circuit to use Mosfets as an AC switch

[joe tiger]

AC input; 120/240 VAC
Load: solenoid valde 50ma.
I need a simple circuit that I can use to turn the solenoid on and off from a micro.
Any ideas. I want to use mosfets because I think it requires the lowest drive power.
Thanks
Joe

 

[tcmtech]

a opto isolator triac is all you need. Its a 6 pin device that only costs a few 10's of cents and will easily do what you need to do.

 

[joe tiger]

Yes but,
I do not have the current to drive the opto. I do not need isolation. I am referenced to the AC Neutral. I can tap about 1 ma from the line using a resistor and a shunt zener diode to get the 1 ma to power whatever circuit i use to drive the switch.
is there a way to drive triac with less than 0.5 ma of drive current from my uprocessor vcc?
It is why I was looking for Mosfet. they do not require much current to drive them.

 

[smanches]

MOSFETs are DC switches, not AC. You would be much better off with a triac or solid state relay.

If you need more current, use another transistor to help drive it.

 

[joe tiger]

Thanks for the reponces,
Solid state relays require few ma of drive. Triac even more. I am sure there is a way to build a circuit with couple mosfets for AC load, one for each polarity. They do it integrated in the Solid state relay, so there must be a configuration to do it in descrete and not use an led to trip the mosfets.
Thanks
joe

 

[tcmtech]

The simplest way to get the added power is to change out the main voltage dropping resistor with a capacitor and zener diode of the right values and have a capacitive coupled power source. You will gain efficiency despite being able to draw more power at the same time.

There are a loads of threads here related to AC line voltage LED circuits and other low power line connected devices as well.