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Energy losses in doorbell

In this article a description about how I reduced the energy consumption of my doorbell.
My doorbell system is existing of:
- Three pushbuttons at the door (three, because I have rooms for rent), all these pushbuttons have a small internal lamp for illuminating the nameplate.
- Four bells (one pushbutton is driving two bells).
- One transformer.

For the annual energy consumption, the power consumption of the bells is hardly important, the few seconds they are in use per day can be neglected.
The energy consumption is determined by:
- The energy loss in the transformer.
- The power for the illumination in the pushbuttons.

Transformer 1.

Years ago, my house had one doorbell with one pushbutton, powered by this transformer.

Input voltage: 220 Volt
Output voltage: 8V (1A)
This old transformer is from the time when the mains voltage was 220 Volt here in the Netherlands, nowadays the mains voltage is 230 V.


Transformer 2

Later I got three pushbuttons at the door, and I started using this larger transformer.
The transformer had to be larger because the old transformer didn't have enough power to drive two bells in parallel

Input voltage: 230 Volt
Output voltage: 6 Volt (2A)
The unloaded output voltage is about 6.6 volt.

Though the bells are 8 Volt types, they also work o.k. on 6 Volt.
The lamps in the pushbuttons however gives less light at 6V


Transformer 3.

To reduce the energy consumption, I replaced transformer 2 by this toroidal transformer.
Such a toroidal transformer has much less energy losses the the previous two EI-core transformers.

Input voltage: 230 volt
Output voltage: 6 Volt (2.5 A)
The unloaded output voltage is about 6.9 Volt.

More info about power loss in transformers can be found here

In the next measurement the input power is measured for the three transformers.
One time unloaded, and one time loaded with the (lamps in the) three pushbuttons.

  Input power
Output unloaded
Input power
Output loaded
Transformer 1 1.86 2.78
Transformer 2 1.85 2.45
Transformer 3 < 0.50 * 0.92

* My energy meter cannot measure below 0.50 Watt.

We see transformer 3 uses the least energy.

To reduce the energy consumption even further, I replaced the light bulbs in the pushbuttons by led's.
The original light bulbs used about  30 mA per piece (at 6.9 Volt transformer voltage).
So the total power for the 3 lamps is: 3 lamps x 30 mA x 6.9 Volt = 0.621 Watt.

Left: the original circuit diagram with the light bulb parallel to the switch.
Right: circuit diagram of the led illumination.

The led's and resistor build in the pushbutton.

I used two white 3 mm led's , with 2800 mcd brightness at 20 mA.
One led illuminates the transparent circle around the pushbutton.
The other led illuminates the nameplate.

The light bulb is disconnected.

The illumination for one pushbutton now uses 3.8 mA a.c., which is quite less then the 30 mA it was first.
Compared to the light bulb, the brightness of the led's is much higher .

The illumination at night.

After replacing the light bulbs by led's, I measured again the energy consumption of transformer 3, loaded with the three pushbuttons.
My energy meter indicated 0.00 Watt, which means the consumption is below 0.5 Watt.
Then I connected a small load to the energy meter, to overcome the 0.5 Watt measuring threshold.
When I also connected the doorbell transformer, the measured power increased by 0.20 Watt, from which I conclude the doorbell system now consumes about 0.2 Watt.
First the power consumption was 2.45 Watt, so this is reduced by more then 10 times.
The saving is 2.45 - 0.2 = 2.25 Watt.
This is a saving of 19.7 kWh per year.

The low power loss of the toroidal transformer can also be noticed when you touch the transformer with your fingers.
You don't feel any heat production, while the old transformers became quite warm.

When the bell is ringing, it can produce high voltage peaks, by which the led's (maybe) can get defective.
To prevent this, I connected a 25 Volt a.c. varistor parallel to the bell.
The varistor reduces the peaks to about 40 Volt, which is a safe value.
The value of the varistor may lay between 10 and 25 Volt a.c.

The bell with between it's terminals the varistor (round blue component).

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