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DC Motor Soft Start: Understand and Creating Your Own

One of the most significant disadvantages of electric motors is that they require a relatively high amount of power at start-up. At least compared to the amount of energy it needs to run during regular operation. If you're using an insufficient power supply, the DC motor may potentially overload it during start-up. The best way to protect your equipment is by implementing a soft start (or soft start-up/starter). The following guide will discuss what a DC motor soft start is and how you can build and implement your very own.


Why Do We Require Soft Starts?

RSBT Soft Starter RSBT Soft Starter Source: Wikimedia Commons


As with most things, motors require substantial energy to start. But once it starts and stabilizes, the amount of power consumption drops and becomes consistent. It's compatible with the momentum conservation principle. Nevertheless, this sudden sharp spike in power consumption during start-up may damage your switches and fuses. This degradation may not happen instantly. We can prevent and fix this by implementing a soft start circuit. 


How Does a Soft Start Work?

Digital Soft Starter with motor protection and built-in pass Digital Soft Starter with motor protection and built-in pass Source: Wikimedia Commons


The soft-start circuit (or soft starter) controls or modulates the DC motor current. It achieves this by minimizing the torque of the motor. Consequently, this results in a more controlled, smooth start-up where the soft start delivers current in phases.

Additionally, while most soft starters for pump motors tend to be mechanical, there are solid-state versions. They function in the very same way but through different mechanisms. In this guide, we'll be constructing such a device. 


Application for Soft Starters

A large industrial motor along with a hydraulic pumpA large industrial motor along with a hydraulic pump


One question you may be asking yourself is: "do we need soft starters for all DC motors?". The short answer is "no." It's only necessary for a few machines and electronics. DC motors don't explicitly require a soft starter to run. However, a soft starter or soft start circuit may help prolong your equipment. Thus, you should not feel any pressure to implement it into your machinery. 

Nevertheless, there is a slew of machines using DC motors that would gain from the inclusion of a soft starter. Of course, some engines may experience more significant benefits than others. Since some DC motors are more at risk of incurring damages or wearing out than others because of excessive amounts of current. This fact is most common in engines for:

  • Conveyor belts
  • Pump applications 
  • Large fans and cooling systems
  • Electric-based helicopters
  • Radiofrequency devices


Advantages of Soft Starters for DC Motors

Small electric DC motors

Small electric DC motors


We've established that DC motors do not necessarily require soft starters to function. However, besides prolonging your equipment, using a soft starter has other advantages. These benefits include: 

  • More efficient usage of energy for smoother operation
  • Reduced risk of power surges
  • Some soft starters allow you to control how quickly or how long you want the start-up to occur.
  • Soft starters may potentially increase the number of start-ups per hour
  • Reduces the risk of motors overheating during start-up
  • Using a soft starter optimizes overall motor and equipment operating efficiency


Designing and Creating a Soft Start Circuit

In this section of the guide, we'll cover how to design and build your soft-start circuit. The following project should be easy enough for beginners but sufficient for practical applications. 


Required Components

To construct the soft-start circuit for your DC motor, you'll need the following list of electronic components:

  • Power resistor (R1)
  • 12-Volt Static Relay (K1)
  • 10µF Capacitor (C1)
  • 100k Resistor x 2 (R2, R3)

If you cannot afford to use a 12-volt static relay, you can use a 1N4002 diode along with a 12-volt relay with mechanical components. Furthermore, you will require a 12-Volt power source to feed the circuit. 


The following list consists of tools you'll most likely need to compile this project. Which means you will depend on your experience and skill set. For instance, you don't have to use a soldering iron along with a solder. Instead, you can use wires. 

  • Work gloves

A technician with Work gloves A technician with Work gloves

Utility knife Utility knife

  • Ruler/measuring tape

Measuring tape Measuring tape

  • Flathead and star screwdriver

Flathead and star screwdriver Flathead and star screwdriver

  • Pliers

Pliers Pliers


Diagram, schematic

Description automatically generated

Explanation and Steps


Once you've ensured that you have the right components and equipment, you can use the following steps to assemble your soft start circuit:

  • Run a wire or solder connection from your 12-Volt power source
  • Next, connect the two 100K Ohm resistors in series from your power source
  • Ground the resistors
  • Next, create a parallel circuit and run a wire (or solder resistor) to the capacitor.
  • Ground the resistor
  • Next, connect the capacitor to the MOSFET
  • Ground the MOSFET
  • Next, run a connection from the MOSFET to the static relay
  • Finally, create a small parallel circuit with the power resistor and connect it to the output pins of the relay

If you have elected to use a mechanical MOSFET along with a diode, you'll need to click the diode in parallel to your mechanical relay. Furthermore, you must click the diode to the input pins of the relay.

Once you're done assembling the circuit, you may connect it to an inverter. You can then connect it to the DC motor. Alternatively, you can join the course to the DC motor. However, this setup would be less desirable. 



As an electronics engineer, creating new and innovative devices is essential. However, we can continually improve upon the design of old inventions. This is the purpose of soft starters. They slow down the spin-up of a motor to stabilize the current a power source delivers to it. It extends the life of your machines. In turn, this will save you money in the long run. While the project we've included in this guide is simple enough to construct, you can clean it up by adding a PCB or breadboard.

Nevertheless, remember to be careful when working with electronic components. As always, we hope that you've found this guide to be practical. Thank you for reading.