Driving a power MOSFET from optocoupler

Discussion in 'General Electronics Chat' started by cmos, Apr 10, 2010.

  1. cmos

    Thread Starter New Member

    Apr 10, 2010
    2
    0
    Hi,
    i am building a small scale inverter. contol part is done through a microcontroller and transformer is driven by a Power mosfet. As the microcontroller power source is 5V and power mosfet needed around 7V min as Vgs, i used an optocoupler with another power source of 12 V. I have attached the circuit too. However it does not seem like the power mosfet switches on correctly according to the input pulse. Am i doing anything wrong here?
    CMOS
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Does the 12 volt source have a ground common with the 24 volts? With no signal in to the IRLED, is the FET turned full on (12 volts on the gate)? What is the value of the current limiting resistor into the IRLED?
     
  3. cmos

    Thread Starter New Member

    Apr 10, 2010
    2
    0
    yes they have a common ground.
    when IRLED is not diven, MOSFET gets fully switched ON.
    when i drive the IRLED with 50Hz squire wave, i get a strange behavior as follows.
    When the output resister value of IRLED is low (1K), the mosfet is ON always! (does not get switched on/off at frequency 50Hz). However when i increase the resister value to say 1M, it started to swith on/off at the 50Hz frequency. any idea why this is?
    Even when it started to switch on/off as required with 1M resistor, the output voltage from the transformer is very low. The transformer is 230V/24V one used in the other direction and it is quite powerful. The supply to the transformer is a switching power supply taken from a computer and is capable of providing around 10-20A. any ideas?

    thanks
    CMOS
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It helps if you place designators on devices, such as R1, R2, etc.

    When you refer to the output resistor , that almost must be the resistor between the 12 volt source and the FET drain. The phototransistor is only capable of switching a few milliamps, so the function with a large resistor value is no surprise.

    What is the current through the IRLED? It should be pretty high, on the order of 20 - 40 milliamps for driving the phototransistor.
     
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