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    1. · Registered
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      87 Posts
      I purchased a relay to turn on my 1000w inverter when the van starts. I then read the above postings and had a couple questions.

      If the relay doesn't have a flyback diode, and I put a diode inline on the wiring to the 12V+ trigger side of the switch, would that protect the van from any reverse surge?

      Also, am I correct that the relay itself would be damaged by the potential arcing (at least over time) because the switch is inside the relay box? If yes, then isn't this a design problem for the longevity of the switch? I wonder how long it would take to fail (seems like maybe it won't for a long time, or how can they sell them like that)?

      Lastly, would I need a diode that could handle 10x the switch load or the 12V?

      This is what I purchased Amazon.com: 12V DC 120 Amp Split Charge Relay Switch - 4 Terminal Relays for Truck Boat Marine : Automotive

      Thanks!
       
    2. · Registered
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      644 Posts
      Others are correct in that the "catch" rectifier (diode) should be included whenever energizing the relay coil from a source that may not be intended for driving a relay. The relays with a rectifier built in are ideal.

      If you need to add one, the "catch" rectifier needs to go across the relay coil, not in series (inline) in order to do its job. You want to short circuit the coil in the reverse direction while magnetic field begins its collapse.

      The peak current from the field collapse can be significant, but the rectifier will conduct for such a short time that you really don't need a large device at all. For a relay coil that will consume 50-100 mA (0.05-0.10 Amps, which is typical) while energized, a rectifier with a 1A rating should work just fine. The 1N4000-series comes to mind, so maybe a common 1N4004 with a 400V reverse voltage rating.

      Any potential arcing of the relay contacts is largely unavoidable, and the severity depends on the load. With a really inductive load (like a motor) a condensor (capacitor) could be added to the load to quench the arcing on the contacts. With a capacitive load like an inverter, an in-rush limiter (thermistor) could be used, but may cause problems depending on the inverter. Some inverters include in-rush limiting as part of their design.

      Use a large enough relay to handle the load (maybe 5X for an inverter which is a reactive load), and the arcing won't destroy it so fast. And use the rectifier to avoid damaging the origin of the ignition tap source from the relay coil "kick back".

      Good luck!


      I purchased a relay to turn on my 1000w inverter when the van starts. I then read the above postings and had a couple questions.

      If the relay doesn't have a flyback diode, and I put a diode inline on the wiring to the 12V+ trigger side of the switch, would that protect the van from any reverse surge?

      Also, am I correct that the relay itself would be damaged by the potential arcing (at least over time) because the switch is inside the relay box? If yes, then isn't this a design problem for the longevity of the switch? I wonder how long it would take to fail (seems like maybe it won't for a long time, or how can they sell them like that)?

      Lastly, would I need a diode that could handle 10x the switch load or the 12V?

      This is what I purchased Amazon.com: 12V DC 120 Amp Split Charge Relay Switch - 4 Terminal Relays for Truck Boat Marine : Automotive

      Thanks!
       
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