Refurbish a Power Designs TP325 triple output power supply with a digital meter.
Link to demonstration video, stored on Google Drive.
5V and 3V3 power is derived from the TP325 channel-3 21mF capacitor following the transformer and rectifier diodes (typically 13V). A resistive divider and a bipolar transistor set the turn-on/off input voltage threshold, followed by a MOSFET to pull the 5V linear regulator low.
An MSP430 microcontroller drives a shift register with the inputs necessary to realize the proper numeric display. The first 4-bits drive the binary-to-decimal decoder and the second 4-bits drive the MOSFET used to multiplex the LED display anodes.
The TP325 output voltage/current is sensed by a current-sense instrumentation amplifier which is fed through a low-pass filter and into the microcontroller ADC.
The microcontroller and ADC are running at a clock speed of 8MHz. ADC samples are taken at a 50Hz rate; the numeric output is updated at a 6Hz rate and is the median of the previous 15 ADC readings. The numeric output has an update deadband of 8mV (about 1 LSB).
The meter was calibrated against an Extech EX330 multimeter via a lookup table in the microcontroller software with a digital code-interval of 32. As can be seen by the chart below, the meter is just as reliable as the EX330 multimeter it was calibrated to.
- Rather than using 4x single-digit 7-segment LED display, I should have purchased a 4-digit display to avoid having to tie them together by hand. Additionally, I should have purchased a common-cathode LED display in order to multiplex each digit with 1 MOSFET rather than 2.
- I initially tried to amplify the voltage sense signal with a generic op-amp (LM324) rather than an instrumentation amplifier. I was able to get reasonable common-mode rejection by hand selecting matching resistors for the input divider, but ultimately the generic op-amp did not have the common-mode range to operate near it's negative supply (ground).
- The chosen microcontroller only has 1kB of flash memory and 99% of it is currently utilized. Designing the software to fit was challenging and ultimately required trading off between the size of the ADC lookup table and the remaining code.
- End-to-end accuracy and noise rejection would also be improved by the addition of a 12-bit ADC and a higher precision reference with which to calibrate it. I suspect the true accuracy of the current design is about 50mV, rendering the final digit of the display entirely useless. With these improvements the accuracy would certainly be below 10mV.