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Using Cheap Asian Electronic Modules By Jim Rowe Mini Digital Volt/ Amp Panel Meters There are many low-cost digital panel meters available which can display voltage and current at the same time. Quite a few have popped up on the market in the last year or so. So let’s take a look at some of the more popular models, see what’s inside them and whether they’re easy to use. T here are a surprising number of these low-cost digital panel meters currently available. Many are quite similar to each other, but a few are noticeably different. This article will focus on a few of the more popular and useful models. We’ll be looking at the meters designed to measure DC parameters this month (ie, DC voltage and current), with a follow-up article to describe those which make AC measurements. The first one is the DSN-VC288 from the Chinese firm Geekcreit (we’ll be seeing more of their products in later articles). It is available in two versions: one with a 0-10A current range using an internal current shunt, and the other with a 0-50A current range using an external current shunt. Both versions have a 0-100V voltage range. The 10A version comes with two plug-in connection leads for around $5.50 plus delivery, while the 50A version comes with both the leads and an external 50A current shunt for around $8.50 plus delivery. The DSN-VC288 is quite small, at 48mm wide, 29mm tall and 22mm deep. Although some of the suppliers describe it as having a 0.56-inch dual LED display, that is misleading. 76 Silicon Chip The three-digit seven-segment displays used for both voltage (red) and current (blue) are each only 7mm or 0.28in high. Despite this, the displays are quite readable. The display ‘window’ is 35 x 18mm. Both versions of the DSN-VC288 can be powered from a supply voltage of 4-30V DC, usually drawing less than 20mA. So if they are to measure voltages in this range, they can be powered from the same voltage source. The only thing to bear in mind is that the DSN-VC288 can only measure voltages which are positive with respect to its negative rail. That also applies to current measurements. Inside the DSN-VC288 The circuit of the DSN-VC288 is shown in Fig.1. It’s all based on IC1, an STMicro STM8S103F3 8-bit microcontroller. This runs firmware which directs it to take voltage and current measurements every 300ms or so, then show them on volts display DS1 and current display DS2. Three-pin connector J3 at upper left is used for both the meter’s supply input (V+ and V-) and its voltage measurement input (Vin). The V+ supply input connects to the anode of diode D1 and then to the input of REG1, an Australia’s electronics magazine ME6203 LDO (low drop-out) regulator, which provides a regulated 3.3V supply for the rest of the circuit. On the other hand, the Vin input from J3 goes to the AN4/PD3 input (pin 20) of IC3 via a 270kW/8.2kW resistive voltage divider, together with VR1 (the voltage calibration trimpot) and a 100nF filter capacitor across the 8.2kW resistor. The meter’s ‘current’ input is via two-pin connector J4, at lower left. Here pin 1 (-) is connected straight to the meter’s negative rail, while pin 2 (+) connects to the non-inverting input of IC2b, via a low-pass filter formed by a 330W resistor and 100nF capacitor. IC2b is connected as a DC amplifier with an adjustable gain between 23 and 25 using trimpot VR2, to calibrate the current range. Resistor RS connected across the current input pins of J4, shown in red, is the internal current shunt. For the DSN-VC288 version with the 10A current range, RS is a 7.5mW (milliohm) resistor. In contrast, the DSNVC288 version with a 50A current range has no internal resistor RS, as the current shunt is external, with a value of 1.5mW. The only other thing to note about Fig.1 is that ‘connectors’ J1 and J2 are siliconchip.com.au Fig.1: circuit diagram for the DSN-VC288 digital panel meter. The internal current shunt RS is only fitted on the 0-10A current range version, the alternative model with a current range of 0-50A uses an external shunt instead. not physical connectors, but actually a row of test points in the case of J1, with the purpose of J2 unexplained. Presumably, J1 is also used to program IC1 at the factory. Using the DSN-VC288 It’s easy to put the DSN-VC288 module to use, as shown in Fig.2. The first two diagrams show the connections for the version with the internal 10A current shunt, with (A) showing the connections when the module has a separate power supply, and (B) showing the connections when it shares its power supply with the load. (B) can only be used when the load supply is below 30V. The other two diagrams show the connections for the DSN-VC288 version with an external 50A shunt. (C) shows the connections when the module has a separate power supply, while (D) shows the connections for a shared power supply. Again, it must be less than 30V. The two short (150mm) connectsiliconchip.com.au ing leads which come with the DSNVC288 are distinguished by both their size and their insulation. The wires attached to the 3-pin connector that plugs into J3 are thin, while the two wires attached to the larger 2-pin connector that plugs into J4 are thicker. But these four connection options are not the only way that the DSN- VC288 modules can be used. For example, if