REVERSE ENGINEERED CIRCUIT FOR PROGRAMMING CONSTANT ADJUSTMENT
I went through the power supply, tracing out, beeping and and measuring components to determine what was different between this 2005P supply and its cousin the 2005 for which I had schematics. In fact it was a very minor set of changes.
I took the schematic for the 2005 and erased the extraneous gear and made the corrections. Now I have a schematic of my unit. I posted this as the prior blog post for anyone who has this unit and is looking for the drawing.
|Portion of 2005P schematic covering adjustment|
STRATEGY FOR CORRECTING TRIM POT RANGE
I will snip out R CAL and test the resulting output voltage. Based on where the output voltage is with the trimmer pot halfway, I will determine an appropriate parallel or series resistor to make the output of the supply match the external resistor in K ohms by adjusting the Programming Constant pot.
I set up my resistance board for as close to exactly 20,000 ohms as I could get and wired it into the supply rear terminals as the programming resistance. After cutting out the R CAL resistor, I was able to adjust to 20.000 volts with the Programming Constant trim pot somewhere in the middle of its range.
I did have to recheck and readjust the Zero Trim pot, then finesse the Programming Constant but once that was done, I could set up my resistance board to any value I wanted between 0 and 20K and I would see a voltage equal to 1/1000 of the resistance. This project is complete.
CROSS CHECKING WITH OTHER MEASUREMENT INSTRUMENTS
My 2005P power supply is only adjusted as accurately as the resistance I hooked up and as accurately as my VOM could measure. When I am next in a lab with multiple higher quality instruments, I will recheck the resistance and output voltage but that will be a matter of a minor adjustment of the trim pot.
|Power supply with 'programming' resistance board on top|