Bilge Control Via Relay

Discussion in 'OnBoard Electronics & Controls' started by IronPrice, Jul 25, 2017.

  1. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    The Manufacturer probably also uses a IRFP06N in their solid state relay. This device has a build in zener diode which clips the voltage to 55 Volt over the MOSfet, but they cannot guess what inductive load is used over the "contact" by the user and therefore plays it safe. Although the inductive resistance maybe low of the bilge pump, due to the speed of switching from the MOSfet, much faster than a normal switch, the inductive resistance is much higher and could exceed the 55 Volt spike and using a too low rate diode can give you a hard time while at sea. Bert
    P.S. A too low selected voltage normally let the diode shorten when overloaded. i.e. select a diode which is 600 or 700 Volt.
     
    Last edited: Sep 13, 2017
  2. IronPrice
    Joined: Jul 2017
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    Location: NZ

    IronPrice Junior Member

    How about these - 1N4007 1A 1000V

    (picture on the page may be a category picture not the actual product).

    Is 1A enough? There is also a 3A version.
     
  3. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    1 N4007 is normally good enough, except if you have a large inductance then it is better to use a 3 Ampere diode. I don't have the specification of the bilge pump you are using. I only know that it is 13 Ampere. If you feel you don't want to take a risk and the price for the 3 Ampere is reasonable, go for it. Bert
     
  4. IronPrice
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    IronPrice Junior Member

    I found diodes rated at 1000v and 6A.
     
  5. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    There are two things important for the diode. Let say your 13 Ampere bilge pump switch on or off by the Mosfet and could create a spike of 80 Volt or more. Good enough to blow your MOSfet in your SSrelay. At 12/13 Volt and 13 Ampere, your overall inductance is 13 /13 = 1 Ohm. Thus for a fraction of a second during on and off switching this relates to 80 Volt divided by a minimum of 1 Ohm , up to 80 Ampere. It will be slightly less, due to the increased inductance, but still, it means that your datasheet should tell you whether your 10 Ampere diode can withstand high currents for a fraction of a second. If not, normally a diode get short circuit at over current and that means it will blow your fuse and that means that you are sitting duck somewhere on the sea, fiddling around with un-soldering of the diode and replacing it. Nobody wants that. The 1N4007 can handle up to 30 Ampere for 8.3 millisecond and probably 100 Ampere for a microsecond. You must just verify whether your 10 Ampere diode can handle high currents for a short period of time. Good luck. Bert
     
  6. IronPrice
    Joined: Jul 2017
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    Location: NZ

    IronPrice Junior Member

    The FR607 diode I have found supposedly has an 8.3 millisecond rating of 300A.
     
  7. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    You can sleep safely at sea. Have a nice journey. Bert
     
  8. owene
    Joined: Aug 2009
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    Location: Nelson, NZ

    owene Mr Owen Charles

    No, no and no! Adding relays is adding complexity to a situation that should be simple and reliable. This may well be a life and death issue and if the relays fail for whatever reason, you are sunk. For what it's worth, put in thicker cables and reduce the risk.
     

  9. IronPrice
    Joined: Jul 2017
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    Location: NZ

    IronPrice Junior Member

    Done, done and done! Working flawlessly too.

    The relays only control the manual circuit - which is already a back-up system. The auto circuits on the bilge pumps have to fail before I rely upon the relays, and if they fail I can bypass them. I can also bypass the float switches in a couple of minutes, if I'm able to get into the bilge pump junction box.

    Because I'm a 'belt and braces' kind of guy ... I built in a manual by-pass for each relay, so I can switch one or both bilge pumps on and leave them on. In a serious situation, there is no danger of them running dry.

    I also have a spare bilge pump pre-rigged with a hose, cables and a DC plug. As an almost last resort, I can plug that puppy into a DC outlet at the transom.

    I also have a bucket ...
     
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