Relay in-line with power switch??

[drtebi]

I am a bit new to electronics... so bare with me please 🙂


I believe I understand the general idea of a relay switch. What I wonder about though is, can a relay be used in conjunction with a power on/off switch, or does that not make sense?


The reason I am asking is because I want to use a certain rotary switch as a main power on/off switch, but it is only rated for 1 amp. The circuit behind it could draw as much as 3 amps.


So I wonder if I could still safely use the rotary switch as a power switch if I somehow integrate a relay... but then I don't see how that would work, since the power switch would still be in line with the relay and be exposed to the full load?


Sorry if this all sounds very immature, but we've got to start somewhere...

 

[rayma]

I want to use a certain rotary switch as a main power on/off switch, but it is
only rated for 1 amp. The circuit behind it could draw as much as 3 amps.

You can't increase the current rating of the rotary switch, but you can use it to control
the coil of a larger current capacity relay, which then does the high current switching.

Just get a 3A (or more) relay with a 120VAC coil. Switch the coil with the rotary switch.

 

[PRR]

> can a relay be used in conjunction with a power on/off switch, or does that not make sense?

Of course. We do it all the time.

However most of the stealable images seem to come from car wiring. No matter: change some labels and get the right relay, it works the same.

 

[drtebi]

Thanks for your answers 🙂


From the drawing it looks like I would then connect the switch to the neutral line, or doesn't that matter? It would have to be within the main circuilt... otherwise I am confused about how the switch applies electricity to the coil if there is no electricity going through the switch?


Maybe I should get out one of those recycled relays that I have and take a picture... draw the circuit.

 

[rayma]

I would then connect the switch to the neutral line.

Switch the hot line (after the fuse) for safety.

 

[drtebi]

Switch the hot line (after the fuse) for safety.

OK...
But can someone explain to me why there is not 3 amps of current going through the on/off switch if I put it in line with the relay?


There is something I am not understanding... my non-electronics-minded brain tells me that if there is 3 amps of current pulled through the circuit, it does so whether there is a relay switch after the power switch or not?


Sorry for being so green behind the ears.

 

[PRR]

.

 

[drtebi]

Here is a picture of the relays I have at hand... would be nice if I could re-use one of these. The circuit would eventually run on 220 Volts. If I am not mistaken, the two blue ones on the bottom right would be most suitable?

 

[rayma]

can someone explain to me why there is not 3 amps of current going through
the on/off switch if I put it in line with the relay?

The rotary switch only conducts the current for the relay coil,
not the current for the load.

 

[drtebi]

OK, it's starting to make sense to me now...
So in layman terms...

I guess one could say that the rotary switch is connected in parallel to the relay, not "in line", correct? So when the rotary switch is switched, no load goes through the switch. When the relay "clicks in", the big 3 amp spark happens at the relay.



Thank you for the drawings!

 

[TMM]

Those relays are no good because while the contacts are rated for mains voltage, the coils are wound for 12V or 24V DC. The coil will burn out (likely spectacularly) when you connect it to mains.

You need a relay with a 120V AC coil.

 

[drtebi]

Alright, thanks!
I didn't really know how to interpret the ratings on the switches... I thought at least one had been in-line with a power switch, but I guess I was wrong.


I suppose I should rather look for one that doesn't have to be PCB mounted anyway, I don't want to design a PCB just for that...

 

[scottjoplin]

I guess one could say that the rotary switch is connected in parallel to the relay, not "in line", correct? So when the rotary switch is switched, no load goes through the switch. When the relay "clicks in", the big 3 amp spark happens at the relay.

Yes, just make sure both the coil and the contacts of the relay are mains rated

 

[PRR]

You need a 120V AC coil relay.

I used this in a recent furnace control project: Uxcell a14072900ux0197 AC 110V-120V Coil Voltage Red Lamp DPDT 8Pin Power: Amazon.com: Industrial & Scientific

I bought two, and was glad, because one was unreliable. The sockets make it easy to swap-out. The LED is a mild convenience. The socket clips on DIN rail but also has two screw holes. There are zillions of these things, just be sure of the 120VAC coil.

Amazon.com: URBEST HH52P DYF08A Plastic Coil DPDT Black Clear Blue110/120V AC 8 Terminals Electromagnetic Relay w Base: Home Improvement

Different but similar: Amazon.com: Zettler Miniature Power Relay 30A SPDT 120VAC AZ2280-1C-120A: Home Improvement

Over-kill: SCHNEIDER ELCTRIC/MAGNECRAFT 92S11A22D-120 POWER RELAY, DPDT, 120VAC, 30A, DIN RAIL: Amazon.com: Industrial & Scientific

Try this link (it may not be portable). 120 VAC SPST (1 Form A) General Purpose Relays | Mouser