How to Make a Softstarter

In this small project we will be having a closer look at appliances that require a softstarter in order to properly work with a limited output current system. The shown appliances in this project include an inverter, a boost converter, a power supply and a motor. We will find out why a big inrush current requires a softstarter and how we can build a simple circuit that can do this job. Let's get started!

Step 1: Watch the Video!

The video gives you all the information you need to create your own softstarter. During the next steps though I will present you some additional information.

Step 2: Order Your Components!

Here you can find a parts list with example seller (affiliate links):

Aliexpress:

1x 12V Relay: https://s.click.aliexpress.com/e/_dXD9RFD

1x Power Resistor: https://s.click.aliexpress.com/e/_dYcUD15

1x 1N4002 Diode: https://s.click.aliexpress.com/e/_d7kxFwJ

1x IRFZ44N MOSFET: https://s.click.aliexpress.com/e/_dZR4bLh

1x 10µF Capacitor: https://s.click.aliexpress.com/e/_Bf6I4EJN

2x 100k Resistor: https://s.click.aliexpress.com/e/_dWqBQrV

Ebay:

1x 12V Relay: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x Power Resistor: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x 1N4002 Diode: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x IRFZ44N MOSFET: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x 10µF Capacitor: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

2x 100k Resistor: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

Amazon.de:

1x 12V Relay: https://amzn.to/2Yq6UWs

1x Power Resistor: https://amzn.to/2YG6AlO

1x 1N4002 Diode: https://amzn.to/2Yq7pQk

1x IRFZ44N MOSFET: https://amzn.to/2GYPCFh

1x 10µF Capacitor: https://amzn.to/2GJajoc

2x 100k Resistor: https://amzn.to/2YJXhkG

Step 3: Build the Circuit!

Here you can find the schematic and pictures of my finished board. Use them as a reference while creating your own softstarter.

Step 4: Success!

You did it! You just created your own softstarter!
Feel free to check out my YouTube channel for more awesome projects:

http://www.youtube.com/user/greatscottlab

You can also follow me on Facebook, Twitter and Google+ for news about upcoming projects and behind the scenes information:

https://twitter.com/GreatScottLab
https://www.facebook.com/greatscottlab

Comments

[codebeat]

Hai Scott, nice circuit but there is a design issue. When you turn the board on you can hear the delay click. However when you turn it off and immediately after off turn it back on, there is no delay (relay turns on immediately). I think it has something to do with C1 that still holds a charge. How to prevent this?

[dave.ramsey]

Hi Scott, can your softstart circuit be modify to control inrush of a 2.2kw DC starter motor which draw about 1600amp .
Regards Dave

[bllourias]

Do you know how i can get a dchematic of an inverter? Mine got shorted and 2 MOSFETs appear to have been fried. I already have the resistors. However, I do mot know enough about MOSFETs to oder the replacements. Any help or suggestions that you might have is greatly appreciated.

[tytower]

But what is a soft starter used for . Why is it needed?

[nicholas.skram]

Its main purpose is to limit Inrush Currents when a system is switched on. These currents may reach several times higher magnitudes than a systems Rated Current, depending on its supply circuitry (see attached image). If you look at trip tables for automatic fuses, you'll see different characteristics/tolerance for magnetic failure caused by Inrush Currents.

The soft-start allows the system to "slowly" build up momentum to avoid spikes in the drawn current at startup.

[nic.bryan.73]

Stuff like compressor motors want a lot of amps at start-up, and if the power supply isn't capable of providing the start-up amps, it needs to force the motor to spin up slower, in a way that keeps it from blowing the breaker. (Or toasting the battery).

[Flyboyron]

That information is in the video. But Orngrimm's answer is correct.

[Orngrimm]

Slow start a Motor, Slow-Ramp up of a incadescent bulp to limit Burnouts and Burnout-Rate

[ajayinstruct]

Hello brother good work, how to implement it on a AC Motor load

[Udon]

Will this really work? In my mind, a soft-starter was something that would ramp up a PWM signal to the gate/base of something made of silicon, and slowly allow more current to run through till the normal value was achieved. In other words, I'm kind of surprised and confused at seeing a relay. Isn't it way too slow for being used in something like this?

[frarugi87]

From what I see, this circuit is used to limit the inrush current only.
For instance, an inverter may have a very big input capacitor. When you connect the power, the current can be very high (even tens of amperes). When you put this circuit, the resistor is put in series with the load; at turn on the current is limited at 12V/3.3Ohm = roughly 4 A. This current slowly drops, due to the fact that the load's voltage slowly increases. After some time (selected by C1, R2, R3, and is around 0.5-1s for these values) Q1 starts conducing, and so the relay closes, bypassing the resistor (and so avoiding wasting power on it)

[Udon]

Ah, I see. That makes more sense. So speed isn't important, we just wait until (our throttled) device current has normalized, then the relay switches the device on normally.

Thanks for the explanation, 87' bro!
(an excellent year, if I may say so myself)

This circuit is of great interest, as I built a high-voltage power supply that has an issue with sketchy startup.

The device contains many capacitors, big and small, and so inrush current is quite bad. Seeing as it goes through the main inductor, the result is significant overvoltage (target is 400 V, actual voltage is.....?). This big voltage then just sits there on the big capacitor and doesn't go away. So it not only messes up the regulation (which is debatable anyway, seeing as its a multiplier...), it might be damaging the load on each power up.

The above circuit might be the solution.

[jack tech]

this is important!!

[DaviDBCoe]

I'm mightily confused here.

What is the large aluminum-housed circuitry for? The Instructable shows that large unit in many of the photos. What is it and how does it connect to that very, very small circuit that the Instructable adds to that larger device (whatever it is)?

[greg10091969]

It is an inverter for converting 12 volts to 240 volts I believe.

[rwlc]

This is a fascinating and very helpful project, Our 240V fridge will often turn the inverter off when the battery voltage is close to the minimum set on the inverter: Sometimes I think the same thing described here is the problem. Thank you very much for the stimulating tutorial!

[Antzy Carmasaic]

Nice project and explanation. I was thinking that you could reverse the RC-Mosfet circuit to make it switch on the relay when power is switched on, and cut off the power when the capacitor charges after 1 second. Then connect the power resistor to NO instead of NC of relay. This way the relay is only energized in the starting 1 second and you save the 0.5W power which is being wasted.

[Malek_GC]

Anyone else is having difficulties simulating this in a simulator?
I am not saying that it doesn't work, I'm just having trouble simulating and want to know if my simulator is bad

[gpietraf]

Very nice design! Can I use this circuit to soft start a Circular Saw (1400 watts)?

[nearly_trainable]

Worth watching for the great soldering technique alone! I had no idea you could "build your own PC board!"