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AudioSan 10th December 2012 09:26 PM

simple soft start
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hello. i need a simple soft start around a CL-60 NTC.
i was thinking RC. is it possible to do it this simple? and how to calculate the delay?
and do i need to break the flow from IEC to NTC after the rel่ kicks in?

metalsculptor 11th December 2012 06:21 AM

No need for C1 the filter capacitors serve that purpose, even if they are on a different transformer winding, inductive coupling makes them load the primary which affects all other windings.

AndrewT 11th December 2012 11:13 AM

You need to ensure the relay pulls in when mains voltage is at it's lowest.
You need to ensure the components do not overheat when the mains voltage is at it's highest.

That pair of operating conditions makes a simple RC almost impossible to work reliably.

Use a proper timer. Either a 555, or a regulated supply to an RC, to remove the effects of voltage changes from the RC.

gmphadte 12th December 2012 04:24 AM

Use CD4541 and a relay. Has everything u need for such an application.

Gajanan Phadte

metalsculptor 12th December 2012 08:15 AM

If a relay were not able to operate reliably at its rated voltage with the usual industrial supply tolerances it would be of little use for its primary application. They do work OK when used to shunt a start resistor provided the resistor is low enough to achieve the pull in voltage of the relay ( 70% of rated voltage for many relays) Using an NTC start resistor of correct size makes it easier to achieve the relay pull in voltage Time is not the variable you are trying to control it is the filter capacitor voltage, and the time which this takes to reach say 80% of design voltage is not constant, it depends on mains voltage There is nothing wrong with time based starters, the majority of motor starters are time based even though they are controlling motor speed and the minor changes in pull in speed with changing mains voltage does not affect them much. As for the suggestions regarding timers, where is the power to run these going to come from if the system transformer has only just been energised and has not reached full voltage because the filter capacitors are charging?

gmphadte 13th December 2012 04:46 AM

U need to use an additional small psu or an additional winding on the main transfomer if available for the timer circuits.

Gajanan Phadte

Bibliophile 13th December 2012 05:10 AM

It seem to me that, at best, the relay may operate briefly as C1 charges.
The time will depend on the values of C1 & R1.
When C1 has charged, the relay will drop out.
Have I missed something?

metalsculptor 13th December 2012 09:48 AM


Originally Posted by gmphadte (
U need to use an additional small psu or an additional winding on the main transformer if available for the timer circuits.
Gajanan Phadte

The problem with using an additional winding to power the timer is that there will be no power available to run the timer until the filter capacitors are fairly well charged then it depends on the timer starting properly with a rising supply voltage, you might as well skip using the timer and use the relay direct and achieve the same thing in less time.

Using a separate transformer just to power a soft starter seems extravagant.

@ Bibliophile That's about it, the original circuit with the capacitor in series will not work as intended and will only give momentary operation of the relay if supplied from a DC source. Remove the resistor and capacitor connect the coil of a 12V AC relay across the 12V winding and it will work as originally intended

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