Here I explore the noble art of making small change with my early coinshrinking efforts.
“Continue reading” for more details and photos.
This is an account of my attempts to shrink a coin using my small 1.5 kJ capacitor bank in the same way a can is shrunk around the middle. I am not sure if this was feasible. The power is certainly much lower than is conventionally used by the pro’s such as Bert Hickman who use 100 kA from Maxwell pulse caps with up to 6 kJ and the best coins.
Things I have learnt or surmise.
The greatest field is right adjacent to the coil. Minimise it and don’t allow the coil to move away.
The force shrinking the coil is the same one acting to push the coil away. Think of a hammer blow as a fair analogy of the forces involved.
Use the most conductive coin with softest metal. A gold doubloon perhaps?
I may have to use a small coin. I know bigger is better..
There need to be adequate insulation as there is 5 kV across the coil. It will very readily short out.
Version 1.
To start I have used a tiny Malaysian 1 sen copper coloured coin as shown in the heading photo. This choice was for availability, size and apparent copper alloy composition. I started with 8 turns of single stranded PVC insulated wire that was too thin.
I covered the taped up coil with a towel and fired. The wire exploded into small pieces and most of it went through the towel. There was no copper left near the coil. When the smoke cleared the coin was unchanged. Unfortunately I didn’t save the current waveform but it was around 10 kA and more heavily damped then the can crushing coil. In fact it was probably truncated as the coil disintegrated.
The top photo of this post shows the pieces that I could find of this first coil.
Version 2
This was heavier with a wood block for support. Still no coin shrinkage. I was subsequently informed that this particular coin is magnetic and indeed it is. Possibly with nickel content and generally higher resistance. The magnetism gives an effect opposite to the induced current and resists the effect of coin shrinking.
Despite heavier wire and a wooden surround it still blew the wood apart and broke the wire in one part (right photo).
This is the oscilloscope shot which shows the current reading which is different to the can crushing one. Current scale is 4250 A/div and timebase 20 us/div. Peak current around 13 kA. The surprising thing is that the first current reversal is GREATER than the initial pulse. This seems most likely due to inductance (L) reduction by a degree of interturn shorting. Note that the magnetic forces act to compress axially (lengthways) and expand radially (outwards). The consequences of this are that this should NOT recur if I prevent interturn shorting or movement.
Version 3
This used an aluminum disc in lieu of a coin. I setup the whole 5 turn coil and coin in a block of ice (made from relatively non- conductive deionised water).
The block of ice was shattered into small pieces but the coil was still enlarged and burnt out.
The “coin” was shrunk by perhaps 0.5 mm with the blue circle being the tracing before and the red after. So no real success yet. I heard that a Japanese 1 yen coin is suitable for crushing being light and aluminium.
Left photo is of the fibreglass and cloth with inter-turn insulation.
The 4 inch PVC reinforced ice block housing the coil is shown after firing with a few ice shards that have cracked off in the shockwave. No exploding shards of ice just the bang of the spark gap. The ice developed cracks through it. After the shot I melted all the ice (similar to ‘a watched kettle never boils’) to get down to the coil.
Unlike the previous attempts, there was no damage at all to the coil, no expansion and no inter-turn shorting. Almost like it didn’t fire (apart from the cracks in the ice). Incidentally I forgot and used plain tap water rather than deionised water (we use rainwater here for water supply). Unlikely to have made a difference.
Unfortunately there was no shrinkage of my aluminum disc either. This is not a ‘coin shrinker’ yet.
The current/time trace.
Sadly, I now had to take apart the coil that took so much time to make to recover the disc.
So, success in maintaining the integrity of the coil but not in shrinking the coin.
So why didn’t it work? The clue may be in the current time curve above. Current scale is 4250 A/div and timebase 20 us/div as before. Peak current is only 2.5 kA for the 10 turns. Which is much less than the 13 kA for 5 turns. The ringing is more prolonged and slower suggesting that energy is not being extracted well from the resonant circuit. I suppose this means a higher Q.
This suggests to me a couple of possibilities:
1 the longer coil of 10 thick turns is not as effective as the 5 thinner ones in concentrating the field near the coin. The higher Q suggests this.
2 that interturn shorting is increasing the peak current and hence effectiveness of the coin crushing. This means that simply winding magnet wire and expecting it to all explode and have some interturn shorting is important in getting the peak currents, greater than if the coil remained intact and well insulated and not expanded. Paradoxically I perhaps should have been encouraging my wire to short between turns rather than going to lengths to prevent it.
Useful testing. Now on to higher powers using my pulse capacitor bank.
Related pages
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Photo Date: 2005