We will also require us to use very high voltages—well in excess of the 2,000 V that can be obtained from the power supplies we built to power the PMT probe. Fortunately, precise regulation at these voltages is not needed, so a high-voltage power supply that can produce over 100,000 V is easy to build. Figure 39a shows a DC-to-AC inverter that is used to drive the high-voltage multiplier of Figure 39b. In this power supply, a push–pull oscillator drives a TV flyback transformer from an old color TV.† The original primary of the flyback is not used. Instead, new primaries are made by winding two sets of four turns each of insulated #18 wire around the exposed core of the flyback transformer. Feedback for the oscillator is obtained through an additional coil of 4 turns of #24 wire wound around the core. The application of 12 V at the input of the flyback driver should produce 100 to 200 kVDC (depends on the flyback used) at the output of the flyback’s quintupler.
Figure 39 Schematic diagram of high-voltage power supply. (a) A push–pull oscillator drives a TV flyback to produce AC high voltage. (b) A quintupler is built separately and operated while dipped in pure mineral oil. Terminal B is the negative output if C is connected to the flyback’s AC high-voltage terminal and D to ground. (c) The quintupler is shown (with cover removed) connected to an Information Unlimited GRADRTV1 to produce over 50 kVDC from a 15-V input.
Figure 40 We use two different screens for our homemade CRTs. We built one out of the screen of an oscilloscope CRT that we cut in half. The other is an Ace Glass flask into which we poured a suspension of ZnS phosphor. Both tubes can be connected to our apparatus through #25 Ace-Thred connectors.
The output polarity of the so-called Cockroft–Walton multiplier depends on the way in which its diodes are oriented. Since some experiments call for both polarities, we designed the multiplier to yield either positive or negative output. If the high-voltage AC output of the flyback is connected to point A of the voltage multiplier, and point B is connected to ground, then the output at point D will be positive. If however point C receives the high-voltage AC, and point D is connected to ground, then point B will be negative. The multiplier should be built on a piece of clean perforated board suspended by nylon spacers inside a plastic container. Banana connectors may be installed on the plastic container and connected directly to points A, B, C, and D. The plastic container should then be filled with pure mineral oil (can be purchased at a pharmacy) to completely submerge the multiplier circuit assembly, which prevents high-voltage breakdown between components. Please note that this is a dangerous device! It produces high voltages that can cause very painful or lethal electrical shocks. In addition, spark discharges can be produced that can ignite flammable materials or volatile atmospheres.
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