Soft start for sub 300 watt units

nightanole · 30th December 2009, 03:33 PM

Im sorry for asking. Ive looked and looked for a simple soft start circuit but all ive found are 20+ component setups and almost need a pcb etched.

Is there something simple i could air wire That could run off of the secondaries or mains?

AndrewT · 30th December 2009, 04:20 PM

the safest way is to run a separate supply from a separate 12Vac to 24Vac transformer.
This is switched on and supplies power to the control circuit. A timer, a resistor and/or thermistor and a bypass relay.

MJL21193 · 30th December 2009, 05:13 PM

This is mine:

SOFTSTART1.png

Doesn't use a separate supply, it uses the positive rail of the supply it works with. About a 1 second delay before the relay shorts the thermistor. Safer than resistor types - the thermistor is capable of carrying the full current even if the relay fails to close.
R6 and R7 value is calculated from the rail voltage and the relay used.

nightanole · 30th December 2009, 08:29 PM

What about this. use 40ohm thermistors at the mains (300 watt load)and put the relay coil across the secondaries with a resistor and diode? a 24vdc coil will trigger at 17-18v. If i put i resistor in line wouldnt it trigger closer to rail voltage?

AndrewT · 31st December 2009, 12:15 PM

Quote:

Originally Posted by nightanole

What about this. use 40ohm thermistors at the mains (300 watt load)and put the relay coil across the secondaries with a resistor and diode? a 24vdc coil will trigger at 17-18v. If i put i resistor in line wouldnt it trigger closer to rail voltage?

this is a really bad way to activate/not activate the relay.
The resistor/thermistor will reduce the voltage supplied to the transformer.
This in turn reduces the secondary voltage from the transformer.
In the meantime the rectified voltage has to pass the resistor and the relay.
The relay might pull in or it might not.
Once the relay has pulled in the primary gets the full mains voltage and the relay+ resistor get the full secondary voltage.
If you want to ensure that the relay does not overheat, you take the risk that that a slightly low voltage on the mains may never pull in the relay.

A proper timed delay for the relay control is far better.

nightanole · 31st December 2009, 01:33 PM

PS for just plain and dirty

TDRSOXP-120V

it has 120vac trigger so shouldnt this be all i need period?

30th December 2009, 03:33 PM	#1
nightanole diyAudio Member Join Date: Feb 2009	Soft start for sub 300 watt units Im sorry for asking. Ive looked and looked for a simple soft start circuit but all ive found are 20+ component setups and almost need a pcb etched. Is there something simple i could air wire That could run off of the secondaries or mains?

30th December 2009, 04:20 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	the safest way is to run a separate supply from a separate 12Vac to 24Vac transformer. This is switched on and supplies power to the control circuit. A timer, a resistor and/or thermistor and a bypass relay. __________________ regards Andrew T.

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30th December 2009, 05:13 PM	#3
MJL21193 Account disabled at member's request Join Date: Mar 2007	This is mine: SOFTSTART1.png Doesn't use a separate supply, it uses the positive rail of the supply it works with. About a 1 second delay before the relay shorts the thermistor. Safer than resistor types - the thermistor is capable of carrying the full current even if the relay fails to close. R6 and R7 value is calculated from the rail voltage and the relay used.

30th December 2009, 08:29 PM	#4
nightanole diyAudio Member Join Date: Feb 2009	What about this. use 40ohm thermistors at the mains (300 watt load)and put the relay coil across the secondaries with a resistor and diode? a 24vdc coil will trigger at 17-18v. If i put i resistor in line wouldnt it trigger closer to rail voltage?

31st December 2009, 01:33 PM	#6
nightanole diyAudio Member Join Date: Feb 2009	PS for just plain and dirty TDRSOXP-120V it has 120vac trigger so shouldnt this be all i need period?

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