How would you interface a relay to control an AC device

Thread Starter

Dadu@

Joined Feb 4, 2022
127
I get very confused when any AC device has to be controlled with a relay, I really don't understand how to do the wiring. I would like to understand it very deeply that whenever such an opportunity for connection comes, I will not forget the basics.

AC supply 230 V : The three pins are earth, neutral and phase.
DC Supply 5V : Two pins are positive and ground.
Relay 5V DC Five pins are C1, C2, NO, NC and common.
AC Device : Two pins are positive and ground.

This is how I think about connections

+5V DC should connect to NO pin,
Ground pin of DC supply should connect to NC, positive pin of AC device should be connect to common pin of realy

I don't understand the rest of the connections. I want an expert to help me understand the rest of the connection
 

eetech00

Joined Jun 8, 2013
3,162
I get very confused when any AC device has to be controlled with a relay, I really don't understand how to do the wiring. I would like to understand it very deeply that whenever such an opportunity for connection comes, I will not forget the basics.

AC supply 230 V : The three pins are earth, neutral and phase.
DC Supply 5V : Two pins are positive and ground.
Relay 5V DC Five pins are C1, C2, NO, NC and common.
AC Device : Two pins are positive and ground.

This is how I think about connections

+5V DC should connect to NO pin,
Ground pin of DC supply should connect to NC, positive pin of AC device should be connect to common pin of realy

I don't understand the rest of the connections. I want an expert to help me understand the rest of the connection
What is the part numbers of the devices involved? Especially the relay.
The relay should be certified for AC line use ( like UL in USA).
 

sagor

Joined Mar 10, 2019
696
Is the 5V to control the 230V device, or is the 230V device to control the 5V device?
Usually, for 5V to control a 230V AC device, the 5V signal goes to the relay coil pins, and the AC is wired to the NO and COMMON ("C") pins. Applying 5V to the relay will then energize the relay and turn on the 230V device.

You have to consider things like AC current load and the relay ratings. Also, for DC controlled relays, you need a back EMF diode across the relay coils unless this is some commercially made relay board which usually has that built in.

Draw a quick schematic on paper as to what you are trying to do, it will make it easier for us to understand. Pictures of the relay would help also.
 

MaxHeadRoom

Joined Jul 18, 2013
25,229
I get very confused when any AC device has to be controlled with a relay, I really don't understand how to do the wiring. I would like to understand it very deeply that whenever such an opportunity for connection comes, I will not forget the basics.
Presumably you are working off of a DWG, either your own or from other source?
Post it as suggested, also what you are trying to achieve or have in mind.
 

BobTPH

Joined Jun 5, 2013
4,943
The AC side of the circuit is identical to a SPDT switch. These are the NO NC and common terminals.

The DC side is the two coil terminals. Put 5V between them to activate the relay.

Bob
 

MisterBill2

Joined Jan 23, 2018
11,923
As an engineer of any variety you should have had at least the basic circuit theory in your studies. A bit of the basic understanding will do a whole lot to improve your understanding of what you are considering doing. An error with MAINS VOLTAGES AND POWER LEVELS CAN BE QUITE SERIOUS.

And as the contacts of the relay are the control output, why did you think that they should connect to the control signal? and as the coil terminals are the control INPUT connection, why did you think that they should be connected to the power to be controlled?
 

MaxHeadRoom

Joined Jul 18, 2013
25,229
If this is a learning experience, Look into reverse biased diode across the coil, as generally a 5vdc coil relay is fed from a semiconductor device and requires suppression of the coil BEMF at turn off.
 

MisterBill2

Joined Jan 23, 2018
11,923
Max, I suggestgoing back and re-reading post #1 before suggesting adding a protection diode. At this point the complexity is a consideration.
 

Thread Starter

Dadu@

Joined Feb 4, 2022
127
My understanding about a simple relay is as follows

When no current passes through the coil, the common terminal connected to the NC terminal which I call relay is inactive.

When an electric current is passed through the coil it generates a magnetic field. the common terminal connected to the NO terminal which I call relay is active.
 

MaxHeadRoom

Joined Jul 18, 2013
25,229
Max, I suggestgoing back and re-reading post #1 before suggesting adding a protection diode. At this point the complexity is a consideration.
I did not imply it was needed for the OP's circuit, but something for him to be aware of in the future.
I'm assuming he intends progressing from this point, maybe?
Otherwise he can simply ignore it!
IOW, something to make note of ! :confused: ,
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
25,229
My understanding about a simple relay is as follows
When no current passes through the coil, the common terminal connected to the NC terminal which I call relay is inactive.
When an electric current is passed through the coil it generates a magnetic field. the common terminal connected to the NO terminal which I call relay is active.
Basically right, but in the circuit you describe in #8, if you were also making use of both N.O and N.C. terminals, the power supply (Live) would normally be connected to the COMmon contact,
 

Thread Starter

Dadu@

Joined Feb 4, 2022
127
Basically right, but in the circuit you describe in #8, if you were also making use of both N.O and N.C. terminals, the power supply (Live) would normally be connected to the COMmon contact,
I'm getting confused here. The connection shown in #8 has been verified by the member. I don't understand why Live should be connected with Common.
 
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