The transistor in an opto coupler can only pull down, so there is nothing to supply the voltage to the FETs gate.
but wont the power from the supply be too much for the gate of the FET?
But how come it worked for led?
Ok, I found this image I dont understand how optocoupler will survive the high voltage.
Put the opto-coupler where R3 is, and parallel it with a zener (or a Esaki diode).
Quotebut wont the power from the supply be too much for the gate of the FET? What does the data sheet say the maximum gate voltage is?QuoteBut how come it worked for led? Because you were pulling current through the transistor not outputting a voltage.QuoteOk, I found this image I dont understand how optocoupler will survive the high voltage.Look at the data sheet for the opto, but 24V is not a very high voltage for a transistor.QuotePut the opto-coupler where R3 is, and parallel it with a zener (or a Esaki diode).Don't, our dear henry specializes in trying to confuse beginners, it makes him happy.
If it has emitter and collector, it is not a MOSFET, it's a BJT (a k a "70's transistor.")BJTs let current through between collector and emitter when there is also a current (the right way) through the base. This means that there will always be current flowing through the base in addition to the switched current when it is on.MOSFETS let the current through one way ("body diode" direction) always, with some body diode resistance. However, when you charge the gate capacitor (meaning apply a voltage and a short spike of current) they will conduct equally well in both directions. When wired with the controlled voltage against the body diode (normal,) this means they are "off" when the gate voltage is low, and "on" when the gate voltage is high.For a MOSFET, the allowable voltage from source to gate (what controls the built-in gate capacitor) is different from the allowable voltage from source to drain (the controlled voltage.) A typical low-voltage device might allow 20V from source (0-reference) to gate, and 30 volts from source (0-reference) to drain. (IRLB8721 for example -- popular 30V D/S device with 4.5V recommended gate voltage and 20V max G?S voltage) Here's an example data sheet showing those voltages:http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDEQFjAA&url=http%3A%2F%2Fwww.adafruit.com%2Fdatasheets%2Firlb8721pbf.pdf&ei=IZiNUKaZM8LtiwKghYHADA&usg=AFQjCNGUrxQ2jGVS0ZaY7Mic2FOCkETjEQ
It says: Vemittercollector = 6, Vcollectoremmiter = 80 (if again im looking at the right thing). Its says thats absolute maximum.
Thank you for explanation. The optocoupler has collector and emitter, so it must be the BJT type.
I don't know of any MOSFET optocouplers
so it must be the BJT type.