Voltmeter and Ammeter

II. Hardware Design

1. Power Supply Circuit
LDO (Low Dropout Linear Regulator) Selection This project uses an LDO as the power supply. Considering that most voltmeter products are used in industrial scenarios with 24V or 36V power supplies, the SE8550K2 with a maximum input voltage of up to 40V was selected as the power supply. The main reason for not using a DC-DC step-down circuit to handle the large voltage drop is to avoid introducing DC-DC ripple interference during the design process, and the secondary reason is to reduce project costs.
2. MCU Selection Analysis
To reduce the learning cost for everyone, this project uses the LCSC CW32F030C8Tx development board (core board) as the main controller, but this does not mean that we will talk less about this section. From the perspective of training engineers, the correct selection of the main controller is very important, as it relates to the overall advantage of the project. Regarding the voltmeter and current meter, the author used STM32/CW32 and some other 32-bit microcontrollers for some debugging and testing. This comparison is only with the STM32F103C8T6 as a reference for device selection, primarily aimed at providing ideas and improving understanding.
Avoid blind selection. When selecting an MCU (Microcontroller Unit) for this project, multiple aspects need to be considered to ensure the chosen MCU meets project requirements.

Clearly define your project needs: Understand the required computing power, including clock speed, processor core type, and whether a floating-point unit is needed.
Identify the required I/O ports and important peripherals, such as ADC peripherals. Since this is a development board project, primarily for debugging and learning, there are no strict limitations on the number of I/O ports: i.e., the associated costs are not considered.

Key advantages of the CW32 in this project

: Wide operating temperature range: -40~105℃;
Wide operating voltage range: 1.65V~5.5V (STM32 only supports 3.3V systems)
; Superior interference immunity: HBM ESD 8KV; All ESD reliability meets the highest international standard (STM32 ESD 2KV)
; Project focus - Better ADC: 12-bit high-speed ADC, achieving ±1.0LSB INL 11.3ENOB; Multiple Vref reference voltages... (STM32 only supports VDD=Vref);
Stable and reliable eFLASH technology.

A detailed explanation of these advantages will be provided in the chapters on ADC sampling and related extensions.
The main characteristics of the CW32 ADC: This project requires a focus on the 4 reference voltage sources. (Content from the "CW32x030 User Manual")
3. Voltage Sampling Circuit:
The voltage divider resistors in this project are designed to be 220K+10K, therefore the voltage division ratio is 22:1 (ADC_IN11).
The voltage divider resistor selection

is designed to measure the maximum voltage. For safety reasons, this project uses 30V (the actual maximum display value can be 99.9V or 100V).
The ADC reference voltage is 1.5V in this project, and this reference voltage can be configured through the program.
To reduce the power consumption of the sampling circuit, the low-side resistor (R7) is usually chosen as 10K based on experience.

Then, the high-side resistance of the voltage divider resistor can be calculated using the above parameters.

The required voltage division ratio is calculated, i.e., the ADC reference voltage. The input voltage is designed; using known parameters, 1.5V/30V = 0.05 can be calculated.
The high-side resistance is calculated as the low-side resistance/voltage division ratio; using known parameters, 10K/0.05 = 200K can be calculated.
A standard resistor is selected: a resistor slightly higher than the calculated value of 200K is chosen. We usually choose E24 series resistors; therefore, in this project, 220K, which is greater than 200K and closest to the calculated value, is selected.

If, in actual use, the voltage to be measured is lower than 2/3 of the module's design voltage (66V), the voltage divider resistor can be replaced and the program modified to improve measurement accuracy. The following example illustrates this:

Assuming the measured voltage is no higher than 24V and other parameters remain unchanged,
calculations show 1.5V/24V = 0.0625, 10K/0.0625 = 160K. 160K is a standard E24 resistor and can be directly selected, or a higher value 180K can be chosen with some redundancy.

If, in actual use, the voltage to be measured is higher than the module's 99V design voltage, a different resistor can be selected. To achieve a wider voltage measurement range, one can choose to replace the voltage divider resistor or modify the reference voltage. The following example illustrates this:

Assuming the measured voltage is 160V, the solution is to increase the voltage reference to expand the range.
Given that the voltage division ratio of the selected resistor is 0.0145, we can calculate 160V * 0.0145 = 2.32V using the formula. Therefore, we can choose a 2.5V voltage reference to expand the range (increasing the range will reduce accuracy).

