The latest victim on the Teardown Bench is a budget Lidl plug-in power meter. It was sacrificed because its plastic parts had literally begun to crumble away. There is nothing particularly remarkable inside, but the subject of electrical power measurement itself is well worth a closer look!
The Silvercrest 9149 energy monitor (also sold as the Paget Trading 9149 or EMC 9149) is a plug-in digital meter that is inserted between a wall socket and a household appliance to measure and monitor the appliance’s electricity consumption.
Key technical specifications
- Voltage: Designed for standard 220–240 V AC mains supplies.
- Measured quantities:
- instantaneous power consumption (W);
- cumulative energy consumption (kWh);
- electricity cost (€ , HUF, etc.).
- Current range: 0.02–16 A
- Power range: 7–4416 W
- Energy display range: 0.00–9999.99 kWh
- Energy cost display range: 0.000–9999
- Maximum load: 16 A
- Frequency: 50 Hz
- Battery: 2 × 1.5 V LR44-type button cells (also listed as 76 V13 or RW82 equivalents)
- Specified measurement accuracy:
- Voltage: ±3% of the measured value
- Current: ±3% of the measured value, ±0.03 A
- Power: ±5% of the measured value, ±10 VA
- Energy: ±5% of the measured value, ±0.1 kWh
- These accuracy specifications apply under the following conditions:
- Frequency: 45–65 Hz
- Normal room temperature
- Voltage/current harmonic distortion: less than 15%
- Power factor: cos φ > 0.2
The user manual can be downloaded here: http://rfelektronik.se/manuals/Datasheets/Paget%209149%20instructions%20in%20English.pdf.
Safety First!
The manual states that “before replacing the batteries, the energy monitor must be unplugged from the wall socket.” It is difficult to imagine how anyone could replace the batteries while the unit was still plugged in, since the battery compartment is on the rear of the device, right next to the mains plug, and is therefore inaccessible while the unit is in the socket.
This gadget is also fitted with a child-resistant socket shutter. The idea is simple: a small rotating plate covers the socket openings so that a child cannot poke a piece of wire into them and receive a potentially fatal electric shock. When a proper mains plug is inserted, its pins rotate the plate out of the way. At least, that is how it is supposed to work. In practice, plugging anything into these sockets can sometimes be an ordeal. Not long ago, the plug on one of my mobile-phone charger adapters simply broke when I tried to insert it into a child-resistant socket, so personally I thoroughly dislike these things:
For protecting small children, inexpensive socket safety plugs have been available for decades. They block access to the socket and can only be removed with the matching key. The only thing required is to actually use them. Of course, in a child-resistant socket one of these safety plugs may itself get stuck or break off, but that is a minor detail.
Let’s Take It Apart!
The unit is easy to disassemble. Once the screws on the back have been removed, the case can simply be opened up. This is what the inside looks like:
You can already see that the plastic is cracking around the lower-right screw boss that holds the case together. The photograph does not really show it, but the housing is deteriorating elsewhere as well: small pieces can easily be broken off with a pair of pliers. It was only a matter of time before the whole thing started falling to pieces. And at that point there would have been a good chance of a short circuit — potentially followed by the rather more serious problem of the device setting the house on fire.
When disconnected from the mains, the unit ran from three button cells, but the battery holder had already given up as well. Some of the screws securing one of the circuit boards had also pulled free:
There are two circuit boards inside. On the rear of the board on the right, a tiny quartz crystal can be seen, secured with a black, tar-like adhesive. The microprocessor is on the other side of the board, and the crystal provides its clock signal:
The processor itself is hidden under the black blob of epoxy resin, as is common in pocket calculators, watches and similar devices. Above it sits the liquid-crystal display, which is connected to the processor by a strip of conductive elastomer made up of alternating conductive tracks — commonly known as a zebra connector. In the photograph above, the display has already been folded out of position; originally it sat like this:
Next to the display are the gold-plated contact pads for the rubber push-buttons. Viewed from underneath, the buttons themselves look like this. The black dots are conductive; when a button is pressed, they make contact with the gold-plated tracks on the circuit board:
The other circuit board contains the analogue circuitry:
The integrated circuits on the board are made by Geneva-based STMicroelectronics, whose logo is clearly visible. The part numbers, however, are rather exotic, and I could not find them in any catalogue. They appear to have been custom-marked for the customer, which is not uncommon.
Among the many tiny SMD components, two things stand out: the yellow capacitor in the mains power-supply section, and, at roughly the same height, the thick, slightly angled piece of wire that serves as the current-sense shunt resistor. All the current drawn by the appliance plugged into the meter flows through this resistor. In principle, that can be as much as 16 A.
The PCB tracks alone could not carry that much current: the copper foil is too thin and would simply burn out. The manufacturer solved this by applying a thick layer of solder over the high-current section of the track. This increases the effective conductor cross-section enough to carry the required current:
There is not much in this device that is particularly useful for salvaging parts or materials for hobby projects. However, like electronic waste in general, it contains a surprisingly high concentration of tin, lead, copper, some aluminium, gallium and arsenic — and, not least, gold.
