In the last post, I quoted Brian Magee. Part of what he said was this:
“It goes without saying that a fuse should not be placed on the secondary of a transformer driving a full wave rectifier if its failure produces a DC component in the secondary.”

He was, of course, considering the once universal arrangement of full wave rectifier that used a centre tapped secondary and a dual thermionic diode.

This circuit diagram is taken from a book of published prac. notes that I used when I was an undergraduate student at Swinburne. It shows its age with the selection of “8uF” as a value for the electrolytic caps. In the earliest days of valve equipment, high voltage electrolytic capacitor values went up in octaves from 1 uF. The location that Brian warns us about is the location with the “link” symbol designated “A” in the above figure. Note that this circuit uses an indirectly heated rectifier as recommended by Brian. When I was a youngster and used to play with “wireless sets” for a hobby, the rectifier was nearly always a directly heated type, 5Y3 or 80. It must have been that in the olden days it was cheaper to put an extra secondary on the power transformer than to provide a heater to cathode insulation with a rating of 450 volts or so.

I notice that, although it is not quite the same, I came perilously close to doing the wrong thing by Brian’s missive in the circuit that I showed in my post: “Fuses 2”. That circuit would have imposed DC on the transformer if one fuse on the bottom winding was open circuit and the 7812 and the 7912 drew different currents. I was only young at the time.

The idea that we avoid drawing DC on a transformer winding (unless we really have to) is not new to any of us. I have to admit that I have never looked into this in any detail. In the same published prac. notes that I used to extract the circuit above, I find an experiment (it was to do with playing with Lissajous figures) for displaying the B-H curve of a transformer iron circuit.

It would be fun to conduct this experiment with an without a half wave rectifier and load applied to the secondary. Any one out there actually done this?

In the days when domestic radio sets were vacuum tube designs, it was almost universal to have a single ended output with a speaker transformer that (obviously) carried the class A output tube anode current. We coped. Right near the very end of the valve era, I owned a Philips brand valve mantle radio that derived the HT (B+) supply from a half wave rectifier. (Hey you young blokes! “HT” and “B+” are both designations for the anode supply in valve equipment.) The reason for this, it was said, was that it was easy to design the speaker baffle for a null response at 50Hz, whereas eliminating hum at 100 Hz would be more of a problem. Although the cost of a centre tapped secondary was saved, surely the power transformer would have had to have more iron to cope with the DC bias. This would be fun to investigate more fully some time.