How to Make a Softstarter

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About: Awesome Electronics Tutorials, Projects and How To´s

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!

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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:

Amazon.com:

1x 12V Relay: https://amzn.to/31a5hsS

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

1x 1N4002 Diode: https://amzn.to/2YJ2eKw

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

1x 10µF Capacitor: https://amzn.to/2YH2ntX

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

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

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    20 Discussions

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    Udon

    4 weeks ago

    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?

    2 replies
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    frarugi87Udon

    Reply 18 days ago

    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)

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    Udonfrarugi87

    Reply 18 days ago

    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.

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    jack tech

    4 weeks ago

    this is important!!

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    DaviDBCoe

    5 weeks ago

    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)?

    1 reply
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    greg10091969DaviDBCoe

    Reply 4 weeks ago

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

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    rwlc

    5 weeks ago

    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!

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    Antzy Carmasaic

    5 weeks ago

    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.

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    Malek_GC

    5 weeks ago

    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

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    gpietraf

    5 weeks ago

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

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

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    TonK5

    5 weeks ago

    This is also usefull for mains applications, I owned a 240V transformer based stick welder which always blew an electronic main cct breaker at power up due to an excessive run in current. Old style melting fuse was not affected since they are "slow blow"

    Only a few modifications are needed: forst and foremost, it needs a plastic housing! also you may want to use a 100 W / 10 Ohm load resistor (with additional cooling, just mount it/them on a metal plate inside the housing). Also, use a timing capacitor that provides about 1.5 seconds delay. (100 Watt is OK since the resistor just needs to reduce the initial peak, after which the current will drop to the idle current) hwich should be about 2-3 Amp. Start welding after the relay has shorted the resistor out....you may add a green LED in parallel with the coil with a suitable series resistor (820 Ohm will give you about 12 mA current through the LED) as an indicator.

    Important: all circuitry needs to be double isolated and therefore be build inside a plastic housing including the now required additional 12V power supply A simple mains plug type adapter mounted inside the box (with i.e. velcron) would work just fine. Use all nylon screws for mounting the PCB etc to prevent a loose wire providing a live voltage to anywhere on the outside of the housing. Furthermore, use double isolated material for i.e. the main power switch on the front of the housing. note this switch replaces the power switch on the welder which now should be "always on" .
    Also, make abolutely sure the construction is in line with the applicable code for electrical apparatus in your country.

    The relay needs to be suitable for your mains application. Often, an old style car Main light relay 25...40 Amp would do nicely since its contacts typically open wide enough for a mains application and the coil and its connections are generally well isolated from the (Live!) switching contact(s). (>= 6 MM is the norm here). Since a power plug type power supply is used and all is inside the insolated housing, it is not extremely important, just an added safety measure...

    Check your own national situation when in doubt or use a relay which has been designed for such application (12 V coil/ 240V AC / 25 Amp contacts).

    Disclaimer: Since the practical construction is beyond my control, following these suggestions will be solely at your own risk!

    When done right it will be a very usefull box in particular if the cct breakers are like 50 M away from your workshop (and also protect the TV set.)....

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    bobang2378

    5 weeks ago

    What if you tried attadhing the positive termital to the inverter first?

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    jeff.verive

    5 weeks ago

    Nice circuit - very well thought out and presented just as nicely. Kudos!

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    tytower

    5 weeks ago

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

    3 replies
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    nic.bryan.73tytower

    Reply 5 weeks ago

    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).

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    Flyboyrontytower

    Reply 5 weeks ago

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

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    Orngrimmtytower

    Reply 5 weeks ago

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

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    OscarM181

    5 weeks ago on Introduction

    Excellent all round description!!! Great presentation and clean photography. Thank you!

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    UncleEd

    5 weeks ago

    Thank you. Been doing electronics since long before semiconductors and had no idea what the term meant. Maybe we geriatrics need more consideration than I would have thought.