(MOS)FET as a switch

Discussion in 'The Projects Forum' started by akis02, Sep 15, 2012.

  1. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    I would like to be able to use a MOSFET, or other FET, or FETs (plural) ) to simulate/replace a relay contact.

    The problem I am having (I am a beginner), is that you need a certain voltage between gate and source to turn it on, eg, say 4 Volts. This means that the gate must be 4V higher than the source which is sometimes impossible, but it would not be a problem with a simple relay.

    I attach a schematic to demonstrate the problem. I chose a random MOSFET with very low Ron from the list on the simulator. The mosfet needs to switch very close to the Vs (12V - 11.5V) but that requires its gate to be 3-4 Volts over the supply!

    Any help appreciated.

    PS In the end I will buy and use an old fashion relay, but until it arrives I would like to experiment. :)
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    I bet you are not going to build that exact circuit in real parts. So, what's your goal? To get 1/2 volt pushing current through 1/2 ohm? Rearrange the batteries so you can use the MOSFET with source grounded and +12 volts to turn it on. Then put the counter emf battery in series with the +12V battery to lower the drain voltage to 1/2 volt.
     
  3. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    Goal is to see if I can replace a relay with some sort of a transistor/mosfet/fet whatever. It is not something I am familiar with.

    My last project is a battery charger and I have used a relay to disconnect the battery when it reaches a certain voltage. At times the relay will switch voltages as low as 50mV and it will pass currents up to 1A. Can I replace this relay with something solid state as I described above?
     
  4. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    Yes but,

    what you presented here is an abstract idea. Better to present the schematic for the real battery charger to get advice on the best way to arrange the parts.
     
  5. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,525
    In a high side switch mosfet(Nmos type) The gate still only has/needs ~10V. But the ~10V is relative to the source voltage, not ground. This is why a gate driver is used and the easiest way of doing it.

    But the real easiest way is as #12 said, make the switch between the load and ground, also known as a low side switch.
     
    #12 likes this.
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