MicroAmp-Meter

Feature Highlights

Operating range:
- 1.8V - 12.0V
- upto 1000mA
Three shunt stages:
- 0 - 1000uA
- 1 - 300mA
- 300 - 1000mA
Low burden voltage (max 30mV):
- Range 1: 30µV/µA
- Range 2: 100µV/mA
- Range 3: 30µV/mA
Ultra-fast auto shunt switching (in less than 200 microseconds) without any glitching
Voltage over range and overload protection
Hardware low pass filter and software butterworth filter
2.4 inch TFT provides the best user experience UI design
1500mAh LiPo battery provides long life
Fast charging rate up to 0.6c. Battery charging time less than 2 hours
MAX17048G fuel gauge provides accurate battery SOC
Buzzer for measured voltage over range indication, overload indication and Low-battery indication
ESP32 dual core controller with built in WebSocket server application for current profiling
Parameters:
- Current
- Average Current
- mAh
- Voltage
- Battery SOC
Output control button to disconnect the load from a power source with the indication of LED
TFT brightness control button
Dedicated button for SD Card logging
Dedicated button for turning on Wi-Fi and establishing a connection to start the WebSocket server

Architecture

The heart of the MicroAmp-Meter consists of a precision Current-Sense Amplifier INA190, three high precision shunts (30mΩ, 68mΩ, 30Ω), and 12bit ADC BU79100G for high-speed analog sensing. The shunt-switching topology is based on David Jones's design EEVblog #929 - Designing A Better Multimeter.
The brain of the MicroAmp-Meter is ESP32 running at 80Mhz. The used variant of ESP32 has two physical cores. Each core is pinned to a separate task in RTOS. The primary core of ESP32 is responsible for reading analog data from external ADC, calculating current, and switching shunts accordingly. Dedicating the physical core to shunt switching makes the process happen in about 200 microseconds. A secondary core is responsible for handling LCD, Voltage, Battery SOC, SD Card, and Wi-Fi functionalities.

CALIBRATION

MicroAmp-Meter is calibrated to the high precision voltage reference LDO output during flashing. The calibration values are stored in the EEPROM of the MicroAmp-Meter. These values are loaded from the EEPROM during the startup of the software and all readings are compensated accordingly.

Functionalities

Display Parameters

The description of the parameters of the main display page is given below:

The latest reading of the current value from BU79100G-LA ADC. Butterworth filter is applied on this parameter to filter noise and make the reading smooth (For raw value use Wi-Fi Mode)
The latest reading of source voltage taken from ESP32 ADC
The average value of current over T-1 to T time interval. The parameter shows the average of raw current values over 1 second
mAh (milliampere/hour). It is commonly used to measure the energy capacity of a battery. The parameter is very useful for the calculation of battery timing of DUT. mAh parameter automatically gets reset on output disable.

Wi-Fi Mode

Wi-Fi mode is the main feature of the MicroAmo-Meter that provides an application to visualize current readings and other parameters over the web browser at a high speed of 4000 samples/second. The webpage application code resides inside the memory of MicroAmo-Meter. MicroAmp-Meter and web browser both need to be connected to the same Wi-Fi network. No internet connection or installation of software is required. For more details visit the section How To Use.

SD Card Logging Mode

SD card logging feature is useful to measure the current performance of a device over a long run. For more details visit the section How To Use. This feature logs the following parameters in the SD card over a time interval of 1 second:

Average current
mAh (milliampere/hour)
Voltage

TIP: LCD backlight consumes almost 40% of the total device current. Turning off the backlight during logging can drastically increase logging time.

Output Control

The output control feature provides access to disconnect the source from the load. This feature is controlled by a dedicated button switching the DUT on and off. LED is also attached to represent the state of the output.

Brightness Control

The brightness of the LCD is adjustable by button. This feature is useful to turn OFF the LCD backlight to save power during SD card logging mode. LCD has 3 brightness levels.

Level 0 - Backlight completely turned OFF
Level 1 - Backlight intensity 50%
Level 2 - Backlight intensity 100%

Overvoltage PROTECTION

The hardware protection circuit for overvoltage is used to clamp the input to the range of the ADC. In this case, the Schottky diodes are chosen for their low forward voltage drop. The Schottky diodes are connected to the ground and a 3.3 V system rail capable of sinking current. The series resistor after the Schottky diodes also helps limit the current into the ADC. There is also indication on LCD and Buzzer for measured voltage over range.

Overcurrent PROTECTION

The Overcurrent/Short protection is handled by software. MicroAmp-Meter automatically turns off output if the current exceeds over 1000mA. The feature is implemented with the highest software priority to execute the routine in less than 150us.

How to Use

Wi-Fi

Create a wifi.txt file in the root of the SD card. In the wifi.txt file, enter the SSID of your Wi-Fi in the first line and then enter a password in the second line. Then Insert the SD card in the MicroAmp-Meter.

Click the Wi-Fi Mode button. MicroAmp-Meter will connect to Wi-Fi and display the IP address on the screen.

Enter the IP address in the browser. Make sure the device is on the same network as of MicroAmp-Meter. The webpage will display the Current Profiler application.

- Run: Start measurement
- Stop: Stop/Pause measurement. This button does not clear the graph or buffer, so measurement can resume by clicking the Run button again
- Clear: Clear graph and reset buffer
Heartbeat: Display the status of the connection. Blinking LED represents the alive connection between MicroAmp-Meter and the browser
Battery: MicroAmp-Meter battery SOC (state of charge)
Buffer Fill Status: The application has a storage buffer that can store values for up to 60 seconds (240000samples). It is implemented as a ring buffer so that when it gets full, new readings start overwriting old readings from the start. The progress bar shows the fill level percentage of the buffer
Load Buffer Button: Load the data of buffer on the graph
Watch Window: Sets the watch window of a graph in seconds
Output control button: Toggles the output of MicroAmp-Meter by enabling or disabling it.
Output Status: Status of the output. Shows if the output is enabled or disabled
Export Selection time: to-tn is the start to end time of the data on the graph which is selected to be exported. For more information about the graph selection feature see the section below
Export Status: Shows the status of export ( Successful, Selection too large, Out of range, etc.)
Download Button: Download selected data in CSV format

Cursor tooltip: Element in conjunction with the mouse pointer to display information without needing to click on it
X-Axis: Shows time value in milliseconds
Y-Axis: Shows current value in milliamps
Reset Zoom: Discard zoom and restore the original view
Tooltip Data: Shows the value of time and current against the position of the tooltip
Data Panel: Shows the values of measured parameters coming from MicroAmp-Meter in real-time

Select watch window
Enable output (if it is disabled)
Click on Run button to start measurement
Click on Stop button
If you want to load the content of buffer on graph then click on load buffer button
Export: Put the cursor on the graph over the region from where you want to start the selection to download data, then press the S Key to start the selection and press E Key to end the selection
Click on the Download button to export current readings from time t0 to tn in CSV file format

Tip: The graph has an interactive Zoom/Drag feature. To zoom along only one axis place the cursor over the corresponding axis and then use the wheel/trackpad to zoom in/out without pressing a mouse button.

SD CARD LOGGING

Press SD Card mode button
MicroAmp-Meter will create a new file in the location /logs/logsxxx.csv
After creating the file, MicroAmp-Meter will return to the main page. It will keep logging in the background, writing the values on the micro SD card at the interval of 1 second
Once done, press again SD Card mode button to stop logging.
Remove the memory card from MicroAmp-Meter and use card reader to access the logs

Microamp-meter

Introduction