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Thread: FAQ: RELAYS; how they work and how to wire it up...

  1. #1
    Maximum Bitrate FusionFanatic's Avatar
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    FAQ: RELAYS; how they work and how to wire it up...

    There seem to be many members who don't know what a relay is, or don't know how to wire one in. this guide is here to help solve that issue...

    what is a relay?
    -a relay is a device that allows you to control a high-current electrical load with a low-current electrical 'signal'. they are usually electro-magnetic, but are also available in solid-state forms. they can be used with a switch (to allow control of a high-current load with a small switch) or they can be hooked up to a switched power source in the car like the ignition or accessory power circuits (to allow power to be switched on/off automatically with the ignition key).

    Why do I need a relay?
    -when hooking anything up to a car's factory wiring, it's important to remember that factory wires are designed to carry the load of only the factory installed components. they are not 'general use' power circuits like the power outlets in your house. for example, the ignition (IGN) circuit is designed to power the car's ignition system and nothing else. hooking up a high-current device to this circuit can create a fire hazard. by using a relay, you can use the IGN circuit to control a high-current device without directly powering it from the IGN circuit itself.

    Is a relay hard to hook up?
    -no... most relays require only 4 wires.

    Where can I buy a relay, and how much do they cost?
    -you can buy standard automotive relays at radio shack, autozone, walmart, or at almost any electronics store or automotive store. a typical SPST relay will cost about $5(US).

    I bought a relay, but I don't know how to hook it up. all I see are a few weird numbers and strange symbols, what do they mean?
    -a standard bosch-style relay will have 4 or 5 numbered leads (30, 85, 86, 87, and sometimes 87a). why they picked those numbers, I have no clue; but I can tell what they hook up to.

    -30 = constant [positive (+)] power (usually wired directly to car battery)
    -85 = coil ground (wired to the negative (-) battery terminal or any grounded metal panel in the car)
    -86 = coil power (wired to the control source. could be a switch, or it could be the car's IGN or ACC circuit.)
    -87 = switched [positive (+)] power output. (when the relay coil is powered, lead/pin 87 is connected to lead/pin 30)
    -87a = [on 5 lead/pin relays only] this lead/pin is connected to lead/pin 30 when the coil is NOT powered.

    here is the bottom view of a standard 4-lead Bosch-style automotive SPST relay


    here is what happens inside the 4-lead/pin SPST relay. on the left, the coil is NOT powered. on the right, the coil IS powered. notice the switch changes positions when the coil is powered. when the coil is powered, pins 30 and 87 are connected. when the coil is NOT powered, then pin 30 is not connected to anything, therefore it is in the 'off' position.


    here is the bottom view of a standard 5-lead Bosch-style automotive SPDT relay. you'll notice that the only difference is the addition of the 87a lead/pin.


    here is what happens inside the 5-lead/pin SPDT relay. you'll notice that instead of an 'off' position, there is an electrical contact (87a). on the left the coil is NOT powered, and pins 30 and 87a are connected. on the right the coil IS powered, and pins 30 and 87 are connected.


    here is a typical setup to control power to a carPC via the car's ACC/IGN circuits. this is generally use for laptop installs or 'no-write' setups (not good for normal carPC because abruptly cutting power without proper shutdown can cause data corruption);


    I want to use the relay to turn on/off with the car. how do I hook that up?
    -connect lead/pin 86 to the car's IGN, or ACC circuit. these circuits can be tapped into in the wiring harness that goes to your car's key switch. the ACC circuit can also be tapped into at the fuse box or in the stock radio harness.

    What is the difference between the ignition (IGN) and the accessory (ACC) circuit?
    -they are both powered when they key is in the 'RUN' position, and they are both not-powered when they key is in the 'OFF' position.

    -the 'ACC' circuit is powered when the key is in the 'ACC' position, but is not powered when the key is in the 'START' position. when the key is turned, power to the relay will turn on as the key passes the 'ACC' and 'RUN' positions, then turn off in the 'START' position, then turn back on as the key is released (springs back to the 'RUN' position. this is not an issue if you're using a manual on/off switch, or have a delayed on startup controller.

    -the IGN circuit is powered when the key is in the 'START' position, but is not powered when the key is in the 'ACC' position. this circuit avoids the issue stated above, but requires that the key is left in the 'RU' position if you want to use the carputer with the engine off. on some cars, this can burn out the ignition coil.

    I bought a 5-pin relay, can I still use it as a simple on/off switch instead of a changeover switch?
    -yes. simply leave pin 87a disconnected.

