Voltmeter and Ammeter - A Beginner's Guide

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 buck converter to handle the large voltage drop is to avoid introducing DC-DC ripple interference during the design process; a secondary reason is to reduce project costs.
2. MCU Selection Analysis :
   The CW32 is selected for its

wide operating temperature range (-40~105℃)
, wide operating voltage range (1.65V~5.5V, STM32 only supports 3.3V systems),
and strong anti-interference capabilities: HBM ESD 8KV, achieving the highest international standard for ESD reliability (STM32 ESD 2KV).
This project focuses on a superior ADC: a 12-bit high-speed ADC with ±1.0LSB INL 11.3ENOB and multiple Vref reference voltages... (STM32 only supports VDD=Vref) . It utilizes
stable and reliable eFLASH technology.

3. Voltage Sampling Circuit:
The voltage divider resistors in this project are 220K+10K, therefore the voltage division ratio is 22:1 (ADC_IN11).


4. Current Sampling Circuit : This
project uses a low-side current sampling circuit for current detection. The low-side of the sampling circuit shares a common ground with the development board's meter interface


. 5. Digital Tube Display:
This project uses a digital tube as the display unit.
6. LED Indicators:
This project includes an additional power indicator and an I/O operation indicator.
 


7. TL431 Circuit Design for Voltage Measurement and Calibration:
This project adds a TL431 circuit to provide a 2.5V reference voltage. This can be used to provide an external voltage reference for calibrating the AD converter. From a product design perspective, due to the inherent ADC performance advantages of the CW32, this circuit is not necessary. This circuit will be designed on the development board for learning related application principles.