Monthly Archives: February 2014

Transformer B-H curve

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.

Fuses 3

A correspondent of mine, Brian Magee, has been drawn in on the discussion about fuses, with particular application to valve equipment that had been so ably prodded along by Nigel Machin.

Brian writes:
Fuses in Valve Amplifiers.
I have never had any problems, so have not been forced to think too deeply about the matter..
The following thoughts come to mind when designing power supplies for valve gear :-

(a) Any starting transients in the high tension supply before cathode emission starts may generate large grid – cathode fields in the tube.
This can reduce cathode life.
In frame grid tube such as the ECC88– EF184 and the E180F the grid wire diameter and grid spacing is so small,  field emission occurs and this  usually destroys the tube.
To prevent this I always put a small neon between the grid and cathode of frame grid tubes.
To delay the onset of high tension I use indirectly heated cathode rectifiers such as the 5AR4.

Under these conditions the greatest peak input current occurs when the switching occurs at the start of a cycle, driving the transformer core into saturation. Toroidal core transformers tend to have smaller primary resistance and so greater peak input

To set the fuse rating on any new design, I simply pull the rectifiers out and switch the thing off  and on about fifty times upping the fuse rating until it withstands the turn on transient. 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.

<end quote>

I do remember learning, years ago, that the initial magnetizing current in a transformer primary will depend on the phase of the mains cycle when the switch on occurs. My memory had been that the magnetizing current transient peak could be up to about twice the continuous peak magnetizing current. One would not expect this to exceed the sum of the continuous magnetizing current and the load current.  I have to admit that I had never thought of saturation in this context. Something new (for me) to think about. I have never been aware of any evidence of excessive current on transformer switch-on. I have been caught out, however, with a potential problem when a transformer is switched off. If there is no load on a transformer, and it is switched off with a switch with low arcing losses, then there is nowhere for the magnetizing current to go, except into the charging of stray capacitances or the creation of a carbon track through the insulation. A bit of a lottery as to which will happen first. After having lost a high quality and expensive transformer by doing no more than swithing it on and then off again with no load, I now have a rule that I never switch a mains transformer off without a load on it (a MOV on the primary will do), or if just powering it up to check secondary voltages etc. then use the variac so that the magnetizing current can be reduced at a slow pace compared with a mains half cycle.