Okay. I am trying to control 28vac using 3vdc Best I have come up with is a triac. Anyway to make the triac closed by default and triggered open with the 3vdc? Basically I am trying to make the triac to fail closed allow the 28vac current to flow that way if the micro-controller sending the signal to the triac fails the 28vac will continue to flow. Then will the micro-controller sends a signal it will open the triac interrupting the 28vdc.

 

You should use an AC opto-coupler to drive the gate of the triac.
Or use an AC solid-state relay.

 

How much current do you need for the 28VAC load? As MC said use a AC-optocoupler.

You want a NC solid state relay. Have a look here: http://search.digikey.com/us/en/cat/relays/solid-state/1048664?k=relay
Select the appropriate parameters, there are quite a few relays available,

 

Thanks for the replies. The 28vac current is about 200 ma. Its driving a motor that I am trying to turn off using a triac. I have run the 28 vac to MT1 and MT2 of the triac. I have 3vdc signal coming from a micro controller to trigger that gate on the triac. I am just trying to make the motor always on, unless a signal comes from the microcontroller to open the gate of the triac cutting current to the motor. So is a normaly closed solid state relay the way to go? I am searching on digi key and looking, but the question is how do I tell if its normally closed or normally open? Thanks for your help.

 

Okay, I think I figured my question out. I think I need a (SPST-NC (1 Form B)) relay.
http://search.digikey.com/us/en/products/LCB127/CLA183-ND/565928

 

(Almost) All solid state relays will be normally open. You can make it normally closed by adding a pullup resistor to the control logic input.

Here is an inexpensive SSR good to 900mA:

http://search.digikey.com/ca/en/products/PR29MF11NSZF/425-2363-5-ND/720212