    My relay has more than 5-leads/pins?
    if your relay has more than 5-leads/pins then is is most likely a DPDT, MPDT, or MPMP relay (M=multiple). it will work the same way, it simply has two or more separate switched inputs/outputs inside it. regardless of the number of switched contacts, it will still use a single coil, so it will still be controlled by a single power source.

    Are there others types of relays other than the standard 'bosch style automotive relay'?
    yes, just like regular switches, there are several different types, sizes, and styles of relays. some have multiple switched contacts, some are circuit-board mountable, some have high-current capacity, some have low-current capacity, some use higher or lower voltage coil power (generally you would only use a 12v coil in a car).

    What is a 'valet switch'?
    -the 'valet switch' is a standard SPST switch wired between the coil power source and pin 86. it allows you to leave the relay turned off, so nobody (including a 'valet' driver) can turn your carPC on. how effective this will be depends on how well you hide the switch (it should be accessible, but hidden from plain sight).

    does a relay take the place of a fuse?
    -no! a relay provides no protection from overload or short-circuits.

    do I need to use a fuse if I use a relay?
    yes! you must still fuse your power wires!


  2. #2
    Newbie tcayere's Avatar
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    Awsome FAQ

    Lots of helpful material, GREAT FAQ

  3. #3
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    oh, so THAT'S what it's called. I was trying to think of what one of those was called because I was building a setup to allow me to flip a single switch to allow my radio, carputer, and amp to run off of AC. Thanks!

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    Question: I understand the use of the relay, but as you said, wouldn't it cut out all power as soon as it is switched off? So then, for most people, what would be the point of using the relay? Is there a way to wire it up so it has a smooth shutdown

  5. #5
    Newbie tcayere's Avatar
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    Quote Originally Posted by Rocket3004 View Post
    Question: I understand the use of the relay, but as you said, wouldn't it cut out all power as soon as it is switched off? So then, for most people, what would be the point of using the relay? Is there a way to wire it up so it has a smooth shutdown
    I think that is what a startup/shutdown controller is used for, so when you turn off the ignition the computer acts like you just hit the shutdown button and it turns off the computer properly

    someone correct me if i am wrong

  6. #6
    Maximum Bitrate FusionFanatic's Avatar
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    Quote Originally Posted by Rocket3004 View Post
    Question: I understand the use of the relay, but as you said, wouldn't it cut out all power as soon as it is switched off?
    a relay is a switch, so it will instantly cut off power to anything hooked up to lead 87 when the coil is not energized.

    Quote Originally Posted by Rocket3004 View Post
    So then, for most people, what would be the point of using the relay?
    the main reason is to isolate your power and/or signal wires from the factory wiring. as I mentioned earlier, the ACC and IGN circuits are not general power circuits. you should keep the additional load to a minimum to avoid overloading the circuit.

    signal (A.K.A. remote, REM, turn-on) wires do not draw much power, but they still draw power which can add up fast when hooking up multiple devices. if you only have one or two devices then a relay is not necessary, but if you have multiple devices to turn on/off with the ignition key then you should use a relay to avoid overloading your car's factory wiring.

    Quote Originally Posted by Rocket3004 View Post
    Is there a way to wire it up so it has a smooth shutdown
    it depends what you're using...

    if you're using a laptop (with the relay controlling power to the DC-DC laptop PSU) then power to the PSU can be cut instantly while the laptop continues to be powered by it's own battery. you can configure the power settings to automatically shut down after a set time without external power.

    with a desktop-type carPC (basically anything without it's own battery), cutting power abruptly can cause serious data loss and could prevent your system from booting. you can prevent this by using a startup/shutdown controller to turn the carPC on/off properly. a relay could control the ACC signal wire and/or turn power on/on to additional devices


  7. #7
    Variable Bitrate
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    Sketch up a diagram for those of you guys wanting to tap into your stereo or cd changers 12V ignition wire to feed your PSU and other pc devices its 12V ignition signal.


  8. #8
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    thanks for this write up! used it to wire up some angel eyes for my headlights to a switch.

    just to make sure, is it ok to connect the 87 pole to lights and to another relay's 86 pole so they both come on with the same switch? (each relay has it's own fused power source)

  9. #9
    Variable Bitrate
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    Yeah u could do that or you can just wire up the coils in the relay in parallel.

  10. #10
    Newbie Robotjam's Avatar
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    Using a relay to control rear defogger

    Hi guys i am new here but have found this thread to be very informative.
    I was wondering if you can help me, I am wanting to hook up my rear defogger on my 90 Lincoln towncar to an accessory output on my remote car starter. I know I have to use a relay but am unsure on how to do this. Can any one post a wiring diagram or some sort of hookup instructions? Thank you

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