Considering the potential fluctuations in the measured power supply, a 10nF filter capacitor is connected in parallel with the low-side voltage divider resistor to improve measurement stability.
Range switching:
In this project, an additional voltage sampling circuit was added. Therefore, we can discuss the significance of range switching for improving measurement accuracy. Multimeters often have multiple range settings for more accurate measurements. By adjusting different ranges, the optimal measurement accuracy of the measured point within the corresponding range can be obtained.
This project requires a combination of hardware and software to achieve this function. When we first use the ADC_IN11 channel mentioned earlier to measure voltages below 30V... If the measured voltage is within 0~3V, use the ADC_IN9 channel for measurement. In this case, the measurement accuracy is greatly improved due to the reduced voltage division ratio. There are many ways to implement range switching, and the development board design provides more design possibilities.
4. Current Sampling Circuit:
This project uses a low-side current sampling circuit for current detection. When learning the common ground between the low-side of the sampling circuit and the meter interface on the development board, please do not solder R0!!!
The design analysis
for this project involves a sampling current of 3A, and the selected sampling resistor (R0) is 100mΩ. The selection of the sampling resistor mainly considers the following aspects:

the maximum value of the pre-designed measurement current;
the voltage difference caused by the 3A current sensing resistor in this project;
and the power dissipation of the current sensing resistor, which should generally not exceed 0.5V. A suitable package should be selected based on this parameter. Considering the power dissipation (temperature) issue under high current, a 1W packaged metal-wound resistor was chosen
. The voltage amplification factor of the current sensing resistor: No operational amplifier is used in this project, so the factor is 1.

The current sensing resistor value can then be calculated using the above parameters. Selection:

Since no amplifier circuit is used in this project, a larger sampling resistor is needed to obtain a higher measured voltage for measurement.
Considering that a larger resistor will result in a larger voltage difference and higher power dissipation, a larger resistor cannot be selected indiscriminately.
A 1W packaged resistor was chosen in this project, corresponding to a power rise of 1W.

Based on the above data, a 100mΩ current sensing resistor was selected. According to the formula, 3A * 100mΩ = 300mV, 900mW.
To address different usage environments, especially those with high current, resistor R0 can be replaced with constantan wire or a shunt, allowing for selection based on the specific application. For safety and educational purposes, this project does not delve into measurements exceeding 3A, but the principle remains the same.
5. Digital Tube Display:
This project utilizes digital tubes as the display unit.
Two 0.28-inch three-digit common-cathode digital tubes are used as display devices. Compared to displays, digital tubes offer better visibility in complex environments. Depending on the specific needs of the application, smaller current-limiting resistors can be used to achieve higher brightness. Furthermore, digital tubes possess better mechanical properties and are less susceptible to damage from external forces compared to displays. They are commonly used in industrial applications requiring stability and reliability. From a development board learning perspective, this facilitates targeted learning of electronic measurement principles and related development.
In this project, actual testing showed that the current-limiting resistors (R1~R6) for the digital tubes were configured to 300Ω, resulting in good visibility for both red and blue digital tubes, with a soft and non-glaring brightness.
Strictly speaking, the current-limiting resistor should be added to the segment; adding it to the digit would affect the display effect. In our actual design, we added it to the digit to save a few resistors, but the impact on the display is not significant. Therefore, we added it to the digit for convenience.
The
driving principle of a digital tube mainly involves controlling the switching state of each segment to display numbers, letters, or symbols. The following is a detailed explanation of the driving principle:

Basic structure of a digital tube:

A digital tube is usually composed of seven or eight LED segments (eight segments in this project). Each segment represents a part of the digital tube and can display numbers 0-9, letters AF, etc.
Digital tubes have two types: common cathode and common anode. The difference lies in whether the common terminal COM of the LEDs (i.e., the end connecting all LEDs) is connected to the negative or positive terminal of the power supply.


Driving method:

Segment selection: The desired number or character is displayed by controlling the switching state of each segment of the digital tube. Each segment corresponds to a control signal. When the control signal is on, the segment will light up; otherwise, it will be off. (a, b, c, d, e, f, g, dp)
Bit Selection: The desired digital tube is selected by controlling its bit lines. Bit line control sets the bit line of the desired digital tube to a high level and the bit lines of other digital tubes to a low level. By continuously switching the state of the bit lines, display switching between multiple digital tubes can be achieved.


Driving Circuit:

The digital tube driving circuit can be implemented using hardware circuits, such as integrated circuits like digital signal processors (DSPs), microcontrollers (MCUs), or shift registers to generate control signals suitable for the LEDs.
These control signals can be in the form of pulse width modulation (PWM) signals, serial data signals, etc. By controlling the frequency, width, and amplitude of these signals, the brightness of the digital tube can be controlled, thereby displaying the desired numbers or letters.


Software Control:

In addition to hardware driving circuits, digital tubes can also be driven by software. By programming to generate control signals suitable for the digital tubes, more flexible and complex display effects can be achieved, such as scrolling numbers or alternating numbers.


