Electronic Energy Meter TS-838

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Official specifications:
This is a power meter from Aliexpress dealer haron.k-2, how well do it work?

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It is delivered in a cardboard box with some specifications on.

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Inside the box is the meter and a 40 pages English instruction manual

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The black strip seen on the pictures above must be removed before the meter can be used.

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The strip is used to prevent current draw from the batteries.

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The mains voltage is not easily accessible inside the battery compartment (good).

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It uses two LR44 batteries.




User interface

This power meter has a very simple user interface, most of the time only one button can be used. Only exception is when the electricity price must be configured and to reset the device.

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The VALUE key will change the top line between:
The Display key will change the center line between:
The bottom line shows the energy consumption for the last 7 days, but the scale only works for devices that uses a lot of energy. My fridge did not register (It uses about 1kWh/day).





Measurements

Measurements

The meter is not for measuring low power devices, the readout works in 0.5W steps. It needs a couple of watt before the readout is acceptable.

Temp4541

M1: 59,1C, M2: 80,4C, HS1: 95,5C

With only about 0.7 watt in power consumption I would not expect anything to get hot, but it looks like most of the power is dissipated in two resistor that gets rather hot. The circuit board has space for a larger resistor where the hottest one is.



Tear down

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Two screws and I few clips before I could open it.
The circuit is directly connected to mains, it do not use any transformer.

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I wonder if the black paper behind the circuit board is to add some isolations for the grills.

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On this side of the circuit board is the power supply, this is a capacitor (C1), resistor, electrolytic capacitor and diodes. The voltage is regulated with a standard 5V voltage regulator (U2:HT7150A). The current shunt is marked SHUNT.
It looks like the current and voltage is adjusted with trimmers (VR1 & VR2).

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The other side of the circuit board contains the "power meter chip" (U1:LM324) and supporting parts.
There is not much power meter in that chip, it is just 4 amplifiers, this means everything is done on the display board.

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The display sections contains the four buttons and the LCD display connected with a rubber (zebra) strip.

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Here is the buzzer for over current alarm.

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The small round metal can is a crystal for the main processor.

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On the display side is the main processor, it is a COB (Chip on board), this means a chip bonded directly on the circuit board and then covered in some usual black stuff.
There is also the domes for the four buttons, with labels that do not match the actual functions.



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Conclusion

For a cheap power meter this one is acceptable, but it is not good at low power and the voltage is a bit off (I will not recommend to trim it, the trimpot is directly connected to mains power).
The hot resistor is a bit problematic, it is not too hot for a resistor, but I believe it is used to measure the mains voltage and a hot resistor is not as stable as a cold resistor.



Notes

The error in readout will probably not be present on all devices, but shows that the calibration is not well done.