AC Mains power supply

This morning I wondered what the output of the AC Mains power supply would look like while the printer was running. I hate leaving questions unanswered, so I took a look:

This is a screen capture from my oscilloscope connected across the unregulated DC supply. The vertical scale is 2.00 volts per division, DC coupled, and the horizontal is 100 milliseconds per division. The single-sweep trigger is set for the negative edge at 8V DC.

With the AC power applied and the regulators turned off, the unregulated output of the bridge rectifier is 10.5 volts. Turning on the regulators drops this to about 10.1V. Printing the same eight-character pattern as before pulls this down to about 6.5V as the motor starts up before stabilizing at about 7.5V with a 0.75Vp-p, 120 Hz ripple. If you look closely you can pick out the seven small dips where the print hammer solenoid fires, with the big valley at the end occurring when the solenoid fires to print the eighth character and is held engaged to trigger the paper feed. Once the motor shuts off the big filter capacitor recharges with the classic RC charge curve.

One of the things that has long confused me is the markings on the original power supply transformer indicating that it provided 5.2VDC at 600mA. Marking a transformer with a DC output voltage seemed strange to me.

With these results it becomes clearer. If the rectified output provides 10.5 volts with no load, and drops by 3 volts with a 340 mA load, then the supply impedance is about 9 ohms. This suggests that with a 600 mA load the voltage drop would be 5.4 volts, which would leave 5.1 volts output. That's consistent with the 5.2 volt output marking on the transformer.

I wonder what will happen when I fire up the VFD filament and +30V supplies, plus a complex FPGA design? The datasheet for the Toshiba TC83230-0015 calculator chip, which appears very similar in function to the original calculator functionality, says it draws all of 6mA max. This FPGA and its regulators and oscillator are already drawing 25mA doing almost nothing. I might need to turn off the VFD while the printer is running to avoid a brown-out reset of the FPGA.