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Which relay do i use

jamest6

Feb 16, 2012
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Hi,

I am designing a circuit for the output of my PLC, the output of the PLC is 24V AC i need this output to turn on 12V LED's and a buzzer. I was thinking of using a relay but i don't know much about them. What relay would be best used for the attached photo?

Cheers
James
 

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duke37

Jan 9, 2011
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Most relays are intended to be driven by DC, AC relays are available but use more power and tend to buzz.
I would put a bridge rectifier in the AC supply to a DC relay. If you cannot find a 24V relay, then a 12V relay with a series resistor equal to the relay coil resistance would do.

As for the contacts, most relays can manage 0.5A, you could even go wild and get one that can switch 1A.
 

jamest6

Feb 16, 2012
4
Joined
Feb 16, 2012
Messages
4
Most relays are intended to be driven by DC, AC relays are available but use more power and tend to buzz.
I would put a bridge rectifier in the AC supply to a DC relay. If you cannot find a 24V relay, then a 12V relay with a series resistor equal to the relay coil resistance would do.

As for the contacts, most relays can manage 0.5A, you could even go wild and get one that can switch 1A.


Hi Duke,

Thanks for the speedy reply, after looking at some relays i notice the contact voltage is rated in AC. Does it matter if i use it for DC?

Cheers
James
 

duke37

Jan 9, 2011
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Hi James,

Relays contacts are rated differently for AC and DC since on AC the arc is extiguished when the current reverses, on DC an arc can be drawn for a considerable time so harming the contacts. Accordingly, the DC voltage rating may be a quarter of that of AC.

In your case, switching 12V DC should be a doddle unless you are using diddy relays intended for signal control.

Duke
 

jamest6

Feb 16, 2012
4
Joined
Feb 16, 2012
Messages
4
Hi James,

Relays contacts are rated differently for AC and DC since on AC the arc is extiguished when the current reverses, on DC an arc can be drawn for a considerable time so harming the contacts. Accordingly, the DC voltage rating may be a quarter of that of AC.

In your case, switching 12V DC should be a doddle unless you are using diddy relays intended for signal control.

Duke

Ok thank you very much for all your help
 
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