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using car relay for wall outlet power

Discussion in 'General Electronics Discussion' started by Vaselinessa, Oct 9, 2011.

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  1. Vaselinessa

    Vaselinessa

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    Oct 9, 2011
    My intention is to use a car relay (e.g. 12 VDC Tyco Relay SPDT 20/30A) to close a circuit between a wall outlet and a cassette player.

    Do you see any problems with this working? I'm an electrical n00b, so this is why I ask. Yes, I'm in the US, so our outlet power is 120 (right?).
     
  2. davelectronic

    davelectronic

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    Dec 13, 2010
    Hi Vaselinessa.
    Why turn a cassette player of with a relay, if you dont mind me asking, auto motive relays are usually for low voltage and fairly high currents, another mains rated relay would be my preference, not a big unit as the current for a cassette player will be low.
    Dave. :)
     
  3. Vaselinessa

    Vaselinessa

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    Oct 9, 2011
    I guess I didn't understand the rating of the car relay. I thought it meant that the coil would switch at 12v. (And thought, but must have been mistaken, to have read that it was rated for 120v.)

    As for the question of why not just open up the recorder and insert the relay after the current gets transformed to a lower voltage, I don't actually own a tape recorder, so I would borrow one, and I'm not comfortable hacking into such a thing.

    I can't think how I would manage the power supply (short of buying batteries) if I wasn't plugging it into a wall outlet.

    But what I really have in mind is a contact trigger (for said treasure hunt) being used to play a short audio recording when a person reaches a certain point. If you can advise me on a better device for playing such a recording, I would love to hear it. The issue is that I'd like to play it loudly, so I'll need some sort of amplifier, and I'd like to have good fidelity (can't just rip the device out of a record-your-own-greeting.

    Moreover, I would rather not use an ordinary AC adapter and then put a low voltage relay on the DC end because the adapter would be drawing power all the while it was plugged in.

    Could you explain what a 'mains rated' relay is?
     
  4. duke37

    duke37

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    Jan 9, 2011
    The relay can probably stand the voltage but it has a very large current capability. 5A should be plenty.
    Auto relays are quite massive and consume a considerable amount of power in the coil which has to be supplied by the control circuit.

    Remember that you have to make an earthed box to house the relay and make it safe to use.
     
  5. davelectronic

    davelectronic

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    12
    Dec 13, 2010
    I did not expect you to hack a tape recorder apart, i asked why turn off a cassette player with a relay.
    An automotive relays works on 12 volts + / - % off rated voltage. the contacts for switching are for low voltage high power or current.

    A mains relay has a low voltage to switch the relay coil and operate the switching contacts, these contacts are intended for high voltage, say mains voltage 120 / 240 volts, most relays coils are DC and low voltage, typical is 5, 6, 12, and other low voltages.
    Dave. :)
     
  6. Vaselinessa

    Vaselinessa

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    Oct 9, 2011
    Thanks, blokes.

    So supposing I bought a cassette player at a 2nd-hand shop, opened her up and inserted the relay after the voltage had been transformed down to 12V DC, and I wanted to use just a few volts for the coil (could I do it with 1.5v?), what sort of relay could I use for a thing like that?

    And, if you don't mind, what does "12 volts + / - % off rated voltage" voltage mean?
     
    Last edited: Oct 9, 2011
  7. davelectronic

    davelectronic

    1,087
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    Dec 13, 2010
    You need a censer circuit so the recording is triggered on the item in the hunt being found, an amp can be a pair of good quality pc speakers or a small stereo amp and speakers.

    Another member can help i expect in the design of a sense trigger circuit, i dont build to many of them, the parameters of distance angle etc etc need working through.
    Dave. :)

    PS, + / -% of the rated voltage is say 5 or 10 % higher or lower than the 12 volts its coil is rated at to switch the contacts.

    A relay with a coil with a voltage of 1.5 volts might be hard to find, and may not switch mains voltages.
    I would say use a 5 volts relay for the coil, and 4 to 6 volts will still switch its contacts.

    Relays.

    http://www.maplin.co.uk/components/relays/general-purpose-relays
     
    Last edited: Oct 9, 2011
  8. Vaselinessa

    Vaselinessa

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    Oct 9, 2011
    Another thing has me confused, I guess: when I look at a relay (like this one), I imagined that 12vdc was the voltage to make the coil switch. Is it the voltage for the relay's principal circuit instead (and what is the name for that curcuit?)?

    In either case, what's the proper way to identify both the rating and the switching voltage?
     
    Last edited: Oct 9, 2011
  9. Vaselinessa

    Vaselinessa

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    Oct 9, 2011
    I'm looking for info on 'censer circuit' but turning up nothing. Is it the same as 'sensor circuit'?
     
  10. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
  11. Vaselinessa

    Vaselinessa

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    Oct 9, 2011
    Thanks much
     
  12. Vaselinessa

    Vaselinessa

    7
    0
    Oct 9, 2011
    Thanks much.
     
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