Side project while waiting for outdoor air to become fit for human life - restore a Power Designs 2005P Power Supply - Part III

 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

As is shown above, the Programming Constant trim pot is R21 and it is wired to deliver 0 to 200 ohms of resistance from one end of the range to the other. Above it, there is a parallel set of resistors, one fixed at 5200 ohms and the other (R CAL) chosen to make the effective resistance correct for operation.

In my unit, the value of R CAL is 100K such that the equivalent value of the parallel resistors is 4,925K. Since the circuit when adjusted all the way to one end of the Programming Constant trimmer pot, where the resistance is 200 ohms, is insufficient to make the output low enough, the equivalent resistance of this pair is too low. 

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. 

FINAL ADJUSTMENTS

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