[SOLVED] drive fet with sinusoidal voltage

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dxtr

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hey everybody , is it possible to drive a FET with a sinusoidale voltage of 220V , and if possible i want it without using a rectifier
thank you in advance
 

Typical FETs allow units of volts between gate and channel, so it is a nonsense to drive the device from 220 VAC. You must use a voltage divider or a transformer. From your question it looks that you have no idea how a FET works; find a good textbook and learn the basics before 220 V AC can kill you.
 

crutschow said:
What do you want the FET to do?
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i need to see if a motor is working or not , i tough about using current sensor or relay (the best solutions) , i just can't find them here where i live
thats why i need a fet , its going to work like a switch allowing 5V to go to the µController when the motor is On.
 


As I wrote, you do not know how a FET works. Now I can see you have no idea ow an AC motor operates. Monitoring the motor AC current is possible but not the way you think.

1. The typical AC motor generates a current spike when switched ON, often <30x higher than the nominal value.
2. The motor current varies with motor load, again over a wide range.
3. A FET is a switching element, not a current transformer.
4. To indicate that an AC motor is running is best done by a mechanical pickup on its shaft. You can use an optical sensor or a simple contact mechanically coupled with the shaft, to close when motor runs.
5. Do not try to touch AC 220V, it can kill you. Using a current transformer sounds safe but the output voltage can easily kill your electronics when motor starts running.
 

i know how AC motor operates !
i don't want to monitor the motor current , i just need to know if the motor is on or not
the 220V that i'm talking about is not the Motor power supply , its the command circuit power supply , i want to monitor the voltage in the contactor (it's just like monitoring the motor)
 

Knowing the voltage presence across AC motor terminals does not reliably indicate that the motor is running.
If you only need to monitor the AC voltage on motor terminals, use an isolated optocoupler to get the ON/OFF indication for your uP. You can use also a 220V AC to 5V DC plug-in power supply.
For such indication I do not see any use of a FET.
 
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    dxtr

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Thank you for help
i know its not reliable but i'll go with it
 

In case you feel like experimenting, see if you can create a 'sense winding' by placing a coil in a likely spot against the motor housing. Or, wrap a few turns of wire around the motor. Put it close to a gap in the housing where your wiring might be expose to a greater intensity of flux field.

Take a volt reading from the coil you created. It may produce a sufficient reading to tell you the amount of power going through the motor. Perhaps a few mV AC.
 

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