Tr£veling-Plasma Jacob’s Ladder

This popular electrical display saw great fame in ihe old Frankenstein horror movies of the 1930s. A con­tinuous traveling arc of electrical plasma climbs a metal ladder, expanding and eventually evaporating into space (see Figure 19-1). At thal moment, the arc reinitiates at the bottom of the ladder and now con­tinually repeats the cycle. It must be noted that these older displays required dangerous and lethal amounts of electricity to obtain the desired results. This property often discouraged their use in public places.

This project generates a plasma arc over 3 inches long that travels a ladder height of nearly 24 inches. It rapidly recycles and contains an arc power adjust­ment

Not only is this a rewarding and energetic display, any shock hazard is greatly reduced by the use of our patented safety shutdown circuitry. Any contact to the ladder elements results in immediate circuit shut­down, avoiding any shock. Even if the shutdown were disabled, contact would only cause a minor burn as the output energy is at a high frequency.

Most parts are readily available and are listed in Table 19-1 at the end of the chapter. Those that are specialized, including a printed circuit board (PCB), may be obtained through www. amazingl. com.

Expect to spend $50 to $75 to complete the project as shown.

Origin

The Bible tells the story of Jacob’s dream about a ladder that extended from Earth to heaven. Jacob, the son of Isaac, was the father of the founders of the 12 tribes of Israel. Among sailors, however, a Jacob’s ladder is a long rope ladder that is hung over the side of a ship so the harbor pilot can climb aboard.

Basic Description

The power supply for this project forms electric arcs across two diverging stainless steel strips (LAD­DER). The 16-inch-long strips are mounted on insu­lating blocks to eliminate possible leakage. The stainless steel strips are separated by about a quarter – inch at their bases and diverge to a distance of about 3 to 4 inches at their upper ends.

The strips form a gap across the secondary wind­ing of the output transformer. After power is turned on, the air dielectric breaks down due to the almost short-circuit state across the lower end of the gap, and an electric arc is formed. As the arc heats up, thermal convection causes the arc to rise up the V – shaped ladder. As the plasma arc ascends the ladder,

its length increases, thereby increasing the arc’s dynamic resistance and thus increasing power con­sumption and heat. This causes the arc to stretch as it rises, and it extinguishes when it reaches the top of the ladder. When this happens, the transformer out­put momentarily exists in an open circuit state until the breakdown of the air dielectric produces another arc at the base of the ladder and the sequence repeats.

Tr£veling-Plasma Jacob's Ladder

Figure 19-1 Jacobs ladder plasma machine

Circuit Descriptions

We suggest you refer to the circuit description sec­tions in Chapter 8, “Handheld Burning C02 Gas Laser,” as this project is very similiar, with the excep­tion of the added safety shutdown circuit shown in Figure 19-2. This circuit detects any abnormal ground currents that would be produced by accidental con­tact to one of the ladder elements. Current now flows through the resistor divider (R5 and R9) and biases diode (D6) into conduction. A rectified voltage is now developed across capacitor (C8) and trimpoi (R4). The trimpot sets the trip point of the silicon – controlled crowbar switch, shorting out the supply voltage to IC1 and turning off the power circuit.

Rssembly Steps

To begin the project, follow these steps:

1. Study Figures 19-3 and 19-4 showing the power board. Assemble as shown, adding the safety shutdown components shown in Figure 19-2.The added parts are Rl, R4, R5. R9, C8, C12, D6, and SCR and are included on the parts list in this chapter.

2. Assemble the heat sink bracket as shown in Figure 8-8 (Chapter 8).

3. Assemble the main power board shown in Figure 19-3.

4. Finally, assemble everything as shown in Fig­ures 19-4 and 19-5, and read all special notes and options.

Tr£veling-Plasma Jacob's Ladder

5. Verify all the wiring for the solder shorts, the correct components, the polarity of parts where noted, and the general overall quality of the assembly.

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