GFI Circuit

[German1981]

Hi,
I am trying to design a Ground Fault interrrupter circuit. I do not want to use a differential transformer; but i would like to use a reed relay with a comparator op amp (741). The op amp will compare the current on both the hot and neutral wires and the output will trigger the switching(opening) of the circuit. This seems to be a good hypothesis, but I am unsure of the details that are involved in this project. I am not that experienced at circuits and I would like some help. I am building this for a project at school. If anyone is willing to help me, I would be greatful. Thanks.

 

[Oznog]

Why would you want to do this? There are huge problems with using an op amp. The voltage on each current shunt is 110v apart so you'd need to do a relatively complicated, difficult, and expensive circuit.

 

[stevez]

What is the voltage/current of the circuit you intend to protect? How are you planning on sensing the current?

Is it safe to presume that you are building this for demonstration purposes only?

 

[German1981]

GFI

I am using this for demonstration purposes, I will use DC to test it.I already have the relay.

 

[Oznog]

Re: GFI

I am using this for demonstration purposes, I will use DC to test it.I already have the relay.

Well, this is just not a simple or particularly reliable circuit. Trying to interface two low voltage signals with high voltage differences has a lot of problems that are tough to compensate for without introducing offset errors. If you can guarantee the voltage diff is less than 30V, we could talk, but for 110V it would require some very technical solutions.

Probably a good way to do this would be two inductors, one for each leg, wound on a common core. Since it's DC, you won't be able to induce AC in a secondary, but maybe you could use a Hall effect switch to determine if there's current differential. There's also DC current pickups that don't use windings, you just put the conductor through the center. In your case, you'd have one wire going in and the other coming back out, the net current should be zero. These devices have a bit of offset error, and cannot tolerate large currents outside of their ratings.

 

[German1981]

GFI

The voltage we are talking is about 5 to 10 volts. I plan on using a relay, which operates at 5 volts, along with a 741 comparator op amp. I will also have a JK flip flop in the circuit for memory. The GFI that I am building will sense a current difference of 3 mA between the hot and neutral wire. I have the overall idea of How I want it to work, but, i'm a little stuck on the details, such as how to configure the JK flip flop.