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Inside the Owon P4603/P4305 linear PSU

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dietert1:
Thinking about how to proceed, i decided to wire a Keithley 2700 with three channels of a 7706 multiplexer to record the P4305 output voltage, the reference voltage and the output of the output voltage measurement (schematic above). The diagrams once more show an up and then down scan of the P4305 set voltage from 0.01 to 30 V and then back. With a 8.25 Ohm load the fan starts running at about 15 V and doesn's stop until the end of the down scan.
Apparently the voltage reference is rather stable even with the fan running. The voltage measurement stage is the culprit and its deviations explain more than 80 or 90 % of the behaviour i reported above. Maybe the SMD resistors they used for the voltage divider aren't good enough.
Also note the P4305 output voltage deviations are now half of what i saw before when measuring voltage on its output terminals, with the load taking up to 3 A. Of course, in order to have the unit perform to 1 mV under load, it will need to operate in four wire mode. I will probably try to add a 4-pin socket on the front panel to support 2-wire mode with a shorting plug and 4-wire mode using external sense lines.

Regards, Dieter

dietert1:
When i took out the rear RS232 interface to fit the missing buffer cap, it was already there.
Meanwhile i took out the six sense amplifier gain resistors and characterized them to be 0.1% 50ppm/K type. The 10K resistors were about 36 ppm/K while the 100k+39k gave -18 ppmm/K. So this explains the 50 ppm/K output deviation seen before.
I inserted 10K, 2 ppm/K and 47K, 5 ppm/K SMD resistors (3x 47K = 141K) and they solved the temperature dependence. Using the description above (by james38) i could calibrate the unit with the new resistors, except below 1 V. There remains some deviation of up to 12 mV. Note that 912 mV / 900 mV * 139,2 KOhm = 141 KOhm.
I also added the 4W connector. It's a small 4 pin connector (binder 719 09-9764-70-04), in order to support 2W mode with a shorting plug. Now i can say it's a 1 mV unit.

Regards Dieter

dietert1:
In order to reduce the residual deviations below 1 V output voltage i ordered some 0.1 % resistors 1M and 57K6. They change a 47K precision resistor to 45K, so one gets closer to the original: 100K + 39K = 2 * 47K + 45K. Now the linearity sweep stays within +/- 1 mV.
The no load case has a minor deviation between upward and downward scan. During the downward scan the 5 sec delay time after each 0.1 V adjustment leaves charge in the output caps to settle.

Regards, Dieter

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