(Prerequisites for this post are the previous two posts: “Fuses” and “Fuses 2”)
The only picture I had of Nigel was not suitable, so I have used a picture that Nigel might like instead.
Nigel Machin seeks to sidetrack us with an interesting comment about my last blog posting. I reproduce part of it here:
“I think that for a multiple output transformer individual output fuses make sense to save the transformer when a regulator or diode bridge fails – this does (rarely) happen. A single fuse at the 230V input may not do this because the secondary attached to the fault may greatly overheat without tripping the input fuse which is sized for the whole transformer’s current consumption. However a thermal cutout inside the transformer could suffice I reckon. In the special case of the teensy weensy winding (a technical term) only a secondary fuse could help. I have had this experience with the transformer for a valve amplifier that has a TWW for -47V bias, its diode failed and it burned into a short and the whole transformer was lost. No primary fuse or thermal cutout would help here.”
As is often the case with Nigel, we might be rewarded with some interesting thoughts if we follow him up his side track.
The example takes us straight back to what I (rather inelegantly) called “fault scenario analysis”.
The whole idea of installing a fuse is to minimize the damage when there is some failure that causes an increase in current. This design task would normally arise in a context in which a kindred task is to try to minimize such failures in the first place. Perhaps when each failure mode is identified, and it is being evaluated to see if it is suitable for fuse protection, it could also be considered for elimination or reduction in the probability of a failure.
Take Nigel’s example of the valve audio amplifier with a separate bias supply. The following considerations immediately come to (my) mind.
1. A bias supply is usually a supply with a quite low current. Maybe it would be acceptable to place a resistor between the transformer secondary and the rectifier in such a supply, with a value high enough that the transformer could provide the limited current into a shorted rectifier indefinitely. Power rating of the resistor to take the short circuit current indefinitely. If a vitreous wire wound (brand with good reputation, or tested in real circuit) type or other with a good surge rating is chosen, the resistor should be reliable. The presence of the resistor would surely reduce the probability of rectifier failure.
2. What about the amplifier?
How will those output valves cope with the bias supply removed? Will the plate current increase? Is the output tranny primary rated for the plate current of a pair of output valves with no control grid bias? (I have looked carefully at valve push-pull output transformer data sheets. I do not recall a continuous primary fault current rating.)
Maybe the increase in plate current when bias is lost will be sufficient to promptly open a fuse in the B+ supply. Indeed, if carefully contrived, perhaps the output stage would be just the sort of fault current multiplier that we might like to ensure prompt fuse opening.
As with many Nigel Machin comments – leaves you thinking.