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Projects Wiring Gurus: Looking for LOW VOLTAGE RELAY

Discussion in 'The Hokey Ass Message Board' started by Detonator, Jul 18, 2013.

  1. I'm looking for a relay that will carry 12V, but only needs about a 5V signal to trip it.

    I'm using the relay to power back-up lights, but the reverse trigger wire from the shifter mechanism is putting out less than 6V -- and it's not enough to trip the coil in a standard Bosch-style relay.

    The shifter is what it is, don't ask. I'm not going to get anymore voltage out of it. Just trying to fulfill a customer's dream.

    Thanks
     
  2. SMOG_GUY
    Joined: Jun 28, 2011
    Posts: 388

    SMOG_GUY
    Member
    from Dinuba

    How about a 6v horn relay?
    6v relay will carry 12v loads with ease. 6v systems carry HEAVIER amps then
    12v. Look at the wiring on a 6v car--it's always heavier han 12v.
    And anyway, horns take lots of current 6 or 12v.
    Don
     
  3. JeffB2
    Joined: Dec 18, 2006
    Posts: 8,420

    JeffB2
    Member
    from Phoenix,AZ

    Try Fry's Electronics they have a vast relay selection.
     
  4. Thanks, but horn relays are triggered by a ground being completed (your horn button). I need a relay that I can trigger by putting voltage to it.
     

  5. plym_46
    Joined: Sep 8, 2005
    Posts: 3,988

    plym_46
    Member
    from central NY

    SO??? Hook the ground to the chassis and switch the power going to it.....................The relay won't care!! A 6v relay should energize on 5 volts withoug a problem.
     
  6. Leviman
    Joined: Dec 11, 2012
    Posts: 201

    Leviman
    Member

    Yeah, you must realize a relay is simply an electromagnet. It doesn't matter if you switch positive or negitive, as long as the current is broken and then connected, you're fine. Also, make sure you get a non-latching relay, aka a horn relay. Also, i can't imagine why the switch is only putting out 5v. I would not be surprised if you hook a relay to that and the switch won't trip it due to dirty contacts or something.
     
  7. SMOG_GUY
    Joined: Jun 28, 2011
    Posts: 388

    SMOG_GUY
    Member
    from Dinuba

    The old horn relays I have are three terminal units I think. On my old stuff that would be 6v pos ground.
    So, two terminals for the heavy work.
    And the third terminal ( signal or trigger) goes to horn button. Button grounds the ckt through third terminal. Ground on this ckt causes relay to close and heavy current can flow through the first two terminals.
    Relay is mounted on metal but does not need to be grounded. No ground ckt inside relay connected to relay case. My best guess.
    Sooo, how would you make that work?
    On a 6v pos gnd sys hitting the horn button connects the relay to ground in this case pos+.
    On 12 v neg ground the first two terms still carry the heavy current.
    In order to trigger the relay you need a neg- ground. That's the third terminal.
    I googled horn relay and found what looks like a old GM four terminal horn relay. Just don't know if that's for 12 or 6v.
    A four terminal 6v horn relay should work, if it exists. You could trigger that with either pos or neg?
     
  8. First, you have to realize that a relay may or may not be rated for a single voltage. The voltage to operate it can be completely different from the rating of the switched contacts.

    Next, if you're putting 12V into the switch on the shifter and only getting 5V out of it, you may not find any relay that will operate. There's a definite problem with those switch contacts, you may lose what remaining voltage you have when you put a load across it. If the load presented by the meter drops that much voltage, the chances of operating anything are pretty close to zero.

    With that said, if you can't fix/replace the switch, find a 5V coil SPST relay with contacts rated for the load; 10 amps should do it. Look for the smallest 'pull-in' current (in milliamps) you can find and give it a shot, but I wouldn't count on success....
     
  9. SMOG_GUY
    Joined: Jun 28, 2011
    Posts: 388

    SMOG_GUY
    Member
    from Dinuba

    Did quick search Rock Auto shows a four terminal horn relay 1954 Dodge CAR. Looks like 99% out there are three terminal.
    I have no idea how that 4 terminal is connected inside but logic says two isolated ckts.?
     
