Wanted: Simple soft start circuit

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#5
No kidding...

I won't even ask how AC is supposed to actuate an electromagnetic switch 🙂

So let me throw out some conjecture:

I assume that initially the entire 120V drop occurs across the resistor while the cap charges. Once charged, the cap shorts the resistor and powers the relay. Once powered, the relay closes and shorts the cap. As long as I get a 120VAC relaty, I should be safe even if the line voltage is say 115V because the relay should engage with say 80% of rate voltage?

OK so what did I get wrong? How do I calculate time delay via R and C, and how are my considerations based on temperature or current?
 
#6
Upon start-up the relay is not energised, as there is no AC. So the resistor is in series with the transformer. Now the transformer draws a lot of current on start-up causing a big voltage drop across the resistor so that the relay is still not energized, as the voltage is not high enough.
When the power supply is fully charged the current through the resistor is minimal and the relay gets the full line voltage. Then the relay energises and shorts the resistor as if it is out of the circuit.
The cap is only to quench the spark.
So the inrush current is limited by the resistor😎
 
#7
Easiest soft start circuit:

two position switch (I prefer rotary) connected to two suitable relays, one kicks on the AC mains into a resistor, the other shorts out the resistor. you turn the switch after the caps charge up enough - use two LEDs or a bicolor to indicate status, and I prefer a 12 or 24vdc relay AC or DC coils. Although I have done it with 120vac relays too.

Next simplest:

your basic RC time constant on the base of a transistor or two (usually two) the larger one is a power device and pulls in the relay coil, the smaller takes the RC filter and swings the base to saturate the transistor, which kicks the bigger one. Use a DC relay coil here, the initial turn on is with a switch that connects the little DC supply to the first relay, then the time constant kicks the second one in that bypassed the power resistors...

Better bet:

Use the same set up as above; DC power supply, DC relays, but use a 555 timer circuit to control the pull in...

these I think are the three simplest methods I know of...

_-_-bear :Pawprint:
 
#8
Let me add the cap charge circuit across the relay will work, but it is pretty difficult to get long delays by that method, especially if the relay is hefty and requires a fair abount of "juice" to pull in...

And this presupposes that the drop across the resistor will be large enough not to pull the relay either...

I prefer hefty relays with multiple contacts for this sort of thing, btw... single contacts are usually a receipe for failures...


_-_-bear
 
#10
Use Resistor and Capacitor, this will have some time to charge to a voltage needed to

Using Resistor and one capacitor, or electrolitic condenser, you will have some time to charge the capacitor, this way, the voltage on its extremes will be going up slowly as it will be charging.... when reach some zenner voltage, some conduction will switch on some transistor that will operate a relay....one relay contact will keep the voltage high in it"s own coil and the other will switch on whatever you want to be switched delayed.

This small and poor system, will use two switches...one is the Mains switch, and the other is the delayed on push button.

As one example...the mains can send power to transformer, and the delayed relay contacts can switch speaker on in a retarded way.

Of course... switching of....everything turns off..... if you need, i do not know some reason,...but if you need to retarded of somewhere..use a big electrolitic in the DC relay Coil.... this one will keep it on for some seconds.

Simple is good, and easy to understand.

Carlos
 
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