Load testing the AC Mains supply

In the previous post AC Mains power supply I asked, "I wonder what will happen when I fire up the VFD filament and +30V supplies, plus a complex FPGA design?" Well, there's no time like the present to find out.

What I did was to take the printer test I described in the post Testing the printer interface and added the VFD display test I described in the post Proof of Life. The two don't interact so this was easy to do.

With the bench output supply set to 7.5VDC, the board draws about 124mA. This varies depending on how many digits and segments of digits are illuminated.

Poking the "FEED" key causes the printer to print the same 8-character sequence as before. I captured a video of the supply's meters, but this single snapshot tells the story well enough: the current draw jumps to a bit more than 500mA while the paper is feeding.

The bench supply output I'm using is only rated to 500mA, so the output voltage dips a bit from 7.5V to about 6 volts. I could switch to the other output and get up to 2.5 amps, but then I'm limited to a maximum output voltage of 6V and the ammeter scale gets pretty small.

What's more important is how this behaves when powered from the AC Mains transformer. For that I again hooked my oscilloscope to the unregulated output from the bridge rectifier (yellow trace) and triggered on the big drop in voltage that occurs when the motor starts running. With the extra 100mA of current consumed by the VFD the unregulated idle voltage is about 9 volts, or about a volt lower than before. With the motor running it seems to have dropped about a half a volt lower. The good news is that the lowest recorded voltage is about 5.25 volts, and the LTC3607 regulator should be able to maintain a stable output down to 4.5 volts.

I also connected a second 'scope probe to the output from the 3.3V regulator, shown in magenta, with a vertical scale of 100mV/division, AC-coupled. There is about 80mVp-p of noise, which is tolerable. The larger spike near the right edge of the image seems to be external noise, as it's present even when the regulators are turned off. But there's no observable change in the regulated voltage even with input swings of over 3 volts.

The important point demonstrated by this test is that the P170-DH calculator's original transformer appears capable of running the FPGA and the VFD even when the the printer is operating. Whew!