  10. Let me ask a few questions; is the 'trigger wire' coming from some sort of electronic assembly, or is this a mechanically-operated switch? If the latter, disconnect the power wire and check resistance through the switch when it's closed. If you don't get zero ohms (at least under 1 ohm) when reading on the lowest ohm scale of your meter, the contacts are bad and you won't have much luck with any relay.

    If it's an 'electronic' power supply, you may have to 'stack' relays; a small one that will accept the low input ((and likely won't have contacts big enough for the load), and use that to drive a larger relay that can handle the load.
     
  11. Dan Timberlake
    Joined: Apr 28, 2010
    Posts: 1,269

    Dan Timberlake
    Member

    4 terminal circuit using European DIN terminal ID numbers. Grounding the relay and feeding a signal to the relay.
    http://binatani.com/wp-content/uploads/2009/07/lexus-relay-wiring-diagrams.gif

    4 terminal relay, grounding thru the horn button as you described.
    http://www.secondchancegarage.com/articles/images/relay/hornrelay.gif

    I think any 4 terminal relay could be wired either way.
     
  12. This car has been from shop to shop, it's now with us. We're horn-and-headlights guys, so this is a little OT. The shifter is one of those electronic push button-servo jobs. It's old, and the company that made it is out of business. I was able to track down one of the guys and he admitted that the low voltage on the reverse-signal output was a "known flaw" with the shifter.

    That said, it is what it is and now it's time to make it work. If it was simply a switch I'd jump 12V into one side of the coil and switch the ground on the other side -- and be on my way. But it's a box full of electronics and way above my pay grade :D

    So, the ideal fix would be a Bosch-style relay designed to carry 12V, but only needs 5V to trip it.

    Not really our style, but an interesting project:

    [​IMG]
    [​IMG]

    My partner did a lot of the aluminum work on the car:

    [​IMG]
     
  13. :eek::eek::eek::eek: i don't think I've ever seen so many freakin relays in a car!!!!
     
  14. davedriveschevy
    Joined: Mar 16, 2011
    Posts: 37

    davedriveschevy
    Member

  15. Fenders
    Joined: Sep 8, 2007
    Posts: 3,922

    Fenders
    Member

  16. Kona Cruisers
    Joined: Feb 4, 2007
    Posts: 1,074

    Kona Cruisers
    Member

    I'm guessing that is a 5V reference wire the was original a signal wire switch to ground in a PCM?
    For the Amperage that it would see on the trigger side of a relay why dont you just use a 5v to 12v step up? less then 10 bucks on ebay
     
    Last edited: Jul 18, 2013
  17. OldFord39
    Joined: Aug 23, 2011
    Posts: 64

    OldFord39
    Member
    from Monroe, Wi

    An Omron relay should work G8P-1C2T-F DC5 Mouser electronics Part # 653-G8P-1C2T-FDC5 The cost is $ 4.99 but they only indicate they have one
     

    Attached Files:

  18. BillM
    Joined: May 26, 2007
    Posts: 247

    BillM
    Member Emeritus

    Here is a link to a 6V automotive style relay that may work; but the coil resistance is 22 ohms which suggests it will need about a quarter of an amp to trigger it. If the 5V signal from your electronic shifter gizmo is meant to trigger low power IC logic signals then I would be hesitant to use that relay; lest you may let the smoke escape from the shifter electronics. The best solution in my mind would be to build a circuit with a transistor triggered by the 5V signal that would in turn switch a common 12V automotive relay, if you really have to use that shifter.

    http://www.newark.com/durakool/dg85b-8011-96-1006-m1/automotive-relay-spdt-6vdc-60a/dp/30M9197
     
  19. d2_willys
    Joined: Sep 8, 2007
    Posts: 4,101

    d2_willys
    Member
    from Kansas

    Find a 5-6 volt relay with contacts that can handle the current you need (don't worry about the voltage rating of the contacts). Check at:

    RADIATION SHACK.JPG
     
  20. customrod48
    Joined: Oct 10, 2010
    Posts: 201

    customrod48
    Member

    Just to muddy the waters.......it seems the coversation is mixing two separate elements here, the voltage needed to "trip" or engage the coil, and the amp rating of the contacts, or amp load consumed by the load. The coil portion is rated in voltage the coil needs to pull in the contacts, the contacts are rated for the amperage draw of the item being energized. So relays are rated by coil voltage and contact amps...too light of a amperage rating on the contacts will result in burnt contacts. If you need a 5v power source for the coil, then use a Ford type gage pack voltage reducer. Or get a relay that has the same voltage as the source you are using (6v or12v), and a contact amperage rating to match your load......
     
