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Thread: 12v to 5v relay

  1. #1
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    12v to 5v relay

    heya,

    is there a way or possibly a component(pref relay) that can drop a 12v dc car current to 5v?

    the reason i ask is i hava 12v relay swtiching my PC on, however every 5-10secs or so the relay will cllick over (i suspect due to fluctuation of the current), this causes an issue that it will keep sending a signal to the M2-ATX to keep it powered on, however if i could drop that 12v to 5v and use a 5v relay surely the current wouldnt fluctuate around 5V?

    any suggestions appreciated!

  2. #2
    North of the land of Hey Huns
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    You say it clicks over, I assume this means that it opens momentairly? If it is a 12V relay, and your voltage is dropping low enough to close it (typically 5-6 volts), then you have some other much larger issue that you should solve.

    Have you tried a different relay? Is it possible the relay is bad? What source are you using to drive the coil on the relay? Are you sure there is enough current capability to drive the relay?

    You can use a voltage divider to get the current down, but you would have to use very large high current resistors, an it would generate a good amount of waste heat. Really you should solve whatever issue is causing your relay to flicker, before it causes even more damage.

    Also, perhaps explain a bit more about how your M2 is electrically connected? (Eg: Why you need the relay)
    "stop with the REINSTALLS, what do you think we got some lame-o installer!!!" - mitchjs
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  3. #3
    Raw Wave
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    Keep it mind that a relay does not convert voltage. It is merely a electrically connected mechanical switch. As long as it required voltage appears across its actuating coil, it will remain closed.
    Relays are typically used as switch amplifiers - ie, use a small switch or current source to switch a much larger current, or remote switches.

    Voltage conversion is different issue. Depending on what (current) is required, they can vary from resistive dividers, voltage regulators and voltage (dc-dc) converters.


    Like Malcom, I'd question why you need the relay.

  4. #4
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    cheers guys, let me clear this up,

    as the dome light in my car is ground switched its got a 12v constant on and when the light comes on it flicks to 0v, so i have the 12v runnign through the relay and the relay to turn it "on". i have a 12v going to the M2 ATX as normally on (on the relau), so aslong as the 12v is going to the dome light (in this case when the doem light is off) the relay will keep in a "off" position, however when the dome light is on and the 12v drops to 0v the relay flicks back to how it is with no current and completes the circuit to turn the m2 atx on, (this is a sp dt relay).

  5. #5
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    what i will do is try putting another external 12v supply onto the relay and check if the problem reoccurs to confirm if it is the relay or not.

  6. #6
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    Dome light 12V source --- Relay Coil --- Dome Light --- Gnd
    Normal Car 12V source --- Relay power ---- m2atx

    Is that sort of how you have it set up? If so, it very well could be that the circuit with the dome light either A: wires can't handle the extra current, or B: the dome light itself is high enough resistance to prevent the coil from being able to activate.

    Why are you running your carpc power off the dome light? M2ATX is a "smart" power supply, so you can hook it directly to power, and run an ignition line to it and it will automatically power on/off depending on the ignition state of your vehicle. There is no need for a relay with the M2.
    "stop with the REINSTALLS, what do you think we got some lame-o installer!!!" - mitchjs
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  7. #7
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    thats sort of how its setup, i think i should clarify, the power going to the m2atx is the signal to tell it to turn on.

  8. #8
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    and i have it on the dome light so its booting before i get in the car, and then a second signal comes on when i turn the ignition on to keep the m2atx sitll on.

  9. #9
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    There are a number of products that you could use to do this, some pre-made and some that you could make yourself. A few sites sell a "DCDC-USB Intelligent DC-DC Converter", mini-box.com being just one of them. They're a little pricy but really capable.
    If you're a do-it-yourself'er you can use a TO-220 Voltage Regulator and build a regulator circuit (you'll need a couple of other components but it should run you less than $10-$15 easily). Sparkfun.com sells them for pretty cheap and they have a nice tutorial on there to build a regulator circuit: Unregulated Power Supply Tutorial
    The Regulators are here: http://www.sparkfun.com/products/107
    I'm using these as regulators to power some low cost car-compliant USB Hubs (normal powered hub that normally runs off a 5v wall-wart).

    If you test your relay and it seems like it's working, you could try putting a capacitor in the circuit to smooth the ripple (probably around a 100uF cap between Latching high and ground). The lighting circuit may be bouncing around a little with the in-rush of draw between your lights/door locks/headlights/etc, although that's sort of a shot in the dark because I wouldn't really expect to see much ripple on that line.

  10. #10
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    PS...
    Quote Originally Posted by shizzlehappens View Post
    ...you can use a TO-220 Voltage Regulator ...
    NippleSlip - TO-220 is a package type. You meant LM7805 (fixed 5V) or LM317 (adjustable) voltage regulators.
    Though I don't think "power supply" is what is required in this case.
    /end of PS


    So you have +12V thru relay coil thru dome light's switch? (Malcom - check this; relay coil is parallel to the light(s), though the +12V supply can be different...)

    And the +12V to thru the contacts to the M2 is +12V battery fused if high current (from which the coil+ can also be taken), else from the dome +12V if merely a signal.

    If dimmers are fitted, the coil- is on the GND switch side, else the coil+ is from +12V if dimmer is on the +ve side.

    Hence the relay is only on with the GND switch closed. (IE - dome light is on though it isn't needed for the circuit to work.)


    Diodes should not be necessary unless there is an alarm or other circuit also using the door switches, though the relay's "output" +12V may need relays if joined to the other +12V that takes over.
    Last edited by OldSpark; 09-28-2011 at 01:37 AM. Reason: ps...

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