"explosion" proof switch

Discussion in 'OnBoard Electronics & Controls' started by BertKu, Mar 24, 2015.

  1. BertKu
    Joined: May 2009
    Posts: 2,480
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi there,
    Often somebody wants to switch high currents , like 300 - 500 Ampere and is complaining about the high cost of a breaker, which isn't actual save to use in a fume filled environment.
    I had to switch high currents, when switching from 24 Volt to 36 Volt for my electric boat. Attached the way I have done it.
    a) for a single on/off switching earth. The bottom half part.
    b) for switching on/off at the plus side The top half part.
    c) for a change over like in my case when I needed to switch a third 12 Volt battery bank to the existing 24 Volt bank, the complete circuit. i.e. 36 Volt output.

    If one needs to switch 300 Ampere, you only need 10 MOSFETS parallel x2 for the changeover. Cost +/- 1.20 Each.
    You need a DC to DC converter at +/- 4 dollar
    2 resistors at 1 dollarcent each., A piece of printed cuircuitboard and some
    Copper or Aluminium busbars.
    Total cost? under the 35 dollar and I assure you, it is safe to use it in a fume environment as there are no sparks. If you want to be very sure, pot it in.

    To explain the Mosfets I use is the IRFP064 N, 9 milliOhm when in ON condition and 114 Ampere each. i.e. 10 x 114 Ampere = 1140 Ampere, but I always like to play it safe.
    For the DC - DC converter I use a B1212LS-1W
    2 x 1,2 KiloOhm 1/4 watt resistors.

    10 MOSFets parallel, means 9 /10 = 0,9 milliOhm x 300 Ampere = 0,27 Volt loss.
    Your heatsink does not need to be that brilliant. It does get hot a little.
    For a 300 Ampere i.e. 10 parallel, you will have 0,9 milliOhm = 81 watt dissipation
    efficiency? reasonble high. the DC-DC only uses 0.2 watt .

    Bert
     

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  2. Anton59
    Joined: Nov 2011
    Posts: 2
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    Location: Greece

    Anton59 New Member

    Some info needed

    This sounds all very interesting but I have some queries.
    What is the max. voltage that can be applied and also why switch from 24v to 36v? Again why a potentiometer and not a switch?
    I will take your word for it but remember, we are not all rocket scientists.
     
  3. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    Do the MOSFETS fail open or closed? What is the worst case scenario if they fail closed?
     
  4. BertKu
    Joined: May 2009
    Posts: 2,480
    Likes: 43, Points: 58, Legacy Rep: 223
    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi Anton, In my case I used a 55 Volt MOSFETs and thus no higher than 48 Volt. In anyway , I believe one should not go over the 48 Volt on a yacht or leisure boat, to avoid serious problems when accidental touching a connection.

    I have the extra 12 volt on top of the 24 Volt to have in an emergency, some extra whoom. Basically I double the power to the prop. Also I use the 12 Volt battery bank as a spare emergency power in case, the main battery bank gets low or for whatever reason.

    I use a potentiometer to avoid any problems by switching a capacitive or inductive load, like an electric motor. Or in my case a large electrolytic capacitor, which is then charged to 36 Volt and then suddenly switched back to 24 Volt. The switching is thus gradually and not suddenly whereby a large spike is created. A switch will create a spike when switching the motor off or in case of a large capacitor, the capacitor is not discharged by the time it switches from 36 to 24 Volt. Trust that will explain why I have done so. Bert
     
  5. BertKu
    Joined: May 2009
    Posts: 2,480
    Likes: 43, Points: 58, Legacy Rep: 223
    Location: South Africa Little Brak River

    BertKu Senior Member

    Yes you are right. Like most of the semiconductors, it fails closed (short circuit). However only one will pack up and melt the internal bonding wires and the short circuit opens up (like a fuse). Because of the speed, the MAIN fuse will not blow except if all MOSFETS fails. I mentioned before, that in case one has some concerns, change the type of heatsinks and connect the MOSFET's on top of each other and pot the whole circuit in. I will be safer than a circuit breaker in a fume fuel filled engine room or environment.
    Bert
     

  6. BertKu
    Joined: May 2009
    Posts: 2,480
    Likes: 43, Points: 58, Legacy Rep: 223
    Location: South Africa Little Brak River

    BertKu Senior Member

    Just to add to my previous reply. In a breaker the contacts can also melt together, if an over current situation exist. The difference is that it will spark. While if one has potted the whole circuit in, my experience is, that you only get a heated marked spot on the MOSfet. But no sparks. One could use a vacuum switch, but they are quite pricey. At the end, a user has to judge whether his application is suitable for MOSfets.
    Bert
     
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