Project / Demo
Exploding resistor
One of my favorite simple demonstrations: catching a carbon-film resistor at the instant it gives up — in stills and at 1200 frames per second.
The exploding resistor demonstration.
Explanation of the demo
A 2900 μF capacitor is charged to 300 V, then discharged through a 10 Ω, ¼ W carbon-film resistor. The resistor doesn't so much explode as violently fly away. Below is the resistor body after it came to rest on the table about 0.5 m away.
The physics
Because the timescales involved are “slow” as far as the electrodynamics are concerned, the electric current can be considered spatially uniform throughout the wire–carbon–wire system. Essentially all of the voltage drop occurs across the cylindrical carbon body, so all of the Ohmic heat is generated in the carbon. The temperature of the cylinder rises rapidly and the wires to either side heat up by thermal diffusion. The cylinder eventually fails and a flaming discharge erupts from the center, turning the body into a tiny rocket. The thrust rips the cylinder off the now extremely hot wires. The sparks are mostly molten wire flying away, and the flash is mostly the flaming discharge; the bang is the rapid failure of the ceramic body.
In the high-speed frames it can look as though the flash is an electric arc between the two leads. But the voltage is only 300 V across several millimeters — nowhere near enough to arc, even at elevated temperatures. So that is not what is happening here.
High-speed video
To see what was going on, I used a high-speed camera capable of 7,500 fps at 1 megapixel — and up to 150,000 fps at reduced resolution.
High-speed footage of the failure.
Setup
It appears you can only use carbon-film resistors for this demo. Here is the type I used. I also tried metal-oxide film and carbon-core types; those would fail, but not spectacularly — they would fizzle or simply short out with no noticeable event.
Acknowledgements
I wouldn't have known about this demo — much less done a systematic study of it — without Kurt Wiesenfeld. And I'd probably have gotten bored and never produced the high-speed videos if not for my partner in crime Balachandra Suri, who ran many of the experiments with me.
Further reading
The carbon-film resistor (resistorguide.com)
A Test Circuit for Long Distance Directional Plasma Discharge Using the Exploding Wire Technique, D. W. R. Smith et al.