Developing a new 4 digit plus mode display for a Power Designs Inc. 6050C power supply capable of displaying current at a milliamp resolution, and voltage at a 10 mV resolution.
After the success of upgrading the original display board for the PD 6050C, I still wanted to look into the feasibility of adding an extra digit to the display.
Design parameters for the upgrade were:
- No modifications to the power supply that could not be reversed by plugging the old display board back in, and re-calibrating.
- Utilize the existing 12 volt AC power to supply the board.
- Large, deep-red 7 segment LED displays to match the original color.
- Milliamp current resolution, and 10 millivolt voltage resolution.
After a bit of searching I found a MAX1447 4.5‑digit, single-chip ADC with LED drivers IC by Maxim Integrated. It includes an internal reference, and a multiplexed LED display driver. An internal charge pump generates the negative supply needed to power the integrated input buffers for single-supply operation, and the ADC is configurable for a ±2V input which is the range needed for the PD 6050C.
For the first revision of the board I decided to keep things simple and utilized the internal reference, instead of the additional expense and time of adding an external higher accuracy reference. The specifications looked reasonable, and I was very happy with the final results.
I utilized a nearly identical rectification and filtering design from the original board, and then regulated that down to 5 volts using a high efficiency Recom buck regulator with some extra filtering on its input and output.
The MAX1447 requires three supply inputs but can operate from a single 2.7V to 5.25V power supply with some additional filtering between power rails. To reduce heating from the LED drivers on the MAX1447 IC, I opted for a 3.0 volt MCP1703 linear regulator for the LED supply rail. I also used a 3.3 volt version of the MCP1703 for the analog and digital power rails, mainly because I had a large quantity of them in stock.
The analog and digital rails require less than 1 mA total, so a much smaller regulator can be used.
Most of the other components are bypass capacitors for the MAX1447 and regulators, with the exception of R1, R9, R10, R11, and R12.
R1 is the trim pot for the brightness current control of the 4 multiplexed LED digits, and R11, R12 together form the second half of a voltage divider for scaling down the power supply output voltage by a factor of 100.
In a future revision I will replace R12 with a 10K multi-turn trimpot, and add the other half of the voltage divider, a 1.62 K resistor to the top of the board.
Resistors R9, R10 are pull-down resistors for the decimal point selection inputs of the MAX1447 and are driven by the position of the front panel “Display Function” switch. I also enabled leading zero suppression in Voltage mode to blank the most significant digit for voltages less than 9.99 volts.
The interface connector for the PD 6050C uses an uncommon 0.15″ spacing 13 pin connector. I was able to find some 6 pin headers with the correct spacing and luckily pin #7 is not used, so I populated both sides of pin #7 which worked out well.
I added 5 test points for testing the individual voltage rails, ground and the 5 volt output.
The 7‑segment LED display is on it’s own socketed board and includes 5 current limiting resistors for the segments of the mode digit.
I decided to change the original “I” and “E” mode indication to a “c” and “E” display so that the “I” could not be confused with a “1”.
On the original board there was additional spacing between the mode indicator and the rest of the digits, but with 5 large displays there is no additional space to differentiate the mode from a fifth digit.
I used mating machine pin headers for connecting the display board to the main multi-meter board for ease of troubleshooting. 12 of the header pins are used for the 4 multiplexed digits, and 3 pins for the mode indicator including ground. The three mode “c” segments are always on and connected to the 3.0 volt LED power rail thru individual current limiting resistors. The additional two segments that add to the always on “c” to make it an “E” receive power from the front panel “Display Function” switch and two current limiting resistors.
All five 7‑segment LED displays are socketed using Mill-Max 14 pin DIP sockets (110–41-314–41-001000) and the total height of the display from the board is just under 0.5″. I used a pair of hex #6–32 threaded standoffs 0.5″ high to replace the threaded clips used to mount the red acrylic lens on the front panel. The display board was then attached to the standoffs using a pair of #6–32 nuts and nylon washers.
I still consider this 4‑digit display modification a prototype. It works well, but needs an additional trim pot for adjusting the voltage divider to calibrate the voltage mode. It also needs better physical mounting of the multi-meter board to the display board. It should work great if sitting on a work bench, but would not be secure for rough shipping conditions.
Cost of components without shipping or taxes was around $57.00 USD
Eagle CAD V9.X ZIP files for both boards