Fixing issues with the 4 digit plus mode revision 1.1 multimeter board.
Although the revision 1.1 boards worked very well, I was hoping that the original Power Designs, Inc. voltage divider circuit would be accurate enough to support the extra digit of resolution. I also opted for bodging thru-hole resistors in the rev. 1.1 multimeter board design to make it easier to change resistors if I did need to make resistance corrections.
I was not terribly surprised when I measured the output voltage against the displayed voltage and saw a 14 mV difference at 10 volts. After several changes of resistors I found a combination that resulted in less than 3 mV of difference between the display and the output with good linearity across the entire range.
After a few hours of work I modified the original revision 1.1 design to incorporate a surface mount voltage divider network with a fine adjust 12 turn trim-pot. I also modified placement of a few other components for ease of assembly, along with correcting some silk screen errors.
Other changes that did not affect the board layout were changing the 5 volt switching regulator to a 7805 linear regulator mainly for cost savings, and replacing the voltage input pin header with individual longer gold (Au) pins. The individual pins allowed me to have a thicker gold plating and a longer pin for about the same price of the original 6 pin header. I will install these pins from the back of the circuit board next time as I did not need quite that much length.
Mounting of the multimeter board securely to the display board was not as much of an issue as I had originally thought. I ended up using a nylon 4–40 machine screw with three nuts to clamp the edge of the display board in place using the mounting hole on the multimeter board.
Calibration of the voltage went well, but I decided that the trim-pot did not need as much precision as what I had originally designed. So I changed the values of resistor R4 to 100 Ohms, and changed the value of the trim-pot R2 to 1K Ohm which extended the range of adjustment.
I also moved a few of the components for additional clearance of the power supply connector along with increasing the trace width of the 4 display cathode lines from the IC to the connector to further reduce the voltage drop. These changes are now included in revision 1.21
These will most likely be my last changes to the board designs as I have run out of PD 6050C power supplies to modify.
Cost of parts for both multimeter and display boards was $80 USD
Excel spreadsheet for BOM revision 1.21
Eagle CAD 9.X board and schematic files for revision 1.21