More generally, a discarded printed circuit board can contain as many as 60 different chemical elements, and metals may account for around 40% of its mass. Estimates suggest, for example, that recycling one tonne of mobile phones could recover roughly 130 kg of copper, 3.5 kg of silver, 0.34 kg of gold and 0.14 kg of palladium.
The figure of 340 grams of gold per tonne is particularly striking. Today, an ore body containing more than 200 g/t of gold would be considered exceptionally rich, while a mine can already be commercially viable at grades of just 2–3 g/t. At Roșia Montană, ore averaging about 1.4 g/t of gold was to be processed using cyanide leaching. Perhaps everyone would have been better off if the company had concentrated on recycling mobile phones, with their roughly 240 times higher gold content, rather than on mining…
I cannot say whether the badly crumbling plastic housing would eventually biodegrade or simply add to the amount of microplastic debris in the environment, but that is beside the point here. The plastic parts could go to a waste-to-energy plant, where they could be used to generate heat and electricity, while the electronics should go into the e-waste stream for recycling.
I am going to separate the parts properly and take them to a recycling centre, trusting that they will then end up in the right place. Most people, however, would simply throw something like this into the household rubbish, from where it would end up in landfill.
Admittedly, this is only a small device and does not add much to the mountain of waste. But over time, the metals it contains can leach out and contaminate groundwater. Of course, one could argue that this no longer happens because modern landfills are lined. And indeed they are. If you attend a professional waste-management conference, you will see excellent presentations on leachate treatment. Then, during the coffee break, the conversation turns to who manages to sweep leaking leachate under the carpet, and how…
How Did It Work?
Now that we have taken it apart and sorted the remains for recycling, let’s take a closer look at how the device actually worked.
A plug-in electricity meter of this type measures voltage and current separately, then calculates power from the two signals. The thick, shiny wire visible on the PCB is a low-value shunt resistor connected in series with the load. Because all the load current flows through it, a small voltage develops across the shunt according to Ohm’s law. By measuring this voltage drop, the electronics can determine the instantaneous current.
The mains voltage is measured from the line itself, typically through a high-value resistor network that scales it down to a safe level for the measuring circuitry. By sampling both voltage and current repeatedly throughout each AC cycle, the meter can calculate the instantaneous power and then average it over time. Integrating this power gives the accumulated energy consumption displayed in kilowatt-hours (kWh).
The distinction becomes important as soon as the load is anything other than a simple resistor. In a purely resistive appliance, such as a traditional electric heater, the current waveform follows the voltage waveform almost exactly: when the voltage rises, the current rises with it, and the two cross zero at the same instant. In that special case, the RMS values can simply be multiplied, so P = V(RMS) × I(RMS). If the load contains inductance or capacitance, however, the current is shifted in phase relative to the voltage by an angle φ. For sinusoidal voltage and current, the useful or real power is then P = V(RMS) × I(RMS) × cos φ. The product V(RMS) × I(RMS) is the apparent power S, measured in volt-amperes (VA), while cos φ is the displacement power factor. Thus, a resistive load has cos φ ≈ 1; an inductive or capacitive load has a lower value because part of the current merely exchanges energy with magnetic or electric fields instead of producing net work or heat over a complete cycle.
Modern electronic loads make the situation more complicated. A switch-mode power supply, LED lamp or inexpensive charger may draw current only during short parts of each mains cycle, so the current waveform can be strongly distorted rather than sinusoidal. In that case, power factor is not generally equal to cos φ. The rigorous definition is PF = P/S = P / [V(RMS) × I(RMS)]. The IEC explicitly distinguishes the fundamental, phase-shift-related component from the component caused by waveform distortion. A meter such as this therefore cannot obtain real power reliably merely by measuring RMS voltage and RMS current and multiplying them together. Instead, it samples the instantaneous voltage v(t) and current i(t), multiplies them point by point, and averages the product over one or more mains periods:
That method automatically accounts for both phase shift and waveform distortion. In the special case of clean sinusoidal waveforms it reduces to the familiar P = V × I × cos φ; with distorted current, it still yields the true active power in watts. This is why two appliances can draw the same RMS current yet consume different real power: their apparent power may be identical, but their power factors can differ substantially.
You can read more about this here: https://www.ti.com/download/trng/docs/seminar/Topic_1_Cohen_Lu.pdf
So despite its unremarkable appearance, the Silvercrest 9149 is doing rather more than simply “measuring watts”. It senses voltage and current separately, takes their phase relationship and waveform into account, and from this calculates real power and accumulated energy. That makes even a cheap plug-in meter a surprisingly capable little instrument — and a useful reminder that AC power measurement is not quite as simple as multiplying volts by amps. Its electronics may be ordinary, and its crumbling plastic case was certainly not built for eternity, but the measurement principle behind it is sound, elegant and widely used in modern energy metering.