This post has only one ending, but it has many beginnings. There are various literary techniques for dealing with this, but I am not skilled in these. Let us just look at all these beginnings and see where that leads us.
Back in the dark ages when I was at high school, probably about third form (now called year nine) there was an interesting event. In one of the science rooms some sixth formers were conducting an experiment. A glass rod was held about 500 mm above the bench with a laboratory clamp at each end. Near the middle, a wire was wrapped tightly around the rod in two places about 20 mm apart. My eyes followed the wires. This 20 mm of glass rod was wired to a power cord. It was in series with the mains and an electric jug. It seemed as clear as anything that the jug was not going to boil water, as the 20 mm of glass rod between the wires was a good insulator. The experimenters lit a couple of bunsen burners and proceeded to heat the glass rod. One of them explained to me that glass was a conductor at high temperature. I was sceptical, but hung around to see what happened. What happened was that a teacher arrived and saw the bare wires on a circuit that was plugged into the mains, and all hell broke loose! Fortunately this was one occasion when I was not in trouble. I made a discreet exit from the room, and that was the end of that, except I often thought about it. It would have been interesting to see that experiment completed.
That was then: this is now. Now you can see that experiment completed at Glass Conducts .
In the 1970’s I used to subscribe to a magazine called “Wireless World”. There really isn’t anything quite like it these days. There was a very wide range of contributers. Wireless world would print contributions that looked really crazy. In those days a frequent contributer was Ivor Catt who wrote articles to explain why he thought that James Clerk Maxwell and Albert Einstein were all wrong. Some years previously, Authur C Clark had put forward the idea of geostationary satellites in a Wireless World article. The magazine carried articles about audio equipment that ranged from the important to the wierd. In the first category were Peter Baxandall, Douglas Self, John Vanderkooy and many others. Many issues contained papers that one would want to keep, and I kept them.
A few years ago, I was doing some design contract work for Omnitron Technologies. Part of this involved revision of sparsely documented work done by others. In the design there were several instances of drive of a large MOSFET or IGBT through an isolation barrier. My predecessor had chosen an HCPL316J for this. You can see the datasheet here. This device includes a fault detection and automatic shutdown scheme, that Avago call “Desaturation Detection”. The saturated power switch pulls down a logic input through an external fast high voltage diode. The idea is that if a fault current arises, that will pull the switch device out of saturation. The rise in voltage is detected, and that initiates the shutdown. The detail that intrigued me is that is is a deliberately slow shutdown. The operation is described in the data sheet like this:
And again, in the more detailed blurb:
Further down, details are provided.
The above extract from the Switching Specifications table does not show the units for the times. The unit is microseconds. Obviously the chip designers set out to respond to the IGBT being pulled out of saturation as quickly as possible, but then allowed 1.5 to 2.7 microseconds for the switch-off transition. Such a slow switching transition, if repeated every cycle in a switching circuit would give excessive switching losses. If it occurs only once, then the rise in IGBT die temperature is clearly considered a lesser evil than the excess voltage from the circuit inductance if the switch-off is faster. It is an interesting idea, and a surprising idea that a compromise switch-off speed can be found by the designer of the chip, who knows very little about what inductance might be in that fault current circuit. Leaves you thinking.
(Now we are getting closer to the actual subject of this post.) I write a column called “Sparks ‘n’ Arcs” for the magazine Australian Model Engineering. In the issue for July-August 2013, I covered the subject of fuses, and explained the significance of the I2.t measure. For your convenience, I have placed an extract of this article here. Copyright for my writing in that magazine is held jointly by me and the magazine. I have the magazine editor’s permission to make this extract available in this blog post. If you would like the whole article, that is obtainable by purchasing that magazine issue from the AME Retail Shop.
(For continuity of my story, I suppose I am assuming you will be interested to read the article extract.)
I have been clearing out a lot of old stuff, and in order to make space (for new stuff) I am going through my old Wireless World magazines and keeping only the articles that I want, and discarding the rest. I just discovered an article “Fuses for the protection of electronic equipment” by R.A.W. Connor, F.I.E.E. He covered the basic material much as I had done in my AME article, but then introduced a thought that was new to me. It was new, but in a way, I had been drip fed vital bits of information since school days.
In many applications for an HRC fuse, there will be sufficient inductance in the circuit for an appreciable amount of energy to be stored before the fault is cleared. It falls on the fuse, then, to not only clear the fault current, but to dissipate the energy that the fault current has stored in circuit inductances. If the fuse opens instantly (an arc forms and then extinguishes) then the inductance might cause an over voltage failure of the very components that the fuse has only just saved from an overcurrent. The designers at Avargo who designed the HCPL316J could tell you that. I have read in many places that the powdered silica that an HRC fuse cartridge is filled with, acts to draw thermal energy from the arc and extinguish it quickly. R.A.W Connor has just filled in a critically important part of the story for me. The arc raises the temperature of the silica powder so that it conducts. It provides a shunt path both in the thermal sense and in the electrical sense. By shunting current from the arc, it promotes rapid arc extinguishing. Then the thermal mass of the rest of the cartridge full of silica powder comes into play and lowers the temperature so that the silica is no longer conducting.
Why hadn’t I grasped this before?
I wonder if the silica has different concentrations of salt doping for different fuse voltage ratings?