Simple soft start for high power mains device

I have seen, but cannot find again, a really simple implementation of a mains soft start circuit. The thing I'm thinking of involves a power resister inline of the mains. When the device is started, current is restricted until a relay is energised, which shorts out the resistor allowing full power

The thing is, I can't remember (and so can't decide on) a good way to energise the relay after a second or so.

Does anyone know of a link to a reference implementation for such a circuit?

Cheers

• There is usually a reason to have such a device, such as large motor where the start current needs to be limited until the motor shaft is rotating. What type of load would this circuit have to deal with? – Sparky256 May 20 '16 at 17:25
• You are right, this is heavy machinery, inductive load. 1800w table saw motor – Wayne Booth May 20 '16 at 17:32
• The reason is the bulk cap, which will take hundreds of amps, if not limited. Inrush current can blow fuses (better case) or burn the device (worse case). But i don't understand the question, the solution is described inside it. – Gregory Kornblum May 20 '16 at 18:28

I have seen, but cannot find again, a really simple implementation of a mains soft start circuit. The thing I'm thinking of involves a power resister inline of the mains. When the device is started, current is restricted until a relay is energised, which shorts out the resistor allowing full power

Something like this: -

Or maybe like this: -

Or just use one of these: -

It's a thermistor that when cold has a resistance of tens of ohms and rapidly warms down to a low on resistance but it's slow enough to take the edge of the inrush current.

• Sorry Andy, but those tiny inrush limiters are for switch-mode power supplies. They would explode trying to start a 1800 watt motor, which consumes large current for a few seconds. In-rush limiters only expect to limit current for 1/10th of a second at most. R4 - R7 are probably 600 watts each, in case the motor has a slow start. I have installed these kinds of circuits, so I know you cannot cheat. – Sparky256 May 20 '16 at 17:44
• @Sparky256 what part of the question mentions an 1800 watt load? OK it's in a comment left after I answered however, I have furnished my answer with two relay circuits so what is the apology about? – Andy aka May 20 '16 at 17:51
• It was a statement, not an apology. – Sparky256 May 20 '16 at 17:56
• This is my work of @Andyaka circuit, apparently the same source: dropbox.com/s/kkc7y1fk9jhqikg/InrusLimiter2.brd.pdf?dl=0 dropbox.com/s/0y6ysv8ce16bysl/InrusLimiter2.pdf?dl=0 dropbox.com/s/ervdqgnmysgj9p6/InrusLimiter.brd.pdf?dl=0 – Marko Buršič May 20 '16 at 18:07
• @MarkoBuršič you've lost me on what you mean – Andy aka May 20 '16 at 18:14

[
This diagram is close but has too many parts:

K1 and K2 are terminal block - or large wire nuts. K1 goes to what should be a 20 to 30 amp 250 volt slow-blow fuse. Assuming Re1 has a 120VDC coil, R1=R2=R3 = 2.2K ohm 3 watts each. C1 can be omitted as can C2. B1 can be a bridge rectifier with a 1 amp 600 volt rating, or make one from 4 1N4007 diodes.

C3 can be 47uF 150VDC, enough so Re1 does not chatter. Increase C3 value if longer delay time is needed. Re1 should be a relay with a 120VDC coil (common) and contacts rated for 20 to 30 amps 250VAC. The high current rating will ensure a long life. R4, R5, R6, R7 should be 2 ohm 300 watt ceramic resistors mounted to a panel or on a fiberglass threaded rod.

They take some of the punishment if the motor stalls, giving time for the fuse to blow.

I would suggest 600 watt resistors but they get expensive (in case the motor is jammed and cannot rotate). These are all bulky brute-force parts designed to last even if the motor stalls. I suggest adding a thermal cut-off next to R4-R7 unless it is built into the motor. ]

Back when high-powered tube-based amplifiers were commonly used by amateur radio operators, it was common to control the main contactor (the one that bypasses the current-limiting resistor) using a thermal time-delay control relay.

There are several ways to do the delayed energize of the resistor bypass relay coil. One method is that there is a separate isolated power supply off the mains that is designed with its PWR_GOOD indicator status to come valid at the end of the 1 to 2 second delay period. This indicator signal can turn on a transistor that energizes the bypass relay coil using the voltage rail of this separate power supply to give power to the relay.

Another method is to design a direct off mains power circuit that is not isolated but can provide power to a 1 second delay circuit and then drive the relay coil.

Some products that need a soft start run on a off mains switch mode power supply. These products often have rectifier diodes right off the line and then the resistor to the main SMPS switcher curcuit operates as a DC rail. On these cases it is possible to use a FET to bypass the soft start resistor.