#289 Voltmeter/Ammeter Module

Test and calibrate a DC Voltmeter/Ammeter panel module.

Build

Notes

I got this DC 0-100V 10A Digital Voltmeter/Ammeter module from a seller on aliexpress at a nice price. But is it any good?

This project is a quick test of the unit. Bottom-line: not bad for an easy voltage/current indicator. For more accurate readings, it is a good idea to check the calibration.

Specifications

Display color: Red (voltage) & Blue (current) LED dual display.
Display: 0.28” LED digital.
Operating voltage: DC 4.5 ~ 30V (for power supply to the module)
Operating Current: <20mA.
Measure voltage: DC 0 ~ 100V.
Minimum resolution (V): 0.1V.
Measure current: 10A (direct measurement, built-in shunt).
Minimum resolution (A): 0.01A.
Measure accuracy: 1% (± 1 digit).
Refresh rate: ≥500ms / times.
Operating temperature: -10 to 65°c.
Operating Humidity: 10 to 80% (non-condensing).
Size: 48mm x 29mm x 21mm.
Mounting cutout: 45.5mm x 26.5mm.

Here is the module, front and rear. Unfortunately it is not possible to determine what kind of processor is used, as the main IC has no markings

module_front

module_rear

Module Power Supply

The module takes a separate power supply to drive the unit and LED display. This is good, as it avoids confounding measurements of the circuit being monitored with the unit’s own power requirements, and allows voltage measurement below the minimum 4.5V required to power the unit.

It’s important to note two things:

the module power supply shares a common ground with the circuit under test, so care is required with the arrangement of external circuitry to avoid a ground short
the 2-pin JST power connector has the opposite polarity from what is commonly seen in JST power connections. There is no actual standard, so this is not a problem per se, and the supplied connector lead is correctly colour coded. But if other power connectors are used, it is important to double-check polarity

External Circuit Wiring

Connecting the module in-circuit simply requires:

parallel connection to the positive voltage supply (red wire)
series connection on the low-side of the load to ground (blue, black wires)

VoltmeterAmmeterModule_connections

Test Load

To run some tests, and in the absence of anything better, I mocked up a dummy load:

12V, 1.5A, 18W DC power supply
an arrangement of parallel 50Ω 5W resistors. I connected between 1 and 5 of these for different static resistive loads
a 9V battery to power the module independent of the load

load_layout

Initial Test Results

It works nice enough, but is not quite as accurate as the claimed 1% (± 1 digit) resolution (i.e. the last digit can be off by one). The unit is obviously not suitable if you need accurate mV and mA readings.

Load	Nom Voltage	Nom Current	Nom Power	Measured Voltage	Measured Current
50Ω (1)	12V	240mA	2.88W	n/a	n/a
25Ω (2)	12V	480mA	5.76W	n/a	n/a
16.7Ω (3)	12V	719mA	8.62W	12.5V	0.69A
12.5Ω (4)	12V	960mA	11.52W	12.5V	0.93A
10Ω (5)	12V	1.2A	14.40W	12.5V	1.17A

Note:

I didn’t take measurements for the 1 and 2 resistor gangs
nominal power per resistor is 2.88W, that’s why I’m using 5W resistors here.
according to my multimeter, the power supply delivering about 12.3V over 50Ω down to 12.05V over 10Ω. Still, readings not quite within 1% resolution.
resistors are quite close to their 50Ω rating. Five in parallel measure at 10.3Ω according to my multimeter.

So Does it Just Need Calibrating?

There are two trimpots on the circuit board. Although not documented, it’s highly likely they are fine adjustment for voltage and current measurements. And indeed they are:

module_calibration