  21. Kona Cruisers
    Joined: Feb 4, 2007
    Posts: 1,074

    Kona Cruisers
    Member

    Or step up the voltage 12v and be done with it... Everyone is overcomplicating this. The 5-12v Step up is SUPER COMMON and i just checked ebay is like 2.99.
     
  22. d2_willys
    Joined: Sep 8, 2007
    Posts: 4,101

    d2_willys
    Member
    from Kansas

    Nice converter, but only good for 5 watts with a 6 volt input. Well 5 watts @ 12 Volts = .410 amperes. Not a lot of current to drive anything!
     
  23. The step up would only trigger the relay, typically about .2A. As far as the amperage load on the contacts, Detonator did say it was for backup lights...not a WINCH!
     
  24. d2_willys
    Joined: Sep 8, 2007
    Posts: 4,101

    d2_willys
    Member
    from Kansas

    Typical backup light (one bulb) 2.1 amps. Way too much for step up converter! And why would someone want to complicate this whole thing by putting a converter AND a relay, when a simple 5V, 4 amp relay will do the job. GO RADIO SHACK!
     
  25. Engine man
    Joined: Jan 30, 2011
    Posts: 3,476

    Engine man
    Member
    from Wisconsin

    It's expensive to find one relay to do it all. I'd use a relay that operates on 5 volt signal to control a regular 12 volt cube relay.

    Here's a low cost [$1.27]relay with a 5 volt coil with contacts rated at 5 amps 30 vdc;
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMtSzCF3XBhmW7qiUsndWlwIX8PrdzcuhNY=

    You might want a socket for it too.

    There are solid state relays that will carry 20 amps DC but they are over $60.
     
  26. Kona Cruisers
    Joined: Feb 4, 2007
    Posts: 1,074

    Kona Cruisers
    Member

    .2 amps triggers the relay, the relay handles the power... Or find an exotic relay you won't be able to find when it fails, that makes since.


    Posted using the Full Custom H.A.M.B. App!
     
  27. Kona Cruisers
    Joined: Feb 4, 2007
    Posts: 1,074

    Kona Cruisers
    Member

    do you understand the purpose of a relay, because the step only triggers the relay. The power side of the relay handles the load.


    Posted using the Full Custom H.A.M.B. App!
     
  28. Engine man
    Joined: Jan 30, 2011
    Posts: 3,476

    Engine man
    Member
    from Wisconsin

    Since when? I'm sure the step up converter will be easy to find when it fails. As I said, I would use a regular cube relay to control the lights and use the 5 volt control relay to control the cube relay so it has no real load and will last a long time. % volt control relays are becoming fairly common these days.

    If the signal from the shifter was designed for a computer control, it will have a very low current output. The computer only needs a very low current signal. There might not be enough current to operate a step up converter. The relay I listed has a coil current draw of .04 amps.
     
  29. Kona Cruisers
    Joined: Feb 4, 2007
    Posts: 1,074

    Kona Cruisers
    Member

    More than one way do do it. Wire a house with 40 different journeymen electricians, you'll have 40 differently wired houses. It's the same this here. They all work right? I'm just stating how I would (and have) wired a 5v reference to operate a 12v load. You have yours I'll have minePosted using the Full Custom H.A.M.B. App!


    Posted using the Full Custom H.A.M.B. App!
     
  30. d2_willys
    Joined: Sep 8, 2007
    Posts: 4,101

    d2_willys
    Member
    from Kansas

    Sorry bud, but do you know anything about relays? You can find 5 volt relays that draw less than 40ma (coil current) and have contact ratings of over 6 amps!

    Digikey # PB356-ND

    So you can blow away the step up converter. Been designing electronics for over 45 years!
     

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