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Engineering: the art of making what you want from things you can get.

Backup power switch

February 02, 2009

Update: My grandpa has requested a second version of this, so I'm likely to draw a circuit and make a PCB layout for it in the near future. I will also upload the source files then.

This was intended to be a small circuit for my grandpa, who was thinking of powering his fridge with "night power"¹, when available. His only problem was that he constantly forgot to plug the fridge into the normal outlet in the morning, or after the weekend.

However, this circuit can also be used for switching between two independent power sources to reduce the risk of power failure (when one power source fails, the circuit switches the load to the other power source).

My solution would have been two SPDT² relays and a transformer. After going to the workshop in the basement, I realized that I didn't have any SPDT relays left. As I wanted to build the circuit as fast as possible (things I put off a while never get done anymore), I decided to build the circuit with two SPST³ relays.

This forced me into a tradeoff:

• As I assume the phase on one line, both plugs need to be inserted correctly into the wall outlets.
• As the circuit won't work as intended with any plug (or both) inserted wrongly, it needs to detect and display these conditions.

I decided to use an AVR microcontroller to detect power conditions and control the relays. The new circuit relies on the fact that the energy meter only measures the current flow on the phase wire. Therefore, the load can be connected between the night phase and the neutral wire of a usual outlet, and will only make the "night power" energy meter count.

The circuit is powered directly from the power lines, the current flow is limited by a "capacitive resistor" (that is, a simple capacitor), then it is rectified, limited to 24 V by a zener diode and smoothed by an electrolytic capacitor. A live phase (one carrying voltage) is detected very easily: it can be connected via a 1 MΩ resistor to the I/O port of the ATmega8 microcontroller. The clamping diodes on the I/O ports limit the input voltage to the range of the microcontroller's supply voltage.

When power is detected on the incoming night phase, the relay switching the normal phase is turned off, and after 2 seconds switching delay, the relay switching the night phase is turned on. When night power is lost, the relays are switched back in reverse order (night off, 2 sec delay, normal on).

¹"Night power"

"Night power" is simply a remotely controlled switch and an additional energy meter on the distribution board. The wire is then connected to a few outlets which are only powered when the electric utility sends a ripple control telegram to the switch. It is usually turned on during the night, because otherwise the electric utility would have to shut down big power plants when there is too little load on the power grid. At my grandpa's, night power is also available during the whole weekend.

²SPDT: single-pole, double-throw

³SPST: single-pole, single-throw

Last modified: August 30, 2009

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