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Technical Relays

Discussion in 'Traditional Hot Rods' started by cammer8, Jan 13, 2020.

  1. cammer8
    Joined: Aug 1, 2016
    Posts: 48

    cammer8

    I 'm converting to 12 volts on my 1934 Ford. Wondering if relays are a good idea for headlights. I know Henry didn't use them. I am using fusable links where needed. Any ideas?
     
  2. cammer8
    Joined: Aug 1, 2016
    Posts: 48

    cammer8

    I 'm converting to 12 volts on my 1934 Ford. Wondering if relays are a good idea for headlights. I know Henry didn't use them. I am using fusable links where needed. Any ideas?
     
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 46,056

    squirrel
    Member

    What bulbs are you using? If they're relatively normal, and don't draw excessive current, there should be no need for relays.
     
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  4. Boneyard51
    Joined: Dec 10, 2017
    Posts: 3,998

    Boneyard51
    Member

    Anytime you add relays, you add potential trouble spots. Use them where necessary, don’t use them unless necessary!






    Bones
     
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  5. If you are using a standard bulb or even if you convert to seal beam you should be fine. A 6V switch is heavier that a common 12v light switch as 6V pulls more amps. if you are going to some sort of high zoot bulb like 200 watt halogens or the like you probably should use a relay.

    here is the deal you use a relay when your amp draw is greater than the capacity of your switch. Generally speaking.
     
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  6. Dave Mc
    Joined: Mar 8, 2011
    Posts: 1,976

    Dave Mc
    Member

  7. klawockvet
    Joined: May 1, 2012
    Posts: 375

    klawockvet
    ALLIANCE MEMBER

    Relays are always best IMO but I never used one on my 33. I did run a circuit breaker on it. When a fusable link goes out your lights are gone. A circuit breaker will usually flash back on long enough to get to the side of the road. Then if you can find the short you will have power to keep going. Ran the car for over 25 years like that.
     
  8. manyolcars
    Joined: Mar 30, 2001
    Posts: 8,262

    manyolcars

    The relay is only going to help if you run heavier wires
     
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  9. jaracer
    Joined: Oct 4, 2008
    Posts: 321

    jaracer
    Member

    On heavy trucks the headlights are split between two relays, one for each headlight. The low beam is connected to pin 87A and the high beam to 87. Power is still connected to pin 30 with the dimmer connected to one of the 84/85 pins and the other 84/85 pin grounded. If for some strange reason one of the relays fail, you still have one headlight.
     
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  10. squirrel
    Joined: Sep 23, 2004
    Posts: 46,056

    squirrel
    Member

  11. squirrel
    Joined: Sep 23, 2004
    Posts: 46,056

    squirrel
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  12. Boneyard51 and squirrel like this.
  13. Mr48chev
    Joined: Dec 28, 2007
    Posts: 26,857

    Mr48chev
    ALLIANCE MEMBER

    Unless you are running high wattage halogens you probably don't need them. High wattage bulbs draw a lot of amps through switches cause switch failure that low wattage regular bulbs don't seem to cause. That from my own experience on my own vehicle that was burning out floor mounted dimmer switches about every 8 months because I was running 55/100 H-4 halogen headlights. Relays fixed that because the amp draw through the switch was then only what it took to trigger the relay.
    Outside of the main power feed from the battery to the fuse block/junction box. I question the advisability of fusible links in multiple lines. Regular fuses in a fuse block or even fuse holders would be a better solution or circuit breakers in a fuse block. Blowing the main feed fusible link means that you most likely saved the system from burning up and need to fix something but blowing a fusible link in a specific circuit on the road means that you may be stuck without a simple fix. You can always pull a fuse out of a circuit you can do without for a while until you get to where you can get more fuses and go again if you blow a headlight fuse but I'd hate to be stuck on the side of the road at night with a blown fusible link to the headlights and no way to fix it on the side of the road.
     
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  14. Still haven't heard what type headlight bulbs are going to be used, on either site.

    The need for a relay is based on whether the headlight switch can handle the draw of the light bulbs.

    Phil
     
    Last edited: Jan 13, 2020
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  15. cederholm
    Joined: May 6, 2006
    Posts: 1,621

    cederholm
    Member

    THIS!

