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Energy losses in modem

This article is about reducing energy losses in my internet modem, and wireless telephone.
The internet modem is of brand: Thomson, the model number is: ST780WL.
In the Netherlands this modem is sold by telephone company KPN, under the name "Experiabox".
You can of course use it for internet, but also can connect a telephone to it (telephony via internet).

Because this modem is used by multiple users, and also because my telephone is connected to it, it has to be switched on for 24 hour per day.

To the modem I connected a wireless telephone, brand: Panasonic, model: KX-A141EXM.

The modem is powered via a mains adaptor of 18 Vac. 1A.
I measured the energy consumption of the adaptor (connected to the modem), and this was 10.0 Watt.

The telephone is powered via a mains adaptor of 6Vdc 500 mA.
The energy consumption of this adapter is measured, and was 3.35 Watt.

So, together modem and telephone consume, 13.35 Watt.

The first energy saving at the modem was to switch off the wireless modem function, which is switched on in default setting.
I have all computers connected via cables to the modem, so the wireless function can as well be switched off.
Switching off the wireless function is done as follows:

Type in the following address in your internet browser:
You then reach the settings menu of the Experiabox.
The menu of my modem is in Dutch language, the Dutch terms (in italic) are translated here.
Click on: wireless
You can see if the interface is switched on (ja / nee  yes / no).
Click on: configureren (configuration).
Remove the tick at "interface ingeschakeld" (interface switched on).
Click on: toepassen (apply).

By switching off the wireless internet function, the power consumption of the modem reduces from 10.0 to 9.3 Watt.

The mains adaptor of the modem gives 18 Volt ac, which is in the modem rectified to 23 Volt dc.
From this 23 Volt are supplied: 2 switching regulators with 3.3 Volt output, and one linear regulator (7812) with 12 Volt output.
The supply voltage of the modem can without problem be reduces from 23 to 15 Volt, without any effect on the operation of the modem.

Volt DC
24 0.312 7.488
23 0.321 7.384
22 0.330 7.260
21 0.341 7.161
20 0.353 7.060
19 0.366 6.954
18 0.380 6.840
17 0.396 6.732
16 0.415 6.640
15 0.435 6.525
In this table the effect on power consumption can be seen if the supply voltage of the modem is reduced.

Reducing the supply voltage of the modem, gives a lower power consumption.


I opened the modem and soldered two wires at the points where the rectified voltage is.
Then the modem is closed again, with the wires through the ventilation holes in the housing.
Now we have a connection at which an external power supply can be connected.

Then I made a power supply for powering the modem.
As transformer a toroidal transformer (2x12V 15VA) is used, because this gives much less energy losses then an E-I core transformer as used in the original mains adaptor (for more info, click here ).

On the toroidal I wound an extra coil with 70 windings (0.4 mm enamelled wire).
This coil gives 5 Volt ac, which is rectified to about 6 Volt dc, to supply the telephone.
So, with one toroidal, two mains adaptors are replaced.

Circuit diagram of the self-made power supply.

All diodes are schottky diodes.
Internal in the modem are also some elco's for smoothing the 15 volt supply.


Picture of the power supply.

The output voltages are:
15 V dc for the modem
6 V dc for the telephone.

The energy consumption of the new power supply is:
7.8 Watt if only the modem is connected
8.5 Watt if both modem and telephone are connected.

So, by this rebuild the energy consumption reduced from 13.35 to 8.5 Watt, a saving of 4.85 Watt.
This is more then 42 kWh per year.
An extra advantage is, there is less heat generated in the modem, which might increases it's life span.

If you are not sure what you are doing, don't begin on such a project.
Wrong treatment (e.g. connecting wrong voltage, or static electricity) can lead to a defective modem, or dangerous situations.
Always work in the modem in a static safe area, with yourself, the soldering iron and the modem connected to ground potential.
Only work at the modem when it is fully disconnected from other electrical circuits.

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