momentary trigger a 240V relay through body capacitance

[marco0674]

Triac trigger

Hi all.
I need some help with circuit in the attached file.
I need to momentary trigger a 240V relay through body capacitance.
Relay must be on just until I keep my finger on the touch plate.
I could design the circuit in file: from the upper part I get 12V and, with the Q9 transistor I can have 12V on C3 as long as I keep my finger on the touch plate.
My problem is: how can I trigger the triac to turn the relay coil on? In other words, how can I fill the "trigger" box? Is a photo coupler strictly necessary, or I can use another solution?
Thanks for any help.

 

[IanP]

Re: Triac trigger

It looks like employing an optocoupler will be a very good idea ..

Rgds,
IanP

 

[erikl]

Re: Triac trigger

It looks like employing an optocoupler will be a very good idea ..

Yes: with the triac connection as shown, an optocoupler is indispensable.
It would however be possible to avoid the optocoupler, if the DC power supply and the triac connection would be changed so that one reference node of the AC supply, the minus terminal of the DC supply and the triac cathode are all on the same GND point.

 

[marco0674]

Thank you both.
It's as I thought.
I will use an optocoupler and let you know.

Marco