Tuesday, March 14, 2017

Canon P170-DH Teardown, part 2

Continuing with the teardown of the Canon P170-DH printing calculator, I'll look in more detail at the power supply.

As I mentioned before, the AC mains go through a 60Hz power transformer that brings the voltage down to 8 VAC (RMS). This then goes through a full-wave bridge rectifier made up from four discrete diodes and filtered with a large electrolytic capacitor. This results in an output voltage of 7.85 VDC under load, as measured on an intact calculator. This much was obvious; to learn more I really had to dig in.

The power supply needed to produce three regulated outputs:
  • +5 VDC for the logic and to drive the active anodes & grids
  • -25 VDC for the inactive anodes and grids
  • 5 VAC @ 190 mA for the filament, offset +7 VDC from the -25 VDC supply.
This being a mass-produced consumer product, cost savings would have been the rule in this design. No high-precision voltage regulators when something cheaper would do, but an inexpensive part custom-designed for this application wouldn't be a problem.

Here's the circuit schematic thus far (it gets bigger if you click on it). I've used the component and signal designators on the actual board for my convenience.

The blocking oscillator is formed by TF1, TR1, R6, R9, and C14. Blocking oscillators are hard on transistors because turning off the current through the primary winding results in a high-voltage spike; I think the purpose of R7/C13 is to absorb the initial transient. I read once that for maximum power transfer you don't want to use a free-wheeling diode across the primary, and ZD1 would provide a path for the excess current while maintaining efficiency.

The +5 VDC output comes from the secondary winding tapped at Pin 7. I found this odd because this pin swings from -9.5 to +6.25 volts, which suggested it was a negative voltage source, but it's not. In fact, the anode of ZD2 is maintained at +5.44 VDC by the regulating action of ZD2 and TR2, which adjusts the bias on the base of TR1. Allowing for the voltage drop across D26, we end up with a reasonably well regulated +5V output. The -25 VDC output comes from the secondary tap on Pin 8, after rectification and filtering. Since this is proportionate to the output of Pin 7 based on turns ratios, it is fairly well regulated too.

The filament supply comes from a separate winding on the same transformer. This provides the AC component of the output, and using the center-tap as a reference provides an even average voltage across the length of the filament. The DC offset from the -25 VDC output is obtained just as suggested in the Noritake-Itron documentation, using a Zener diode with a capacitive bypass.

There are still aspects I don't understand. For example, what is the purpose of D9/C22? As best I can tell they don't connect to anything. My initial thought was that they're left over from a second 5 VDC output, but then you'd think they'd have deleted the components from the design to save the cost if that was the case. It's a mystery.

Although I won't go into the operation of the ASICs that make this board into a calculator, there is still circuitry to be explored before I'm ready to design a replacement board.

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