Simple wiring diagram for small craft

Discussion in 'OnBoard Electronics & Controls' started by CDK, Jun 8, 2009.

  1. DaveJ
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    DaveJ Senior Member

    I agree with Tim B with the use of MOSFET's over relays for the following reasons.

    1. 9 times out of 10 they are cheaper
    2. More reliable
    3. Are enviromental sealed (execpt for the leads, but your going to have the same problem with a relay)
    4. You don't get contact bounce
    5. With a small RC (resistor and capacitor, also know as a time delay) curcuit attached across the gate and drain you can achieve a soft on/soft off flow of current. ie. You can have a long time delay to faid lights on and off.
     
  2. CDK
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    CDK retired engineer

    I've nothing against MOSFET's but for most boaters even a relay is a mystery.

    A disadvantage is that the rugged and cheap ones are N-Channel, so you switch the ground lead. Not a problem but you must get used to it.
    For the soft turn-on the dissipation needs some consideration, a heavy load will heat up a Mosfet very quickly.
     
  3. Ike
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    Ike Senior Member

    I agree with Gonzo. Those knife switches may be cheap and simple, but is it worth saving a few bucks and losing your boat or your life?

    Ignition protected switches are readily available, but they do cost a little more.
     
  4. Dean Smith

    Dean Smith Previous Member

    cdk, nice but surely folk want it simple, here is one I did so the owner can read it:) all boards supplied BEP marine NZ Was told other day that can only buy black tinned due to new fire regs re coatings for wire
     

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  5. whitepointer23

    whitepointer23 Previous Member

    hi everyone. i have an electrical question. my boat has a seperate house battery and 1 starter battery for each engine, at the moment the house battery is wired to the starboard starter battery and it also runs through a solenoid which all the lights and accessorys are connected to. what i want to do is isolate the house battery from the start batterys like it is supposed to be. can i just hook up another solenoid that engages with the ignition so the house batt gets charge while the engines are running. also in referance to knuts idea about the diode for bilge alarm, a very simple bilge alarm is a float switch hooked to a light or horn, simple and works well.
     
  6. Bglad
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    Bglad Senior Member

    Check out voltage sensing relays on Blue Seas Systems web site:)
     
  7. CDK
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    CDK retired engineer

    That is not a good idea. You can only start the engine when the ignition is powered. Your solenoid is also powered at that moment and you'll draw starter current from the house battery.

    Bglad's suggestion is a good solution. Cheaper, but less efficient is the use of (Schottky) diodes. The voltage drop causes a lower charging current for the house battery.
     
  8. bernd1972
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    bernd1972 Holzwurm

    By the way, for do-it-yourself guys try the freeware-program TiniCAD for documenting your wiring. It helps if you have to find out later what you did a few years ago.
     
  9. BertKu
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    BertKu Senior Member

    Morning CDK, I am often wondering whether the use of Schottky diodes are really that much inefficient, versus the advantage of using them. A generator charges at 14,4 Volt (depending on ambient temperature) and the extra loss over the Schottky diode is 0.1 - 0.13 Volt. Depending where the current usage is on the curve. As soon the Lead acid batteries are charged, the voltage will be 13.8 Volt maximum. In my view all what happens is that it takes a few seconds longer to fully charge a battery.
    But for me personnally, two batteries parallel is a no go for me. Even only during charging.
    Bert
     
  10. CDK
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    CDK retired engineer

    Schottky diodes have more fwd voltage drop Bert, 0.3 V as a rule of thumb. Look at the application sheet here:
    http://www.st.com/stonline/books/pdf/docs/4354.pdf

    These 0.3 V doesn't seem much, but because the charging current is determined by Vch - Vbatt and the internal resistance, it reduces charging current with approx. 50% at the end of the cycle.
     
  11. BertKu
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    BertKu Senior Member

    Yes, I had a look at it. Still better than silicon diodes, but indeed for higher currents still too much of a loss. 380 mVolt to 0.4 Volt is too high. In that case I would use 2 x MosFets IRFP064N and put a small 12 Volt battery on the VGS and when the big battery has to be replaced, change the little battery at the same time. Tonight I will measure the leakage current and see whether I am talking crap or that it is a good solution. The loss Forward Voltage is then only about 160 mV at 60 Ampere ( 2 in parallel).
    With all the hunderds of batteries I supplied with our battery charger and Schottky diodes for the Horsy People, I never had any problems. I had to charge up to 10 batteries of 12 Volt at the same time parallel and used schotky diodes.
    Bert
     
  12. BertKu
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    BertKu Senior Member

    Yes, I made a little test circuit.
    I used a 12 Volt penlite remote control Alekeline battery and a 1 MegaOhm resistor.
    I would make a little box with two stainless steel bolts. connect 3 or 4 (cheap ) Mosfets IRFP064N and connect them parallel. Then with the 12 Volt small battery permanently connected in this box via a 1 MegaOhm resistor between VGs (gate)and Source and leave it permanently in the box for 5 years. It may be better to avoid possible oscillation, to place a 180 Ohm 1/4 watt rsistor in each gate of each parallel placed MOSFET. The otherside is then soldered to the 1 MegaOhm resistor. The 1 MegaOhm resistor is connected to the plus of this small alkaline battery. The minus of this alkaline battery is to the source of all MOSFET's paralleled

    You have then a diode which can handle 60 Ampere at only 110 mV loss or 450 Ampere at a loss of only 180 milliVolt or pulsed maximum 1500 Ampere.
    I do not believe that the 110 milliVolt is a problem for charging a large battery bank.
    Bert
     
    Last edited: Mar 24, 2011
  13. CDK
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    CDK retired engineer

    I'm sorry, but no you don't.
    the IRFP064N with a positive gate conducts almost as good as a copper wire, but it cannot be reversed beyond 1.3 VDC, so it is not a really a diode.....
     
  14. BertKu
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    BertKu Senior Member

    I am to differ from you CDK. Maybe with Vgs = 0 volt, but not at + 12 Volt Vgs. (which is the case, because we have a battery 24/7 connected . The reverse voltage is at least 10 Volt.
    Also I do not expect a 11 Volt batteriy to be swichted parallel to a 13.8 Volt fully charged battery. Even then the voltage is still acceptable. Therefore you need diodes to prevent this. Or otherwise said, you could do so if you use diodes.
    Were did you see reverse voltage is only 1,3 Volt??
    Bert

    Note: indeed it does not work refer reply # 46
     

  15. BertKu
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    BertKu Senior Member

    CDK, let me test this. I have enough batteries to have a reverse voltage of 12 Volt. Will let you know.
    bert
     
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