Automatic Transfer Switch with timer and grid outage failover

Started by electron, Mar 29, 2024, 12:49 AM

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The idea is to have your solar power system and inverter power your loads during the day, and a little into the night, then you switch over to the grid until morning.

You can keep your batteries a little more charged for emergencies this way, depending on how you set the timer.

You set the timer by trial and error for a typical day. You would also have to watch it manually on cloudy days, or just let it switch by default back to the grid when the inverter cuts out at low battery.

In this case the inverter cuts off at about 20V, and STAYS OFF. It's that type of inverter. If it were to come back on, you might get a on/off cycling and would want to add some sort of delay circuit to prevent that. You also want most of your stuff on a UPS if it can't handle the "brownout" during switchover. And the inverter should be a sinewave type if possible.

This circuit could be scaled up and down depending on your loads and the size of your Automatic Transfer Switch (ATS, which is really just a big 120V relay in a box).

The small relay could be one of these 120V DPDT for $1.75 :

The small relay will bypass the timer if the grid power goes out at night and put you on inverter power. As long as the inverter is running of course. The relay is always energized if the grid power is up.

It only powers the coil on the ATS, and so does the timer so those contacts will not get much loading and should last a long time.

I would use heat shrink tubing on connections to that relay.

That relay above could also work as the ATS relay for a small system where your loads won't exceed about 1000W since the contacts are rated at 12A. You really don't want to push it beyond that.

The timer should be one of those digital wall switch type timers for AC that uses a battery to do everything. You can do your own search for "Honeywell Econo Switch 7 Day Programmable Timer Switch for Lights and Motors" "Model # RPLS730B1000/U" and find something like it.

CAUTION: This is a high voltage AC circuit, never work on it with the power applied. This is for example purposes only, you are responsible for any damage this may cause your system so check this circuit yourself and do your own homework.

Timer ON + Inverter ON� = SOLAR
Timer OFF + Inverter ON = GRID (late night)
Timer OFF + Grid OFF + Inverter ON� �= SOLAR (power outage)
Grid ON + Inverter OFF� �= GRID
Grid OFF + Inverter OFF� = SCREWED (flashlights on)