24v ac delay

[AMay]

I have a thermostat with a built in delay of 5 min after a power outage to protect my air conditioner. The power must be off at least 15 seconds before the thermostat recognizes that the power is off, presumably the time necessary for its caps to discharge.

I am acquiring a standby generator to power the whole house, and it aromatically turns on in 10 seconds, not enough for the thermostat to notice. The thermostat runs by stealing a little power across one or more of three relay coils (Heat, AC, and fan.) One hot wire and 3 relay control connections.

Easy to put a n/o relay on the hot wire and and delay closing it when the power comes on, but that is not elegant. I don't fancy the idea of energizing a relay for ever when I hope I will never need it. And house current is not handy by the thermostat.

So I would like to use a N/C relay, and use the thermostat power, (a super cap should keep everything on long enough) and a pic as a timer.

The ac sensor that has me stumped. I can't connect 24 volts directly to an a/c optoisolator, and I am not comftorable dropping 24v to 2 volts with a resistor. If I were to make an ordinary power supply as would be needed for the pic, the filter caps would keep power on too long just like the thermostat. I have to be careful how much power I steal or I will triger on of the control relays.

Any eligent ideas?

[Charles Linquis]

I just noticed your post.

There is no problem dropping 24V for an optoisolator. We do it regularly. I would put a 2.2K .5W resistor and a diode in series with the LED in the opto, and another diode across the LED in the reverse direction. Put a 10K pull-up on the opto collector and a 10uF across the opto. The time constant of this combination is 100 mSec.

[amgen]

not as interesting as a pic ckt,
but for about 10 to $15, you can get an adjustable delay on break cube type relay, that just hooks in series with the compressor run relay coil (2 terminal device) and protects the comp from short-cycling every time power is removed.
don
amgen

[AMay]

I didn’t find any relays for $10 –$ 15 which would delaystart up when all power is out but I did find an Omeron G3VM61A1 solid state two pole single throw relay rated for 60 Vac/dc, 500 ma for $2.15 (Mouser.)The A/C compressor relay drawsless than 300 ma at 27 volts AC, so there is plenty of headroom. I added a snubber circuit for protection fromthe compressor relay coil.

I spent more time I care to admit trying to figure out howto sense a power out condition, when the power was out, without batteries. Thenit dawned on me that all I needed to do was wait until power is restored, have along start up process in a PIC, and then send a pulse to a latching relay. I use the second pole of the above SS relay tolatch it on. When the power goes out,the relay opens, and must wait for a pulse from the PIC to close again. I use 3pins to select the delay time with a jumper, 4, 4 ½, or 5 min.

So far it works fine. It has been tested by 4 actual poweroutages and several 2-3 second power “blinks.”

[amgen]

sounds like a good circuit, does it always add a delay when starting the compressor ? sometimes thats annoying waiting for cooling.
for future reference,

http://acsuperstore.com/Merchant2/4.12/00000001/catalog/c605.html

I use to use these alot when I was working...

[AMay]

Correction, the relay I use is a G3VM-62C1. It costs $3.19, quite a bargin I think.

And yes, it always delays a restart after the power goes out, but only after the power goes out. As long as to power is on, the relay stays latched. The thermostat will not restart faster than 4 - 5 minutes on its own, but it has no provision for delay after a power loss.

[AMay]

One more item re the generator. I want to know when it is running. A little Rf remote system seemed the way to go, but I had to sense when the unit was running. Since it is an emergency unit, I did not want to tap its electrical system, for fear a short would would mess it up, or might affect the warranty.

My solution was to mount a reed switch near the alternator which controls power for a battery powered pic controlled transmitter. The receiver is in the house, mounted in a hollow book I got at a hobby store. It works great so far.

[Demon]

Moved from Schematic.

Robert