What?  Adding a digital display to a vintage power supply in under an hour. The analog gauge was moderately inaccurate and not granular (0-55V in 1.8 inches). Digital Display added using AVR (10 bit a/d) and parallel lcd. Documenting this in case it requires Cal / modifications in the future. Teaser Images

Component	Purpose	Image
parallel lcd	Simple Cheap lcd Character Display. HD44780 Compliant. Found in unknown state
atmega 48	Super Cheap DIP Microcontroler. Programmed with AVR ISP MK2. Tacked to a protoboard.
7805	linear regulator
Misc Components	Resistors, Caps, led, potentiometer (10K)

Component	Purpose	Image
Routing the outputs	For some reason the outputs of the supply were in the rear, some low gague wire and scavenged banana jacks later, they were re-located to the front. The supply was also mounted onto aluminum risers to allow proper ventilation, the probably source of the failure of the previous supply
Finding Some Power	To drive the lcd, microcontroller i will need 5v. After some trace-following and careful observing, a low voltage winding from the main transformer was rectified and dumped into a 2200uf cap. nominally, the cap stayed at 10V DC. Upon probing the cap, i found that there was little AC ripple. Next i made some assumptions and tested them. i banked on having the lcd and micro would consume 100ma at 5V. to simulate this (from the 10V cap) i used a xxx ohm resistor. THIS IS IMPORTANT. I checked that the power supply preformed well with this extra internal load. There was no observable difference in the power supplies output, even under load. I now had 10V, one 7805 away from a stable 5V supply. It is also important that the ground of the cap is the same as the ground of the supply output
Testing the LCD	As this LCD was found in an unknown state, it was a good idea to test to see that it worked before designing everything around it. Emily K was nearby, thus the lcd statement. LCD is driven in 4 bit mode (instead of 8 bit mode). Contrast was set by small blue potentiometer (as seen behind lcd screen)
Operating at Higher Voltages	The A/D on the AVR is 5 V tolerant, not 55V, a simple resistive divider from the output of the supply was used.  Note at this point the lcd was temporairally mounted into aluminum rectangle-bar.
Calibration	A multi-meter (outside of image) was used to calibrate the multiplier used to convert ADC counts to Voltage outputs

Dane.Kouttron
Rensselaer Polytechnic Institute 
Electrical & Electrical Power
631.978.1650