Design: AC Power Monitor

I use a UPS to backup the power of my alarm system, on top of the alarm’s battery. It’s redundant, but my alarm panel (Vista 20P) won’t notice a power outage until the UPS gets depleted. At which point, my Envisalink will be dead since it is also on the same UPS.

The trick here is to use a relay tied to the AC power and connect it to a zone input on the alarm. Now the relay would be actuated most of the time (drawing power) and when power goes off, the NO would open and trigger the zone.

I had an idea of how to drive a 5V signal relay for my alarm that would be always on 24/7 that might save power, which saves money and generates less heat.

Relay economizers are common for larger relays, and the theory is you save power by energizing the relay at full voltage and once the relay is actuated, drop the voltage down to maintain it. A common method is to do PWM, which is complex. The simple method which isn’t as efficient as PWM is the use a resistor and capacitor.

I will be using an Omron G5V-1-DC5 relay, which is a 5V powered relay that turns on at 4V. Once on, it will stay on as long as you feed it 1V.

Here is my schematic idea for the relay: Notice the inline parallel resistor and capacitor on the relay coil. The resistor reduces the steady state voltage supplied to the coil and the capacitor provides an initial surge at power on to actuate it.


The values shown above are actual values that worked. My initial calculations were values that were about half of that for resistance & capacitance. Since I didn’t have a big variety of resistor values or caps, I just added a second resistor and cap to the board.

The coil voltage scope trace below shows the relay turning on.


Now, you see a few things here. First the coil voltage peaks at 5V, but drops to 1.83V once the capacitor charges up. After 20 ms the voltage reaches the steady state. The data sheet says the relay turns on in 10ms max, so it actuates likely in the 5-10 ms timeframe, at which point the voltage drops to 3V.

So what’s the point of all this? Well, if this relay is on 365 days, it will have consumed about 1.4 kWh. If I add this resistor and capacitor to my circuit (about $0.50 in parts, qty 1), I can drop the yearly power to 0.7 kWh.

When you run the math on the energy savings, not as impressive as I had hoped, but in end, I could have tweaked the values further and gotten closer to the calculated values and saved a little more.

Now these values are tailored to this relay, other relays would require calculations and testing in order to get the right RC values.

I designed a small PCB that basically is the circuit above with a USB input for the 5V power source. I also put a LED on there to show me that the 5V is on. I am planning on using a small USB power adapter from an old iPhone.

Actually, I went a bit overboard, and made it a dual USB input with dual relays and used a pair of diodes to OR the incoming 5V. I had another use case for a circuit like this, to be covered later. I always look to combine PCBs for different projects when the projects are similar.

The final circuit is below:


The board turned out to be 1.25″ x 1.00″, but I could have used more space for the terminal blocks.


When I built this board, I only stuffed one of  USB connectors and LED circuits. The photo below shows my non economizer build.


The 3 bare boards from OSH Park were $6.25 and showed up in 16 calendar days. I built one with the economizer jumped with a wire, bypassing it and I built one with the economizer circuit.

Both boards worked, one drew about 32mA, the other drew about 16mA.

I installed the board in my alarm panel and it’s working perfectly so far. I will now be notified when my power goes out. I use an Envisalink EVL3, and I set my “Zone Follower” to be the zone tied to this relay.

I will now be notified when my AC power goes out and then maybe get another notification when my UPS power goes out, depending on what devices can stay on for a few extra milliseconds.

The other 2 boards will become a future project in my home automation. I want to monitor a power circuit using ZWave to do some automation. More to come.


2 thoughts on “Design: AC Power Monitor

  1. I found this while searching for a solution to be alerted to a power loss on a Vista 20P with an Envisalink 3 – so you can imagine how thrilled I was to stumble on it. But I’m not electrically inclined – at least not for building. Is there any chance I could buy one of these from you?


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