you want to measure lower currents than their nominal 10A or 50A, you can do that. Bear in mind that the current range of the DSN-VC288 is really just a 0-75mV voltage range, with the firmware scaling this range to show the current passing through the shunt. So you can get a lower current range The underside of the 50A current range version of the DSN-VC288 module. Australia’s electronics magazine December 2020  77 FIGURE 2 by changing the shunt resistor value. This is easier with the version using an external 50A shunt, but it’s also possible with the other version if you’re careful. For example, if you’d like to use the 50A version to measure currents between 0 and 50mA, replace the big 50A shunt with a 1.5W 0.1% resistor. The meter’s scaling will then simply provide current readings from 0-50mA instead of 0-50A. The same approach could be used to give the meter current ranges of 0-500mA or 0-5A, although the decimal point will be in the wrong position. If that doesn’t worry you greatly, the shunt values to use would be 150mW for 0-500mA, or 15mW for 0-5A. If you have the internal shunt version of the DSN-VC288, to change its current range, you’ll need to remove the internal 10A shunt. This is a stout U-shaped wire soldered to the meter’s PCB just to the right of J4, looking from the rear. This is what you need to desolder to change the meter’s current range. Since the internal 10A shunt has a resistance of 7.5mW (providing 75mV when 10A is flowing through it), the scaling firmware in this version will turn 75mV into a reading of “10.0”. So you can change its current range to 0-10mA by replacing the internal shunt with a 7.5W resistor (ideally with 0.1% tolerance). Or again, you could give it a range of 0-100mA by using a 750mW shunt, or a range of 0-1A by using a 75mW shunt. But in both cases, the decimal point will be in the wrong position. Testing I ordered a couple of 50A versions of the DSN-VC288 from Banggood and put them through their paces. Both worked exactly as claimed, with an operating current of 20mA, a voltage measurement accuracy within ±0.1% and a current measurement accuracy of ±1%. In both cases, the readings could The external 50A 75mV shunt is in the foreground, with a similar 100A shunt behind. 78 Silicon Chip Australia’s electronics magazine siliconchip.com.au The Current Shunt Story The stout U-shaped wire (circled in red) is what needs to be removed to change the meter’s current range. be made ‘spot on’ compared with my reference instruments using little trimpots VR1 and VR2. So bearing in mind that the DSNVC288 is very compact and has relatively small readouts, it is very practical and useful, as well as being great value for money! The PZEM-051 meter module The PZEM-051 is one of a range of measurement modules made in China by Ningbo Peacefair Electronic Technology, based in Ningbo City, Zhejiang Province. It’s available from various suppliers via online markets like AliExpress, eBay and Amazon for between $9.00 and $14.95 plus delivery, depending on whether you want the 50A version or the 100A version. There is also a very similar module with a 20A current range available from Banggood for $21.00 plus delivery, designated the PZEM-031 (siliconchip.com.au/link/ab5h). Also, Banggood has another version In the not-too-distant past, voltages and currents were measured using “moving needle” analog meters (ie, moving-iron and moving-coil meters). The current shunt was developed to allow these meters to measure currents that were higher than their basic sensitivity. For example, if a meter needed 1mA to give a full-scale reading (ie, 1mA FSD), it could be used to measure currents up to say 1A by connecting a low resistance ‘shunt’ across its terminals. The resistance was chosen so that it would carry 99.9% of the current, leaving just 0.1% to flow through the meter itself. This effectively converted the 0-1mA meter into a 0-1A meter. Similarly, the meter could be used to measure currents up to 10A by shunting it with an even lower value resistor which would carry 99.99% of the current, leaving just 0.01% to flow through the meter itself. The current shunt would conduct all of the current at 10A, except the 1mA needed for the meter to achieve full-scale deflection (FSD). The name “shunt” comes from railways, where a train is shunted onto a parallel section of track, just like how the current shunt parallels the pre-existing current path through the meter. Working out the required resistance of the current shunt was fairly easy, once you knew the resistance of the meter itself, and the fraction of the current which needed to be diverted past it. For example, if the shunt needed to take 999 times the meter current (999mA/1mA), it would need to have a resistance of only 1/999 that of the meter itself. So if the meter had a resistance of 100W, the shunt would need a resistance of 0.1001W or 100.1mW (100W ÷ 999). In the same way, to take 9999 times the meter current, the shunt would need to have a resistance of 10.001mW (100W ÷ 9999). So that was the purpose of current shunts back in the old ‘analog’ days. But things changed with the advent of digital meters. Since these essentially respond to voltage rather than current, the role of current shunts needed to change as well. Instead of just taking the major proportion of the