     
  16. lowrd
    Joined: Oct 9, 2007
    Posts: 239

    lowrd
    Member

    When I rewired my Chev, I used two relays, one for low beam, one for highs. The headlight switch just triggers the relays and does not carry the heavy current for illumination. This avoids any high amperage coming through the fire wall lessening the chance of a fire if the insulation rubs through.
     
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  17. cammer8
    Joined: Aug 1, 2016
    Posts: 48

    cammer8

    I am looking at Speedway Motors standard high/low bulbs. 50 candle power high, 32 low.
     
  18. Ziggster
    Joined: Aug 27, 2018
    Posts: 579

    Ziggster
    Member

    The only reason to run relays is if the headlight switch cannot handle the current draw of the headlight. I'm guessing if you are using the original switch, and the original circuit didn't have relays, there is certainly no reason to run relays if you are going to 12V as the current will be half of what it used to be everything else being equal.
     
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  19. cammer8
    Joined: Aug 1, 2016
    Posts: 48

    cammer8

    Just running a Speedway incandescent bulb. 50 candle power high/ 32 low.
     
  20. trollst
    Joined: Jan 27, 2012
    Posts: 1,722

    trollst
    ALLIANCE MEMBER

    The real answer is.....YES! Run relays for your headlights regardless of the type of switch you have, a relay feeds power directly from the battery to the bulbs, easier on the bulbs and in my opinion they last longer, easier on the rest of the wiring in the circuit, (less load), and the biggest reason, your lights will be as bright as they can possibly be on a dark night. Works for me.
     
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  21. I say for 50 watt bulbs, no relay needed to protect the switch from being overloaded.

    I think the math goes like this:

    100 watts ÷ 12 volts = 8.5 amps

    Others agree?

    Phil
     
    Last edited: Jan 13, 2020
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  22. Mimilan
    Joined: Jun 13, 2019
    Posts: 560

    Mimilan
    Member

  23. Ziggster
    Joined: Aug 27, 2018
    Posts: 579

    Ziggster
    Member

    Adding relays when not required is adding complexity, and more possible points of failure. If you do decide to use them, avoid the cheap parts store stuff, pick up some Bosch ones. If mounted in the engine compartment, get some sealed ones. I've picked up sealed ones from the wreckers for use on my truck with added my LightForce lights.
     
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  24. My way of thinking of this issue, is that you would want to run at least one relay to protect the contacts in the light switch from the much heavier current load of the lights. This way, the light switch is going to last much longer, because it is only switching a minimal current.
    You could use a double pole relay, which would have a single input controlling the separate outputs to split the load to the two headlights.
    Bob
     
  25. morac41
    Joined: Jul 23, 2011
    Posts: 532

    morac41
    Member

    Mr48chev X 2.....I relay nearly everything...better not to draw current through any switch if you don't want voltage drop...
     
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  26. VANDENPLAS
    Joined: Dec 14, 2009
    Posts: 2,362

    VANDENPLAS
    Member

    I like using relays on higher draw stuff.
    Head lights etc.

    bit if your switch is sized correctly and your using regular sealed beams your ok.

    relays can and are safer then a standard switch, but now your running a lot more wiring to accomplish the same task.
     
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  27. squirrel
    Joined: Sep 23, 2004
    Posts: 46,056

    squirrel
    Member

    ....because, you know, old light switches only last for like, 70 years or so.

    sheesh
     
  28. I am not stating that the switch would fail, but most of the high end cars today use relays for the lights. In my personal experience, I have had to replace two light switches, both on trucks with nothing more than the standard two halogen headlights. All the modern vehicles I have owned in the last thirty years, had relays,
    From my experience, I am just stating how I would wire the system.
    Just sayin'
    Bob
     
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  29. gene-koning
    Joined: Oct 28, 2016
    Posts: 1,812

    gene-koning
    Member

    Yep, the old one will probably last 70 years.
    I wouldn't want to bet on the new ones making it anywhere near that long. They need all the help they can get. It took 4 new switches just to get one that functioned correctly on my son's 57 Dodge wagon. The original switch went through s flood (really) and it was toast. The current new stuff isn't even necessarily good right out of the package.
    Add the relays. Gene
     
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  30. squirrel
    Joined: Sep 23, 2004
    Posts: 46,056

    squirrel
    Member

    Or shop for old light switches. That's what I prefer.
     

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