Panel-mounted 100 V 10 A ammeter-voltmeter

I’ve already posted a review of a very similar device, and even the review title was almost the same. This time, though, the little meter is a bit cheaper, and its quality is definitely no worse than the previous one. So, here’s a quick review of a useful little gadget for electronics hobbyists.


Before getting to the review itself, a few words about the price. The title lists a price of $1.72, but the total comes to more because shipping is paid. Shipping to Russia is cheaper than to our country: $2.51 versus $3.97. Since the order included about ten meters, though, the difference didn’t add up to much in the end.

Specifications
Voltage measurement range: up to 99.9 V
Voltage measurement resolution: 0.1 V
Current measurement range: up to 9.99 A
Current measurement resolution: 0.01 A
Accuracy: 1% (±1 digit)
Supply voltage: 4–30 V
Current consumption: less than 20 mA
Refresh rate: about 3 Hz
Dimensions: 48 mm * 29 mm * 26 mm
Operating temperature: -10° to +65°

Each ammeter-voltmeter comes in its own antistatic bag.


The package also includes a set of hookup wires.


The dimensions are standard: 48 × 29 × 26 mm overall, with a 45 × 26 mm cutout.


The display is covered by a brown plastic filter. The wires are about 20 cm long and have the appropriate connectors on the ends for plugging into the board.


The meter is very easy to take apart: just bend back the plastic tabs on the side of the case and free the protruding part of the circuit board.


Though, to be honest, you probably won’t need to do that, since all the components except the display are mounted on the back.
The board is neatly soldered, especially considering how inexpensive the meter is.


A closer look at the circuit board.


1. Measurement and display are handled by a Nuvoton N76E003AT20 microcontroller.
2. An LM358 op-amp amplifies the signal from the shunt. It’s not the best choice, but still better than having the microcontroller do the job itself.
3. The shunt is wire-wound, which I think is better than a low-value SMD resistor.
4. The circuit is powered through a 3.3 V regulator. The small diode visible above provides reverse-polarity protection for the power supply.


The connections are standard for this type of meter:
Thin red: meter power
Thin black: meter power ground
Thin white: voltage measurement input
Thick red: current measurement input
Thick black: current measurement ground, connected to the power supply ground


The first thing you notice when you switch it on is the animation across the display’s outer segments. I don’t think I’ve seen that before.


And, of course, the tests.
First, let’s check the current consumption and minimum operating voltage.

1. At about 2.6 V, the meter turns on, but the readings on the display are nowhere near accurate.
2. Raising the supply voltage to 3.6–3.7 V brings the meter to its operating point, as shown by the current consumption leveling off. Current consumption is about 10 mA.
3. At an input voltage of 28 V, current consumption stays at roughly the same level.
4. Peak current consumption is about 12–13 mA.
5. At an input voltage of 3.7 V, the readings are already accurate. In the photo, the meter’s input is connected to its own power supply.
6. The readings are slightly low—by just 0.1 V—but this also happens at higher supply voltages.

The test showed that:
1. Current consumption is clearly below the stated “up to 20 mA” and is actually closer to 10–13 mA, which is pretty good in my book.
2. The minimum supply voltage is 3.7 V, not 4 V, so that’s good too.



The voltage measurement accuracy test showed that across the 0.5–62 V range, the meter reads about 0.1–0.2 V low, which is also within the stated 1% accuracy.
At least in the 0–20 V range, the readings are very close to the actual values, since even a slight increase in voltage corrects the issue. You can see this in the second and third photos, where I applied 1 V.


The current readings are noticeably worse. Although the error is almost within the stated tolerance, it seems rather large to me: at just 5 A, the difference was 0.06 A.


There are a couple of trimmer resistors on the board for calibration. Judging by their middle positions, though, nobody even tried to adjust them.
The adjustment is quite coarse, which is inconvenient. I recommend calibrating at the highest possible current and voltage, but since the meter's input voltage can be as high as 100 V, you need to be careful.


Because the adjustment is so coarse, the result was a little worse than I expected. Readings at 50–60 V became completely accurate, but at lower voltages the voltmeter still underreported by 0.1 V.


Even the first attempt at correcting the current measurement gave a noticeably better result: the difference is just 0.01 A, and even that only at some points.


There isn't much else to say, so here's a brief summary:
The meter is quite easy to calibrate, although the voltage measurement error was not very large to begin with.
It has very low power consumption and an operating voltage range starting at about 3.7 V.
On top of all that, it's very affordable.

Considering these advantages, I can confidently recommend it.

That's all for today. I hope this mini-review was useful.