current, they became a current-to-voltage converter. Their resistance value is chosen to cause minimal disturbance to the circuit in which the current is flowing, while still providing enough voltage drop to allow accurate measurement. And the voltage level chosen was 75mV (millivolts), so most modern digital meters are designed to have this full-scale voltage sensitivity on their current ranges. It is still relatively easy to work out the resistance value of a shunt for any particular current range. For example, if a meter needs a 0-10A current range, the shunt value required would be V/I or 7.5mW (0.075V ÷ 10A), according to Ohm’s famous law. Or if you wanted to give the same meter a 0-1A current range, you’d need a current shunt with a value of 75mW (0.075V ÷ 1A). So that’s the function of a current shunt nowadays – to provide a small but accurately measurable voltage drop when a particular current is flowing through it. Front and rear views of the PZEM-051 module. As shown by the label on the back, this meter has a voltage range of 6.5-100V DC and a current range from 0-50A or 0-100A depending on the external shunt used (see opposite). siliconchip.com.au Australia’s electronics magazine December 2020  79 serial EEPROM which is presumably used to store measurement and display settings. So the design of the PZEM051 is quite elegant. Trying it out An inside view of the PZEM-051 module. The main controller for this board is a Mixchips MXM11P62 (U3; lower middle) which is an 8-bit microcontroller. called the PZEM-015 (siliconchip. com.au/link/ab5g), with extra displays including a bar chart display and measurements of battery capacity and internal resistance. That one comes with a 50A-300A shunt and costs just over $18.00 plus delivery. The common PZEM-051 is somewhat larger than the DSN-VC288, at 90mm wide, 50mm high and 25mm deep. It has a display ‘window’ measuring 50 x 30mm, and the display is an LCD with blue LED backlighting. As you can see from the photo, it offers four-digit displays of both voltage and current, plus two additional four-digit displays: one for power (in either watts or kW) and the other for energy in either watt-hours (Wh) or kilowatt-hours (kWh). Other features include switching the display backlighting on or off, resetting the energy indication to zero, setting a voltage alarm level and configuring the PZEM-051 for use with either a 50A or 100A current shunt. These functions are changed using the small pushbutton just to the right of the display window, via various long and short button press combinations. The button is recessed slightly to prevent accidental presses, and can only be pressed intentionally using a small screwdriver or stylus. All of these settings are stored in non-volatile memory, and are retained even when the power is turned off. The operating voltage range of the PZEM-051 is 6.5-100V DC, and it can measure voltages within the same range. The current measurement range is either 0-49.99A or 0-99.99A, de80 Silicon Chip pending on the version and the current shunt. The power measurement range is 0-10kW, with a display format of 0-999.9W for levels below 1kW, or 1000-9999W otherwise. Similarly, the energy measurement range is from 0-9999Wh for levels below 10kWh, or 10-9999kWh for levels of 10kWh and above. I couldn’t find a circuit diagram for the PZEM-051, but once the 100A version I ordered from AliExpress arrived, I carefully opened its case to take a look inside. As you can see from the internal photo, there is not a great deal in it. At its heart, there’s a Mixchips MXM11P62 8-bit microcontroller (U3) with 14KB of one-time programmable ROM, 256 bytes of SRAM, an ADC with 24-bit resolution, 18 bidirectional I/O pins, three 8-bit timers and a UART. There’s also a Holtek HT1621B LCD interface chip (U2) which links the MCU to the four 4-digit displays on the LCD, and a K24C02 (U4) two-wire Using the PZEM-051 is just as easy as the DSN-VC288, as you can see from Fig.3. The two uppermost screw terminals need to be connected to the voltage/power source, while the two lower terminals are connected to the ends of the current shunt. The two inner terminals must be connected to the negative side of the power source and the current shunt, respectively. Note that the screw terminals are located at the rear of the PZEM-051, at the left-hand end. They’re shown at the front in Fig.3 purely for clarity. I measured the PZEM-051’s voltage readings as 0.16% high, while the current readings were just over 2% high. The latter may be due to the current shunt tolerance. There was a pleasant surprise when I measured the meter’s own current draw, which was just below 3mA with the backlight switched on, falling to around 1mA when it was switched off. Therefore, despite its extra functions, the PZEM-051 is much more energy-efficient than the DSN-VC288, due to the use of an LCD rather than LED screen. To summarise, then, the PZEM-051 multifunction DC measurement module can only be described as both extremely useful and decent value for money. Coming up As mentioned earlier, a future follow-up article will describe some of the newer AC-measurement meter SC modules. Fig.3: a simple example of how you can use the PZEM-051 meter to measure DC power, voltage, current and energy consumption. Australia’s electronics magazine siliconchip.com.au