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Thread: Relay advice

  1. #1
    Senior Member RHCP's Avatar
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    Default Relay advice

    I need a relay, but im not sure how relays are named....

    I Plan on running two different 12v devices (both should be below 1 amp) using a common/shared 12v supply. Device 1 and device 2.

    I have 240v power supply for a 3rd device, when this is on i want the relay to power device 1, and not device 2....when the 240v is off, i want device 2 powered and not device 1.

    Thanks, RHCP.
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    Senior Member mickc's Avatar
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    sounds like you need a relay with 1 n/o (normally open) contact
    and 1 n/c (normaly closed) contact
    device 1 will be on the n/o
    device 2 on the n/c

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    Err.. that's single pole double throw ?
    1 not 2
    2 not 1
    and
    3 not 2

    Normally a single double pole double pole switch could do all of this... but I think the point your trying to make is that your circuit should detect when 240V is being drawn by a device and switch the relay when it does ?

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    Default

    You can achieve this with a Single Pole Double Throw relay. SPDT for short.



    240V across the coil.
    N/O (device 1)
    N/C (device 2)

    When the Coil is energised it will Close the NO contact powering device 1, when you remove power from the coil it will go back to the NC state powering device 2.

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    Senior Member RHCP's Avatar
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    Thanks for the replies, especially the diagram (makes it very clear). I'm just not sure which SPDT is the correct one for the job.

    The voltage across the coil will be 240v AC, while the voltage across the NO/NC is 12v DC...

    So i assume i'm just looking for a device which is rated for 240v ac across the activation terminals. None of these are ?

    "Relay SPDT 12VDC/240VAC10A PCB" That means that a 12v will switch the 240v ac devices? Which is the opposite of what i'm trying to achieve.

    Thanks, RHCP.
    Last edited by RHCP; 18-07-08 at 03:13 AM. Reason: typo
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    Quote Originally Posted by RHCP View Post
    Thanks for the replies, especially the diagram (makes it very clear). I'm just not sure which SPDT is the correct one for the job.

    The voltage across the coil will be 240v AC, while the voltage across the NO/NC is 12v DC...

    So i assume i'm just looking for a device which is rated for 240v ac across the activation terminals. None of these are ?

    "Relay SPDT 12VDC/240VAC10A PCB" That means that a 12v will switch the 240v ac devices? Which is the opposite of what i'm trying to achieve.

    Thanks, RHCP.

    This one will do the job , most of these general purpose relays are DPDT ( double pole double throw ) but you can just use the contact on one side or join them together for twice the current load.

    240AC Coil 5A Contact DPDT Relay


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    Senior Member z80's Avatar
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    If you have spare contacts you can wire them up to a small indicator light to show that it is "on"

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