Highest Volatge advisable

Discussion in 'OnBoard Electronics & Controls' started by BertKu, Jun 21, 2012.

  1. BertKu
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    BertKu Senior Member

    Join the club.

    I use, where ever possible MOSFETS 100 Ampere each for 2 dollars and a heatsink. Relays switching 100 Ampere are expensive. Did you solve your problem with your charger en your diesel generator?

    P.S. Mosfets don't spark
     
    Last edited: Jul 2, 2012
  2. mydauphin
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    mydauphin Senior Member

    Where can I buy such animals for that price?
     
  3. Submarine Tom

    Submarine Tom Previous Member

    China, online.
     
  4. BertKu
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    BertKu Senior Member

    I don't know whether International Rectifier produces them in China, I thought in the Silicon Valley or in Arizona. Type number IRFP064N , 8 milliOhm. I buy them from Mantech Electronics in quantities. Internet buyng is normally more expensive, but have a look at www.mantech.co.za and enter IRPF064N. Rand 7 = 1 dollar.

    At 100 Ampere and 8 milliOhm, it means 80 watt dissipation. You need good cooling. I use up to 6 parallel and therefore the dissipation is only 13 watt and the heatsink is not that large.

    The disadvantage is one need 12 Volt for the gate. Quite a lot of Mosfets need only 4-5 Volt for the gate and can thus be driven by a microprocessor.
    Bert
     
  5. sdowney717
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    sdowney717 Senior Member

    here is the datasheet
    http://www.irf.com/product-info/datasheets/data/irfp064n.pdf

    so if you put 12 volts dc on the gate it will close the circuit from source to drain?

    it looks like gate is plus or minus 20 volts DC?
    So you could use this as a relay in a switch circuit?
    How does this differ from a solid state 12 volt coiled relay?
    will it only pass DC current from S to D?
     
  6. BertKu
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    BertKu Senior Member

    Yes. You need to keep an eye on the heat development in the mosfet when switching 80 or 100 Ampere.
    The formula is easy. Current x current x resistance. i.e. the resistance is 8 milliOhm at 80 ampere = 80 x 80 x 0.008 = 51,2 watt. If you touch a 60 watt globe, you will burn your hand. So is it also when you dissipate 51.2 watt in your Mosfet. Thus you need to place it on a sufficient large aluminium heat sink. I use MOSfets parallel with a 10 Ohm resistor in the gate to avoid oscillation and have up to 6 MOSfets parallel. Immediately your power dissipation will go down. Example 4 MOSfets parallel means 2 milliOhm thus the power dissipation is now only 80 x 80 x 0.002 = 12,8 watt which is much easier to cool and need a smaller heat sink.

    There is a trick, if you want to use a N type MOSfet between the plus and a motor or a battery or a stove or fridge. In that case you need to place the 12 Volt gate voltage on top of you supply voltage. To do that, you buy a small DC to DC isolated inverter.(about 3 dollar each) In my case I have a brushless motor and need to switch 3 wires from ground to the 3 coils and also to switch the same 3 coils from plus to the coils. I lift thus the 12 Volt with the inverter on top of the 36 Volt to the brushless motor coils.

    I think they mean approx 20 Volt. Probably for indication the leakage current at + 20 Volt and the leakage current at minus 20 Volt

    Yes, but I will attach an example on how to do it. Per example, to switch 2 batteries parallel for charging on a normal 12 Volt alternator and to switch it to 24 Volt to use it for your winch.
    No idea, I have never used solid state relays, but assume that if the drain/source is open, the current can flow in both directions. I assume, ask CDK
     
    Last edited: Jul 10, 2012
  7. BertKu
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    BertKu Senior Member

    My apology, the attachment were not attached.

    Here they are.

    1) The best is to use a micro and the micro will make sure that the switches will not accidental short circuit the batteries.
    2) The principle drawing can be used, but be very aware about switching a switch at the wrong moment. It is mainly created to show you the actual working.
    3) one can do it for 12,24,36,48 Volt. Like I made it to test electronics at various voltages.

    In your case, you like to switch the charger off, when the batteries are nearly charged. You can simulate this with one toggle switch and a MOSfet on a heat-sink. The battery is a small 12 Volt remote control battery. Bert
     

    Attached Files:

  8. Fanie
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    Fanie Fanie

    The reason why one would use higher voltage is to reduce the current for certain application, as mentioned before, higher DC voltage is a problem switching and spikes is an electronics killer.

    12V items draw so little current, that it's almost not worth it to consider higher voltages.

    Batteries charged in series does not last well and it is not good practice, the slightly better battery always ruin the others.

    There is almost nothing that cannot work off 12V, you name it, everything - except a microwave oven.

    My 2 worthless ZA cents are, stick to the standard 12V, it's cheap, safe and readily available everywhere.

    Bert, there are things you can switch with electronics if you provide for some protection against PIV's from coils and switching and if you waterproof the electronics - especially at sea.

    Midas Auto Spares and probably Autozone too sells inexpensive relays that can be used to switch heavy currents, the ones I bought there can switch 150A or there abouts, never had a problem with them. It just makes it so much more robust and durable.
     
  9. BertKu
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    BertKu Senior Member

    I fully agree with you, but I have one problem. My Marsh motor at 48 Volt is drawing 100 Ampere. The 4 other brushless motors can go up to 72 Volt (If I really want to, but I don't want to draw 7,5 KW) and the others are up to 36 Volt. Thus I wanted to see what the forum members has to say about the highest voltage. Very interesting answers. I will visit Autozone and see whether they have in this town also those relays. I could use some for testing. Midas does not have higher than 30 Amp. (here at our place)
    Thanks Fanie , Bert
     
  10. Fanie
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    Fanie Fanie

    Sorry for being stupid, but what is a marsh motor and what are these motors for ?
     
  11. Fanie
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    Fanie Fanie

    Bert, I think those relays are called 'solenoids' by the motor mac's. They have a coil that pulls a large contact in that sits over the coil. They look just like the heavy duty relays you see in most boats for the trim & tilt and starter motor.
     
  12. BertKu
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    BertKu Senior Member

    Marsh motor is an electric DC motor with magnets on the rotor and the coils are mounted on the stator. The efficiency is high. The motor can run at full power of 4,5 KW continuous. They cost incl. import duties, VAT and freight Rand 5000,-
    Bert
     
  13. Fanie
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    Fanie Fanie

    We use trolling motors for quiet moving around fish, but I don't think they are economic enough re power usage and not powerfull enough for powering boats. Few things beat the old iron horse. I bought a 5hp Parsun... very impressive for it's size, 2 stroke of course.
     
  14. BertKu
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    BertKu Senior Member

    Your slogan is: Water !, just gimme water !.

    It will be in my case, : Juice !, just gimme batteries !!
    Bert
     

  15. Fanie
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    Fanie Fanie

    Hello Bert,

    One must always look at what has been done and used and how successfull it is with what is available. Batteries are not developed to the stage where I think they are usefull as a main power source...

    When powering a boat you will need full power for a prolonged duration like in bad weather with wind. Keep in mind you may experience heavy overcast weather where little charge will give any usefull power to the batteries.

    If you want to use batteries, at least have an outboard at the ready to take over. Use the batteries for good weather when sun is a plenty and wind is low, but you need something reliable for all the other times.
     
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