Theory of Operation of the Quarter Shrinker by Stoneridge Engineering

What is Stoneridge Engineering's Quarter Shrinker & how does it work?

All About Quarter Shrinking

(or "Makin' Small Change" )

Updated 03/30/25

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This page is a fairly technical explanation about how our Quarter Shrinker works.

You can also download a simpler one-page PDF summary.

Theory<br>of Operation

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References

Other Questions? See our Shrunken Coin FAQ

Theory<br>of Operation:

The Quarter Shrinker uses a technique called high-velocity<br>electromagnetic forming.<br>This is sometimes called "magneforming" or magnetic pulse forming, and<br>is a high-energy-rate metal forming process. High-energy rate<br>processes apply a large amount of energy to an object for a very short<br>period of time. The technique was originally<br>developed by the aerospace industry in conjunction with NASA, and was commercialized by Aerovox,<br>Grumman, and Maxwell Technologies (now a subsidiary of General Atomics). EM forming uses pulsed power technology to quickly discharge high-energy discharge capacitors through a coil of wire to generate a brief, but extremely powerful, rapidly-changing magnetic field to re-shape metals inside or near the coil. Although electromagnetic forming works best with metals that have<br>good electrical conductivity (such<br>as copper, silver, or aluminum), it also works to a limited extent<br>with poorer-conducting metals or alloys such as nickel or steel.

In order to<br>shrink coins, we charge up a high voltage capacitor<br>bank consisting of two to four large "energy discharge" capacitors. These capacitors are specially constructed low-inductance, steel-cased capacitors that can each deliver up to 100,000 amperes (100 kA) at up to 12,000 volts. Each capacitor measures<br>30"<br>14" x 8", and weighs about 180 pounds. These<br>robustly-constructed capacitors are rated for over 300,000 discharges at<br>100,000 amperes per shot. A double-pole double-throw high voltage relay<br>is used to connect a variable high voltage AC power source through a 40 kV full-wave bridge rectifier<br>to charge up the capacitor bank. After the bank is charged to the<br>desired voltage, the HV relay disconnects the capacitor bank from the<br>charging supply to prevent possible damage to the rectifiers when the system is fired.

The<br>charged capacitor bank is then quickly discharged into<br>a single-layer ten-turn work coil wound from high-temperature<br>(polyamide-imide double-build 200C) magnet wire. The coil has an inner<br>diameter that is slightly larger than the initial diameter of the coin.<br>The coin is centered within the coil and held in place by a pair<br>of non-conductive polymer cylinders. The cylinders position and hold the<br>coin so that it is subjected to the strongest portion of<br>the coil's magnetic field. The cylinders also prevent the coin from<br>twisting or being prematurely ejected from the coil during the shrinking<br>process. The ends of the work coil are securely bolted to a pair of<br>heavy copper bus bars. A spark gap is the only affordable switch that can hold off the high voltage and then efficiently<br>switch the huge currents used during the coin shrinking process.<br>For many years, we used a custom three-terminal triggerable spark gap called a "trigatron". The<br>trigatron was "fired" by applying a fast rising 50,000 volt pulse to a trigger electrode, which then<br>caused the main gap<br>of the trigatron to fire. However, in order to increase the range of operating<br>voltages and reduce spark gap maintenance, we converted to a solenoid-driven high-current spark gap<br>with 2.5" diameter brass electrodes. When switched, the solenoid drives<br>the movable electrode towards the fixed electrode. Once the gap between<br>electrodes becomes small enough, the air gap breaks down,<br>triggering a highly-conductive arc that connects the capacitor bank to the work coil. The<br>electrodes don't<br>quite<br>make mechanical contact, so welding between the molten areas on the electrodes is prevented. Unlike the earlier<br>trigatron switch, the solenoid-driven spark-gap switch consistently<br>fires, never self-triggers (i.e., no high-energy<br>"surprises"!), and it requires considerably less<br>maintenance.

Once the spark gap fires, current climbs in<br>the<br>work coil at a rate that can approach five billion<br>amperes per second. As the work coil current increases, it creates a rapidly increasing magnetic field inside the work coil.<br>The resonant frequency of the resulting LC circuit (the capacitor bank<br>and the combined inductance of the work coil and the rest of the system) ranges between 7.8 and 10 kilohertz (kHz) depending on the diameter of the coin and the work coil. Through electromagnetic induction ("transformer action"), a<br>huge circulating alternating current is induced within the coin. However, due to skin effect,<br>the induced current in the coin is forced to flow...