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Thread: Relay Question

  1. #1
    Join Date
    Apr 2007

    Relay Question

    Hey, I want to power 2 amps and a m2 atx and was having trouble understanding how to work a relay.

    1)I wont have a hu so ill be using the stock wiring harness connections to connect to the relay. Will that work out alright or do i need thicker wires?

    2) Can I connect the output from the relay to multiple devices as shown in the diagram?

    3) Do I need fuses? and how do i figure out what size relay i need?

    Heres the diagram:

    Thanks in advance.

  2. #2
    Maximum Bitrate crooper's Avatar
    Join Date
    Nov 2006
    Everett, WA
    1) stock wires are fine for switching the relay, takes very little power to switch a relay
    2) yes
    3) fuse size is relative to the size of wire used, although I'm not much of an expert here, 1 amp should be fine for the switched rem wire, since it only takes like 200ma to switch the relay

  3. #3
    Variable Bitrate
    Join Date
    Jan 2007
    East Central FLA
    I doubt it. The current draw will be too high to drive two amps. What you can do is use the relay to drive the remote turn-on wire for the amps and probably the power supply, but I would suggest you do some investigation of the effect of the back surge off a relay and how it would affect the power supply. There are diode and other solutions (to the surge issue), but someone with more circuit design experience than me should help with that.

  4. #4
    DM2 is offline
    Newbie DM2's Avatar
    Join Date
    Apr 2007
    Houston, Texas

    Relay Sizing


    The first thing you need to do is figure out how much load (i.e. Amps) each piece of equipment draws. Some equipment will specify the load in watts which means you'll need to convert this to amps. To convert to amps divide the watts by the voltage the unit requires. Generally speaking a 12 volt device will draw almost twice as much current as a 24 volt device will draw (for more information on electricity check out the HowTo web site page: ).

    Once you've totaled up the load for all three (3) items, and assuming there all the same voltage, you'll know how much current will be passing through the relay contact.

    Next you need to check what the maximum load rating of the contacts on the relay are and make sure that it is at least 125% of the total load you're trying to pass through it is.

    1. PC requires 150 watts at 12 Volts DC (150/12= 12.5 amps)
    2. Amp 1 requires 100 watts at 12 Volts DC (100/12=8.33 Amps)
    3. Amp 2 Requires 100 watts at 12 Volts DC (100/12=8.33 Amps)

    12.5 Amps + 8.33 Amps + 8.33 Amps = 29.16 Amps

    29.16 Amps x 125% = 36.45 Amps (Minimum Relay Contact Rating).

    I suspect the relay your using is likely rated at 10 Amps, so you'll need to use either more relays, or in the case of the PC a relay with larger contact ratings.

    I would recommend that you fuse each amp with a separate fuse and the PC with a fuse as well. Here again use the 125% rule when sizing your fuse (i.e. PC at 12.5 Amps x 125% = 15.6 amps).

    I would also recommend fusing the current to the relay. Relay current varies among manufacturers but it should be marked on the relay itself. A 12 Volt relay is likely more than 0.050 Amps (50 mA) but less then 0.200 Amps. Again use the 125% rule.

    Wire sizes specified in AWG (American Wire Gauge) has current limitations and you what to make sure you use a wire with a current capacity higher than the fuse rating otherwise the wire will melt before the fuse blows.

    Wire Size Max Ampacity
    10 AWG 40 Amps
    12 AWG 30 Amps
    14 AWG 25 Amps
    16 AWG 18 Amps
    18 AWG 13 Amps
    20 AWG 09 Amps

    The above recommendations are based on 600 Volt Insulated wire.


    Real world knowledge isn't dropped from a parachute in the sky but rather acquired in tiny increments from a variety of sources including panic and curiosity.

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