Simple milliohmmeter

This project uses TL431 to generate a constant current, which generates a voltage drop when it flows through the cable to be tested. It is collected and amplified by INA199 and output to STC8H1K08. After AD conversion and calculation, the milliohm value is output on the digital tube.
 
The power supply part uses IP5303 for charge and discharge management. Since the low current in standby mode will trigger the automatic shutdown of IP5303, the P3.2 pin is used to regularly send a trigger signal to an NMOS tube to ground the KEY pin of IP5303 to achieve the effect of power supply maintenance. , and at the same time, this function is used to realize automatic shutdown after timeout.
 
In addition, a mode button is set up. When entering the external test mode, the resistance of the external test lead will be measured first. After the pre-reading is completed, the subsequent measured milliohm value will be subtracted from the resistance of the test lead and then displayed. That is, what is displayed is the resistance of the object under test. resistance value.
 
ADC acquisition occurs every 0.5 seconds.
When the power is on and in normal working mode, the standby flashes slowly (1Hz) to display "-", and when the measured value is within 1000 milliohms, the measured value is always lit.
 
Press and hold the button for more than 2 seconds to enter the external measurement mode, and wait for the resistance value of the external test lead to be measured by flashing slowly. After short-circuiting the external test lead, the milliohm value measured will be displayed with a medium flash (2Hz).
At this time, the unstable test results in the first 2 seconds will be discarded. The 4 measured values ​​within the next two seconds will be averaged as the benchmark. The measured values ​​after 4 seconds with an error within 5% of the benchmark will be entered. The 4th measurement will be entered. After that (a total of 8 results including the benchmark), the average value is obtained to obtain a valid external test lead resistance value, and the resistance value will flash (4Hz) quickly and the resistance value will be displayed.
At this time, you can release the test leads and the external test mode standby value 0 will be displayed. Then you can use the test leads to measure resistance, etc. What is displayed is the resistance value of the external test object (excluding the test leads).
 
Short press the button to exit external test mode.
 
Since the MCU itself only has a 10-bit ADC, it does not have very high accuracy, but it is roughly enough for daily measurement of various USB cables, wires, cables, etc. The maximum range displays 999 milliohms, and the displayed value is "- if it exceeds 1 ohm. --" is out of range, and when no cable is connected, the standby value "-" flashes and displays (the standby value 0 is displayed in external measurement clearing mode).
 
Since 8H1K itself has a USB download mode, the P3.2 pin is used to switch between the MCU download mode and the normal mode with a switch. At the same time, the serial port pins are retained, and you can use the serial port to download if needed.
 
The project itself supports the use of 8G1K and 8H1K. When using 8G1K, R24 is not soldered. When using 8H1K, just connect R24 with a 0 ohm resistor or a piece of tin. (The attached program only supports 8H1K) The
 
TL431 constant current source is set to a constant current value of 40ma. Connect a multimeter to adjust R9 to make the current reach this current value. 40ma is a compromise choice taking into account the current temperature drift, the accuracy to be measured, and the accuracy of INA199 acquisition and amplification.
The 1000 milliohm voltage drop is 40mv at 40ma. After being collected and amplified 50 times by INA199, the output voltage is 2000mv. The reference voltage provided by TL431 to 8H1K is 2500mv (actually slightly lower, mine measured 2488mv). The accuracy and range are appropriate. interval.
 
Using the free printed shell from Jialichuang 3D Monkey, a battery installation area is reserved at the bottom. The maximum supported size is 35cm wide and 30cm thick. However, the shell is difficult to install, mainly because the power switch SK-22D07 is too long to fit in. It needs to be cut short, but if it is cut too short, it will be blocked by the shell and cannot be opened manually. I can only cut it to a suitable length and force it into it.
 
PCB physical diagram, the current version has been slightly modified. The reference terminal of TL431 is used to provide the reference voltage for the Vref of STC8H1K08. The actual measurement accuracy is higher (my own physical version is solved with flying wires).
The digital tube is connected to the Vref using a 2.54 bus pin. PCB, easy to install and disassemble; external terminals use screw-free push-type terminals, convenient to use
 
the measurement cable
 
external measurement mode