If you have a 5V MCU, then it will work perfectly with its VDD at 0V and its VSS at -5V.
It's only a problem if you have inputs which go from 0V to +5V (which I don't think you do)
If you have a 3.3V MCU, then use a LM337, which will withstand -37V on the input.

 

See also Fairchild AN-3006 & AN-3003.
Max.

 

I think this thread has been over analyzed.

I need to switch 24VAC solenoids for garden sprinklers.

Instead of triacs use a OptoMos SSR. Such as a CPC1906Y, see the spec sheet.
https://www.digikey.com/htmldatasheets/production/57770/0/0/1/cpc1906y.html

 

I think this thread has been over analyzed.

Instead of triacs use a OptoMos SSR. Such as a CPC1906Y, see the spec sheet.
https://www.digikey.com/htmldatasheets/production/57770/0/0/1/cpc1906y.html
View attachment 228047

I did start looking at SSR's but this particular part is $7.74 each at Digikey Australia - $61 for 8 channels!

I also need to think about some sort of surge protection on the solenoid lines given they are in the ground ie. ligtening outdoors. Would a suitable TVS diode suffice, some garden sprinkler control boards have spark gaps on the PCB as well.

 

I would agree. TVS diode seems reasonable.

 

I did start looking at SSR's but this particular part is $7.74 each at Digikey Australia - $61 for 8 channels!

Too bad you are in Australia, I have a whole raft of Opto22 modules c/w plug in board units.
4, 8 &16 unit boards.
Max.

 

Too bad you are in Australia, I have a whole raft of Opto22 modules c/w plug in board units.
4, 8 &16 unit boards.
Max.

Apart from SSR prices, being in Australia right now isn’t a bad place to be

 

I think this thread has been over analyzed.

For me it has been about learning more on the topic of AC and TRIAC use. I have gained a lot from the thread and thank everyone who has given me a hand.

 

You'll learn more from wiring up a triac, triggered either by an opto-triac or a negative DC supply, than you would by buying a solid-state relay!
And at 24V AC, it's not going to destroy as many pcb tracks and make as much soot as when it goes wrong on 230V mains!

 

See

 

Does that simulate the rather lengthy turn-on and turn-off times?

 

See