC build a voltage divider, where voltage variations indicate changes in the temperature. If the temperature increases at the NTC side, the output voltage increases, and vice versa. C6 is a decoupling capacitor to reduce noise.

There are two Relay circuits here, both are identical. I explain one of them. Q1 is a Si2302 Mosfet [5] to drive the K1 Relay. R10 is a pulldown resistor for the Gate pin of Q1 to prevent unwanted triggering of the Mosfet. D2 is a 1N4007 diode [6] to protect the Mosfet against reverse currents of the Relay’s coil. D4 is a 3mm Red LED to indicate the fault condition. C8 damps the voltage spikes of Relay activation/deactivation. R9 is a 27R-20W power resistor to limit the inrush current, you can use other values depending on your application.

PCB Layout

Figure 2 shows the PCB layout of the design. It’s a two layers PCB board and I used a mixture of SMD and through-hole components. In my opinion, component placement is the first rule in a good PCB design. In this PCB, the separation between the power and logic circuits is clear.

Figure 2

PCB layout of the 30A AC Soft Starter (Inrush current limiter)


I use the Arduino IDE to write and compile the MCU code. I installed the MicroCore [7] in the library manager to be able to compile the code for ATTiny13. I have already provided the compiled HEX file which you can download from here [8]. The only thing you need is just connecting your programmer to the ISP header on the PCB board and program the MCU. Fuse bits must be set on the 9.6MHz internal clock, with no clock division (disable DIV8). 

If you plan to modify the code, it is as follows:

#define Relay1 1
#define Relay2 2

analog_pin_t potDelay = A3;
analog_pin_t NTC = A2;

unsigned int rawTemp = 0, rawPot = 0, delayms = 1;
unsigned char counter = 0;

void setup() {
  pinMode(Relay1, OUTPUT);
  pinMode(Relay2, OUTPUT);
  digitalWrite(Relay1, LOW);
  digitalWrite(Relay2, LOW);

void loop() {
  rawTemp = analogRead(NTC) + rawTemp;
  rawPot = analogRead(potDelay) + rawPot;
  counter ++;
  if (counter == 10)
    rawTemp = rawTemp / 10;
    rawPot = rawPot / 10;
    delayms = map(rawPot, 0, 1023, 1, 1000);
    digitalWrite(Relay2, HIGH);
    if (rawTemp > 800)
    counter = 0;
    rawTemp = 0;
    rawPot = 0;

void fault()
  while (1)
    digitalWrite(Relay1, HIGH);
    digitalWrite(Relay2, LOW);

Assembly and Test

Figure 3 shows the assembled PCB board. You should wire a small 10K NTC from the board and mount it on the power resistor. The best method to fix the NTC is to use some silicon glue (thermal conductive glue). Simply adjust the potentiometer for your desired delay. Pay attention to the status of 3 LEDs (Yellow, Green, and RED). Please watch the YouTube video to learn how to test/adjust the board.

Figure 3

Assembled board of the 30A AC Soft Starter (Inrush current limiter)

Bill of Materials

Figure 4 shows the bill of materials for the project. The Octopart website is not just a fast search engine for electronic components but also a very nice tool to build any kind of BOM for free.

Figure 4

Bill of materials for the 30A AC Soft Starter (Inrush current limiter) project


[1]: ATTiny13 MCU: https://octopart.com/attiny13a-ssur-microchip-77761976?r=sp

[2]: 10D561K MOV: https://octopart.com/mov-10d561k-bourns-19184788?r=sp

[3]: HLK-PM12: https://datasheet.lcsc.com/szlcsc/1909111105_HI-LINK-HLK-PM24_C399250.pdf

[4]: 78L05 SOT-89: https://octopart.com/ua78l05acpk-texas+instruments-525167?r=sp

[5]: Si2302 Mosfet: https://octopart.com/si2302cds-t1-ge3-vishay-43172315?r=sp

[6]: M7 Diode: https://octopart.com/m7-diotec-30502012?r=sp

[7]: Microcore: https://github.com/MCUdude/MicroCore

[8]: HEX File: https://drive.google.com/file/d/15XfKhBr-9R_P4GLeZIM4XyvKbay-QVaU/view?usp=share_link

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