A.C Soft Starter help

Thread Starter

tjones91

Joined May 6, 2020
19
Hello, I have a site here that explains an Ac soft starter circuit with a schematic I will post below. The article explains very well what components do what, But i am trying to get an overall explanation of what happens as soon as you turn on AC load and most importantly is what order everything happens is.

fig1_3IsWchIkZw.jpg
 

dendad

Joined Feb 20, 2016
3,569
Have a go at working it out and post your effort. Then we can help you.
It is worth you trying first, otherwise you don't learn much if we just give you the answers.
Break the circuit down into parts to see if you can figure it out.
For instance, starting from the left, what happens up to C2?
 
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Thread Starter

tjones91

Joined May 6, 2020
19
Have a go at working it out and post your effort. Then we can help you.
It is worth you trying first, otherwise you don't learn much if we just give you the answers.
Break the circuit down into parts to see if you can figure it out.
For instance, starting from the left, what happens up to C2?
Thanks for the response,
I believe when the 220v switch turns on, a timing circuit created by R5, R6 and C3 acts as an RC circuit to turn on the NPN transistor thus activating the relay, here's the part that gets cloudy....This timed relay only happens when the circuit turns on for a short specified amount of time but I'm not exactly sure how this relay slows the inrush current. My best guess is that when the relay and the dc part of the circuit are activated, R7 and the Relay act as a current divider and while the relay is activated it is not pumping as much current into the load until the timing circuit is finished and the load has had a chance to establish itself. Sounds right to me but I'm not sure. Thanks
 

dendad

Joined Feb 20, 2016
3,569
And please note.
Do not attempt to build this. Direct connected to mains power supplies are very dangerous and not an allowed subject on this forum. An admin may close this post.
Add an isolation transformer to the input and that it may pass, maybe.
 

Thread Starter

tjones91

Joined May 6, 2020
19
And please note.
Do not attempt to build this. Direct connected to mains power supplies are very dangerous and not an allowed subject on this forum. An admin may close this post.
Add an isolation transformer to the input and that it may pass, maybe.
I wasnt going to build this, Just trying to learn more about circuits
 

dendad

Joined Feb 20, 2016
3,569
Yes, the circuit starts up with R7 in series with the load. So the load current is reduced.
After the time delay, R7 is shorted so full power in now applied to the load.
 

Thread Starter

tjones91

Joined May 6, 2020
19
Yes, the circuit starts up with R7 in series with the load. So the load current is reduced.
After the time delay, R7 is shorted so full power in now applied to the load.
Oh so you are saying that when the circuit is first turned on the relay is off? So R7 is reducing inrush current then after the timing circuit the relay is activated and this provides another path for current thus the current rises according to the draw of the load?
 

Thread Starter

tjones91

Joined May 6, 2020
19
Have you built much in the way of electronics?
Yes, I have built many simple circuits, PWM controller using 555, sun-tracking solar with 4 LDR's and a microcontroller, Mini tesla coil, simple (INSTRUCTABLE)-type projects but i want to get more serious about other types of circuits even if i don't plan on building them. I want to try and build more power circuits to get a better understanding of electronics but i don't have an oscilloscope yet.
 

dendad

Joined Feb 20, 2016
3,569
Oh so you are saying that when the circuit is first turned on the relay is off? So R7 is reducing inrush current then after the timing circuit the relay is activated and this provides another path for current thus the current rises according to the draw of the load?
Yes.
 

Thread Starter

tjones91

Joined May 6, 2020
19
perfect explanation, i also just had someone explain it also and it totally makes sense.
One thing that i still dont get is are (4 and 1) on P1 both hot terminals. I get that normal 120v AC the current switches with one leg acting as the hot and the other as the nuetral-return. But how does it work with the 220? wouildt there need to be another leg so that it is 3-phase 220v? and what are (3 and 2) on P1 and why are they bridged?
 

dendad

Joined Feb 20, 2016
3,569
An oscilloscope is a great tool to have. Often called "A Truth meter" as it lets you see what is going on.
There are some traps in using one too, but I think they are a must have tool if you want to do work on electronics. You can get by without one but it is a lot easier with one.
I am in the process on building my first Tesla coil, it is about time as I've been playing electronics for over 50 years now. I want to make a musical one :)
 

dendad

Joined Feb 20, 2016
3,569
It is single phase 220V as shown, not 3 phase.
P1-1 and P1-4 are the mains connections. I do not think the "GND" connections should actually be GND, but floating, all just commoned together as the relay 0V line.
P1-2 and P1-3 may actually be to "switch" the mains on when plugged in. As the other part of the circuit is not shown, it is hard to say.
Here in Oz, we use 240V, not 120V, so it all depends on where the example circuit came from.
Also, in USA, it could be 220V split phase. That is still not 2 phases, but single phase, 120V + 120V.
Here if we use 2 or 3 phase, the 240V becomes 412V as the phases are 120degrees offset. 240 + (240*120degrees offset) = 415V.
I do not know what the 3 phase setup is in USA.
 

Thread Starter

tjones91

Joined May 6, 2020
19
An oscilloscope is a great tool to have. Often called "A Truth meter" as it lets you see what is going on.
There are some traps in using one too, but I think they are a must have tool if you want to do work on electronics. You can get by without one but it is a lot easier with one.
I am in the process on building my first Tesla coil, it is about time as I've been playing electronics for over 50 years now. I want to make a musical one :)
are you going with a spark gap or solid-state?
 

dendad

Joined Feb 20, 2016
3,569
are you going with a spark gap or solid-state?
I may start with a spark gap, but will end up using solid state.
Somewhere in my shed is an old transformer from a microwave oven and that will be the power supply. Often ovens are tossed out so I can get another one easily enough.
I just need to get my roundtoit working ;)
So far I've 3D printed ends for a tube and will make up a quadrature counter for my lathe so the number of turns can be noted. It is a bit cold here to work in my shed for long now as we are in winter and I've not cleared out all the junk that surrounds the wood heater as yet.
 
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