Driving Common Cathode and Common Anode Digital Tubes:

For common cathode digital tubes, the common cathode pin is connected to the negative terminal of the power supply, and the control pin is connected to the output pin of the control chip. When a number needs to be displayed, the control chip outputs a corresponding encoded signal to the control pin, causing the corresponding LED segment to light up.
For common-anode seven-segment displays, the working principle is similar to that of common-cathode seven-segment displays, except that the common-anode pin is connected to the positive terminal of the power supply, and the control pin is connected to the output pin of the control chip.


Encoding Display:

To display the corresponding number or character on the seven-segment display, the segment data port must output the corresponding character encoding. For example, to display the number "0", the character encoding for a common-anode seven-segment display is 11000000B (i.e., C0H), while the character encoding for a common-cathode seven-segment display is 00111111B (i.e., 3FH). The specific encoding depends on the actual seven-segment display.


Dynamic and Static Display:

Seven-segment displays can use either static or dynamic display methods. In static display, each of the eight segments of each seven-segment display is connected to an 8-bit I/O port address. As long as the I/O port outputs a segment code, the corresponding character is displayed and remains unchanged. Dynamic display, on the other hand, lights up each segment of the seven-segment display one by one in turn, achieving simultaneous visual display through rapid switching.



In summary, the driving principle of a digital tube is to display numbers, letters, or symbols by controlling the switching state of each segment of the digital tube, and to achieve display switching between multiple digital tubes through segment selection and digit selection. Simultaneously, the driving of the digital tube can be implemented through hardware circuits or software control, and common cathode or common anode digital tubes can be selected for driving as needed.
This project actually uses dynamic scanning display to drive the digital tube.
Calculating the required current for the digital tube :
Since this project uses dynamic scanning display to drive the digital tube, at any given time, a maximum of only 8 segments of the digital tube (or LEDs) can be lit, or in other words, only one digit can be lit. According to the design, the required driving current is approximately 11mA (IO port high-level voltage 3.3V ÷ 300Ω).
At this point, it is important to ensure that the selected MCU has sufficient current-pull/sinking capability.
Analysis of the datasheet shows that the CW32 has no problems. (Some chips are not suitable.)
6. LED Indicator Lights:
This project additionally designed a power indicator light and an IO working indicator light.

LD_PWR is the power indicator light.

Since chip I/O often has a greater sinking current capability than a pulling current capability, LED1 is designed to be active low (on).
To reduce the current consumption of the LED, some LED brightness is sacrificed, the number of component parameters is reduced, and the current-limiting resistor for the LED is chosen to be 10K.
7. Button Circuit Design:
There are various design methods for button control circuits. Thanks to the fact that the CW32's I/O port can be configured with pull-up and pull-down resistors internally, the button control circuit on the outside of the chip does not need to be configured. One end of the button is connected to the MCU's I/O, and the other end is grounded. When the button is pressed, the I/O is pulled low.
8. TL431 Circuit Design for Voltage Measurement and Calibration:
This project adds an extra TL431 circuit to provide a 2.5V reference voltage, which can be used to provide an external voltage reference for the chip to calibrate the AD. From a product design perspective, due to the inherent ADC performance advantages of the CW32, this circuit is not necessary. This circuit is designed on the development board for learning the relevant application principles.
The TL431 is a relatively "old" device, a classic with wide applications, and still found in many electronic products.
Many beginners may be encountering this device for the first time, so we'll briefly explain its principles to help you better utilize the TL431.
TI defines it as a "Precision Programmable Reference," and we can focus on several key characteristics on the first page of the references.
Precision: This indicates its output voltage is extremely accurate. The TL431 I used has ±0.5% accuracy, and at room temperature, it measured 2.495V on the board. This is a world of difference in accuracy compared to common Zener diodes. In application circuit diagrams, the TL431 is internally represented by a Zener diode symbol.
Adjustable Output Voltage: The adjustable output voltage is between Vref and 36V. We use the output Vref voltage in our project. Vref voltage is approximately 2.5V. Therefore, we use 2.5V in the description, which is approximately equal to Vref.
Sinking Current Capability: This refers to how much current the output voltage pin can provide. This is greatly related to the resistance value (R13) in the application circuit. It should not be less than 1mA. If there is no need for sinking current, do not design the current too high, causing unnecessary power consumption.
****************************************************************************** Physical Verification **********************************************************************************
PCB Physical Current and Voltage Calibration (only a partial calibration process)
For daily use,
the image shows a multimeter voltage screenshot of a nearly depleted 1.5V battery.
This image shows the voltage and current meter screenshot of the project using a nearly depleted 1.5V battery.
 
This can demonstrate the accuracy and feasibility of this project.