Hi i am get the following short circuit protection circuit from the website it is working fine .The problem is when the 12v supply is on it is started from NC operation after pressing the push button only the output will come and if it is short circuit at the output the relay get turn off and it is change to NC.I want to modified this circuit as first it work with NO and if it is short circuit it is work to NC that is i want to avoid pressing push button at the start up and automatically it has to work to NO at first and if it is short circuit at the output it has to work with NC.i am unable to find the way to implement this logic.Please guide me to achieve this solution

 

You do understand that a relay isn't a "short circuit protection" device don't you? A fuse or circuit breaker is what you need for short circuit protection, not a relay.

https://en.wikipedia.org/wiki/Relay
https://en.wikipedia.org/wiki/Fuse_(electrical)
https://en.wikipedia.org/wiki/Circuit_breaker

 

Put your load though the relay NO contacts, so then when you press the coil button,your relay closes and puts power to your load, then if you get a short circuit, the relay coil will power down and remove your load..

 

The TS stated that they do not want the need to push a button. My recommendation is to use a circuit breaker instead. Then you will not need a relay or a circuit breaker. And you will have a more effective protection arrangement.

 

From what I gather, using the relay to cut power when a short occurs, IF it were to be hooked up the way the TS wants then as soon as there's a short it will kick the relay in and remove the short. BUT AS SOON AS THE RELAY KICKS OUT THE SHORT IS GONE AND THE RELAY WILL KICK BACK IN AGAIN. AND AGAIN; AND AGAIN. Over and over until something burns out.

Fuse or breaker. That's the right way to go about it if you don't want a "Start" button.

 

It would be the same if you wired the relay coil through the NC contacts. As soon as power is applied the relay will pull in and shut power off. As soon as power is off the relay will pull in and turn the relay on again and shut power off again. The relay will act like a buzzer.

 

It would be the same if you wired the relay coil through the NC contacts.

But not through the NO contacts as Dd suggested.

 

What I meant to say is that the relay self powers when off. When on it shuts off. When off it turns on. Old style buzzers operated on the same exact principal where when you pushed the button the buzzer would turn on, but because turning the buzzer on meant breaking contacts the buzzer would shut off and go on and off and on and off very very fast, producing the buzzer effect.

 

From what I gather, using the relay to cut power when a short occurs, IF it were to be hooked up the way the TS wants then as soon as there's a short it will kick the relay in and remove the short. BUT AS SOON AS THE RELAY KICKS OUT THE SHORT IS GONE AND THE RELAY WILL KICK BACK IN AGAIN. AND AGAIN; AND AGAIN. Over and over until something burns out.

Fuse or breaker. That's the right way to go about it if you don't want a "Start" button.

What I saw was a circuit that had the relay latched in by the button, and a short circuit would drop the coil voltage and release the latched in relay. A very poor design, but very simple. Certainly not anything that I woukld ever consider using in that manner.

 

What I saw was a circuit that had the relay latched in by the button, and a short circuit would drop the coil voltage and release the latched in relay. A very poor design, but very simple.

It might work okay for a current limited supply that can't tolerate a continuous short circuit.

 

It might work okay for a current limited supply that can't tolerate a continuous short circuit.

It can sort of work for an installation with poor connections and undersized wires. It may even be common in some parts of the world. But none of that makes it other than a very poor choice.