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High current 12 volt relay

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  • High current 12 volt relay

    I need to power a 12 vdc fan on a hyd oil cooler, this fan draws about 40 AMPs cont. but the 40 Amp Bosch type relay I am using now does not last more than a few days. I have several of these relays with matching sockets so I was wondering if it would work to use two or more in parallel to carry the load?

  • #2
    We parallel relays all the time for the pre-charge circuits in our servo drives. I don't think it is the perfect solution because the contacts never make all at the same time, but is it does seem to work well enough especially if the current inrush can be limited. Most of these are automotive relays.

    If you are handy with electronics a power FET circuit (solid state relay) may be a more reliable solution than dry contacts. SSR's are commonly available but not really cheap, but you can build an equivalent circuit pretty easily.
    Last edited by gzig5; 02-19-2013, 12:55 PM.

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    • #3
      Starter relay?

      Tim

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      • #4
        Parallel rarely works well this situation (contact timing on both start and release).. The problem is not the continuous current, but the startup, or, there is no snubber across the contacts. Personally, I'd toss it and put in an SSR. Maybe $10-$20...don't forget the heatsink.

        Of course, as suggested above, a starter motor solenoid (find one on an old rider mower etc) is dirt cheap and will most certainly have enough capacity.

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        • #5
          I've used the old style Ford starter relays that mount on the inner fender in many apps.
          "Let me recommend the best medicine in the
          world: a long journey, at a mild season, through a pleasant
          country, in easy stages."
          ~ James Madison

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          • #6
            If the relay has sockets then it does not seem like a 40a rated relay, unless that is an AC inductive load?
            For 40amps DC I would think the minimum is a Special Purpose contactor, these are specially made for high duty cycles and other features.
            A 40amp inductive load is a considerable load to switch, a DC contactor is usually required, these have special conductors around the contacts for arc blow out, this can also be done with a blow out relay with a magnet.
            Also suitable arc chutes are fitted between contacts.
            A solid state solution should also work.
            Max.

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            • #7
              For low voltage DC applications the contactors used on golf buggys are some of the cheapest and most rugged I've found .

              The coils are in the 12vDC ->48vDC range and the contacts are usually rated in the 100-400amp range.

              http://www.ebay.com/sch/i.html?_saca...lf+cart&_frs=1

              Rob

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              • #8
                I thought of using a starter solenoid (relay) but all that I have found, including the Ford, are only rated for intermittent duty. There are automotive factory fan relays out there and aftermarket fan relay kits but even the factory fan relays alone, at least those with the high current ratings, are around $150. Since I had the 30 Amp relays and some wired sockets on hand I thought maybe the parallel idea might work but apparently not and for the same reason I was already concerned about, failure of both to carry the start load equally.

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                • #9
                  Originally posted by MrSleepy View Post
                  For low voltage DC applications the contactors used on golf buggys are some of the cheapest and most rugged I've found .

                  The coils are in the 12vDC ->48vDC range and the contacts are usually rated in the 100-400amp range.

                  http://www.ebay.com/sch/i.html?_saca...lf+cart&_frs=1

                  Rob

                  Yes, can't beat those. And they are continously rated.

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                  • #10
                    Originally posted by MrSleepy View Post
                    For low voltage DC applications the contactors used on golf buggys are some of the cheapest and most rugged I've found .

                    The coils are in the 12vDC ->48vDC range and the contacts are usually rated in the 100-400amp range.

                    http://www.ebay.com/sch/i.html?_saca...lf+cart&_frs=1

                    Rob
                    We must have been typing at the same time.


                    I just took a very quick look at that link but that looks like exactly what I need, I had searched E-Bay for relays and only found more of the same (or similar to) of what I had. I did not think to search for continuous duty solenoids. Thanks

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                    • #11
                      It took me about 30 seconds to find these 80 amp 12 volt relay, http://www.jamestowndistributors.com...12579&type=pla and not expensive

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                      • #12
                        The old style Ford fender mount starter relay, available in both remote and case grounded versions is also offered as a continuous duty version by most suppliers like Standard. Last time I looked in the book, there was an asterisk by the part number in the line leading to the continous duty version p/n at the bottom of the page. They work, they are cheap and convenient to mount and use. I have used them in remote battery setups for years.

                        The difference is higher resistance in the primary windings for the continuous duty version, easily checked with an ohmmeter.
                        Last edited by jim davies; 02-19-2013, 04:19 PM.

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                        • #13
                          40amp DC SSR.
                          http://www.futurlec.com/RelSS.shtml#DCDC_SSR
                          Max.

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                          • #14
                            The intermittent duty and continuous duty automotive-type solenoids are the same except for the coils. The continuous duty solenoid has a higher resistance in the coil so it won't get hot when it's on for a long time.

                            You could probably use one of the intermittent duty solenoids if you put an external resistor in the circuit to the coil. Be careful with the Ford solenoids that have a snubber diode on the coil circuit. Reverse polarity will ruin it, and the solenoid won't work.
                            Any products mentioned in my posts have been endorsed by their manufacturer.

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                            • #15
                              So long as you use the *DC* variety, AND so long as you can deal with added losses and voltage drops, the DC SSR is fine.

                              A mechanical relay has little voltage loss, and needs no heatsink.
                              1601

                              Keep eye on ball.
                              Hashim Khan

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