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In need for a high current switch

Discussion in 'General Electronics Discussion' started by Sabin, Jul 2, 2013.

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  1. Sabin

    Sabin

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    Jul 2, 2013
    Now, it's been many a year since I've done any electronics work, and so I'm pretty much back to total newb when it comes to this stuff.

    I'm faced with the need for a high current solid-state switch to handle the loads that an RC helicopter handles, so we're talking 45 amps minimum. I could go with a relay, but those things are bulky and heavy. I was hoping something FET based would be lighter.

    Also, weight and space is a consideration so it needs to be able to handle the demands without any excess.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    A solid state relay comes to mind, or a mosfet capable of the current.

    How often would it be switching on and off, and what is the source of the signal to do so?
     
  3. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    Last edited: Jul 2, 2013
  4. Sabin

    Sabin

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    Jul 2, 2013
    Once before the flight, to supply the speed controller with current, and once after the flight has finished. The source will just be a toggle switch.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, in that case you should be able to use a mosfet with a low Rds(on) with a very small heatsink (possibly none at all if it it attached to the body of the model or if it is in airflow).

    Connect a 100k resistor between the gate and the source, and a 10 ohm resistor between the gate and the switch. The switch should connect this to either the +ve supply or ground.

    The 100k resistor provides a path to turn the mosfet off, but it MUST be turned off hard by the switch or you will likely kill the mosfet.

    Use an N channel mosfet, connect source to ground and connect the load between the drain and the +ve of your battery.

    Select a TO-220 mosfet rated at (at least) double your battery voltage and get a diode rated for the load current to place across the load (reverse biased) to protect the mosfet from transients caused by inductive spikes.

    I don't have time right now to point you to an acceptable mosfet or diode.
     
  6. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    Ok if you use a TO-220 like THIS ONE which comes first in mouser if you sort by RDSon (Drain to source on resistance) , it says it has 1.3mili Ohms

    So if i remember the equation correctly power dissipation will be

    P= I^2 * R (P = power dissipation in wats I = amps R = RDSon in Ohms)
    so
    P=40(A)^2 * 0.0013 P=2.08w

    if the load is 80A then P= 8.32w
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That might be a reasonable choice.

    What's the supply voltage?

    If it can ever exceed 20V (and that includes transients) then you need to have a way to limit the maximum Vgs.

    With 2W dissipation you need only a tiny heatsink, with 8W a small one (small = 10 degC/W)

    Also remember the diode. Something like this would be a possible choice.
     
  8. Sabin

    Sabin

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    Jul 2, 2013
    Source power will be a six-cell lipo pack at maximum charge, 25.2vdc
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, since the maximum voltage exceeds Vgs(max) you will need to include some extra circuitry to limit the gate voltage.

    I'll try to draw you a circuit to do it shortly...
     
  10. BlinkingLeds

    BlinkingLeds

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    Feb 23, 2013
    He wants something that will only switch on and off for long periods so won't a resistor and a 18v zener do the job?
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It may do. However you would still want a resistor from source to gate, and the resistor chosen for the gate needs to allow switching in such a way as to stay within the limits of dissipation.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Sorry about the delay.

    This should be able to do the job you want.

    [​IMG]

    Excuse the diagram...

    The NPN transistor should be something with reasonably high gain. A BC547 should be appropriate.

    The diode across the BE junction of the transistor can be a 1N4001.

    The diode across the load needs to be capable of carrying the same current as the load, but will do so only briefly, so no heatsink should be needed.

    The 10R resistor shown as the gate resistor should be calculated as Vcc/IC(max). If the transistor has a 500mA IC(max) then this value would be about 50 ohms.

    Driving the base of the transistor from a voltage source lower than the maximum gate voltage means that the gate resistor can be quite small. The zener and resistor between gate and source are not strictly needed, but I wouldn't remove them.

    The actual gate current is more likely to be limited by the base current and gain of the transistor. A darlington transistor used here would be beneficial, but probably not necessary for infrequent switching.
     

    Attached Files:

  13. Sabin

    Sabin

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    Jul 2, 2013
    I think I can make out most of the symbols on that one. Am I seeing two zener diodes in the schematic?
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes. The two diodes with voltages marked beside them are zeners. 0.5W types should be suitable.
     
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