A simple choke tester measuring inductance and Q-factor based on Atmega128 (Open hardware and open source DIY hobby construction)

I developed a simple tester of inductors, which measures the inductance and Q-value by damped oscillations after a charged capacitor is connected to the inductor. I found some inspiration at Danyk's page, however, I decided not only to count the number of oscillations but to do more measurements. I decided to use ATmega128 for this device, which might be an overkill, but then I was able to re-use a previously designed board to a large extent, to save some work.

The tester connects a charged high-Q capacitor (polypropylene) C201 to the DUT inductor via a mosfet Q201 with very low R_DS_on and low enough gate threshold voltage to be directly controlled from the MCU. The oscillating voltage is tested on the positive input of the built-in analog comparator of the MCU, while the negative voltage of the comparator is connected to a DAC output realized by an internal timer-controlled PWM, a RC filter, and an op-amp buffer. Note that the device cannot be used for in-circuit measurements due to the 5V voltage the capacitor is charged to.

The software then uses analog comparator interrupts and a timer to measure the period of the oscillations, which yields the inductance. To measure the Q-factor, the number of oscillations with amplitude over a certaing voltage threshold is measured for two values of the threshold voltage V0 and V (program-controlled by the PWM). Then Q = PI *(difference in the number of oscillations) / ln(V0/V). The accuracy od the device is not comparable to professional LC meters, but it is quite usable for hobby purposes.

The open hardware schematics is here in PDF and a complete Kicad design including gerbers is here in ZIP. The open source firmware source code is choketester.c and binary for flashing is choketester.hex





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