Automatic Inverter Control


In the past I made a simple transfer switch to move a load from grid power, to inverter power.  This would happen automatically if the grid went down.  When grid power returned, the load would transfer back again.  Worked well... but sometimes the grid is unstable when it comes back online.  It might bounce on/off many times before settling down.  This plays havoc with electronics.  It also causes the GFCI on the inverter to pop.

I would rather have the load be off during this timeframe, and only transfer back once the grid has stabilized.  To accomplish this, I put a time delay relay on the power switch.  This little relay circuit was an eBay purchase, for under $3.  As long as the grid power is bumping, it will keep resetting the 15 second  time delay relay.  Years ago, I would have gone to Radio Shack, bought all the necessary parts, and built it myself.  But most of those stores are long gone.  Knew they wouldn't last when their main interest went from parts for the hobbyist, to selling cell phones and toys.

When it cools off this Fall, I'll run wires in the attic to the house fridge, etc.  Just too hot this time of year!


The time delay relay will mimic manually engaging the power switch on the inverter control panel.  To accomplish this, I simply connected the two switch contacts on the inverter control board to wires, which will be connected to the Normally Open contacts on the time delay relay.

A different angle to better view the switch.

Because this control board sits recessed in the body of the inverter, I couldn't route the control wires out the side or back of the board without a major mod to the inverter.

A rubber grommet dresses up the hole in the panel.

This is the $3 timer circuit.  It uses a 555 timer, resistor, and capacitor to adjust the time delay.  It's intended for 0-10 second delay, and is adjustable via the onboard potentiometer.  Mine is closer to 15 seconds.  By adding a larger capacitor, I could increase the delay, but I'm satisfied with the stock configuration.

I couldn't build this for $3 if I had to buy all the parts separately.

The time delay relay was put in a project box.  Easy to connect.  I have another relay that provides 12VDC when the grid goes down, that will feed power to this circuit.

I'll have another webpage to discuss what all this control board does.  But for this project, there's a wall wart plugged into the grid, which keeps the first relay energized.  As long as it stays in this state, the inverter will not power up automatically.  As soon as the grid goes down, the relay de-energizes, and then applies 12V to the time delay relay.  After 15 seconds of stabile delay (power hasn't bumped on/off), the inverter will power up.

Now you can see why the control wire needed to exit the face of the inverter control board.  The control panel sits recessed into the body of the inverter.

The top inverter is now automatically powered up with the grid goes down.  I will have the house fridge, and some other critical circuits, switch over to this power source during an outage.  The inverter just below that is always powered.  It runs the office, much of the garage, charges the golf cart, etc.  The garage fridge will operate from this soon too.  I didn't want everything trying to run on one 1800 Watt inverter.






Last updated 08/17/16    All rights reserved.