This instructable details the construction of a Jacobs' Ladder display.

If you are not familiar with the Jacob' Ladder, it is a display where an electric arc between two electrodes travels upwards until it reaches the top end of the electrodes, where it extinguishes. The arc then starts again at the bottom, repeating this indefinitely.

I wanted my Jacob's Ladder to be enclosed in a nice looking display case, with any dangerous high voltage points kept out of the reach of anyone.

At this point I have to insert a safety disclaimer.

THIS PROJECT USES HIGH VOLTAGE WHICH CAN ELECTROCUTE YOU.
Make sure that power is disconnected when working on or making adjustments to the unit. Unplug the unit from the wall outlet to be safest. Do not be complacent with safety on this project. Be very careful if you are displaying this to small children or anyone who does not apprectiate the danger.

The construction of a Jacob's Ladder is very straightforward, and there are probably many other plans for this kind of display on this site already. I've detailed my construction in the following steps.

Step 1: Materials

Elelectrical Parts List:

*120V power cord. Buy or salvage from an old computer or monitor.

*Receptacle for 120V power cord. I wanted a neat implementation, so I wired the 120V input to a receptacle for the common type of power cords found on computers and monitors.

*Safety Fuse and appropriate fuseholder. I used a common 1 1/4" by 1/4" diameter type fuse and holder. Size the fuse so that it will burn out for currents in excess of the maximum draw from the transformer primary. I used a 5 amp fuse.

*SPST Switch. The switch must be rated to handle more than the maximum current drawn by the primary of the transformer.

*Neon Sign Transformer. The unit I used has a 12 kilovolt secondary output. Transformers will also have a VA rating (volt-amp rating). Divide this number by 120 Volts to get an estimate of the maximum current that will be drawn on the primary side.
The transformer I used is a 360 VA unit, so the maximum current on the 120 volt side will be about 3 Amps.

The best place to find neon sign transformers at a reasonable price is ebay.

*High Voltage Wiring. This is the wiring that leads from the high voltage output terminals of the transformer to the electrodes. I found special high voltage wiring at this web site, Surplus sales of Nebraska:
http://www.surplussales.com/Wire-Cable/HVWire-1.html
The wire I used was the 40kV rated type It is kind of expensive at about $3 per foot, but you only need 2 or 3 feet, so it is worth it. This wire is basically just wire with extra thick insulation.

*Binding Posts . These are used for electrode connection and adjustment. They can be obtained from almost any source that sells electronic components.

*1/8" diameter brass rods for electrodes. The ones I use are for brazing, and can be found where welding supplies are sold. The length will be determined by your specific setup.

Depending on your implementation, some of the items above could be eliminated. For example, you could wire a 120V cord directly to the primary of the transformer without a switch or fuse or removeable socket for the power cord.

Other Parts:

I chose to build a cabinet for my unit. If you choose to construct a cabinet for yours, it will most likely be different from mine, so I won't provide a detailed parts list here. In general, it is constructed from 1/2" and 3/4" plywood for the case, and 1/4" Lexan plastic for the windows. I varnished the wood pieces and added other features like hinged doors and carrying handles.

Step 2: Schematic and Explanation of Operation

The circuit itself is fairly simple. The transformer steps up 120 volts AC to 12kV AC. This voltage is high enough to break down the air and cause an arc to form between the two electrodes where they are closest together.

The arc heats and ionizes the air, which then rises. This hot ionized air is a lower resistance path between the two electrodes, and so the arc continous to follow this path. The arc rises along with the hot ionized air, until it reaches the top of the electrodes where it cannot rise any higher. The arc then extinguishes, and forms again at the bottom where the electrodes are closest together.

Step 3: Construction

Transformer 120 volt Connections

On this transformer, the 120 volt (Primary) connections are all on one side (on the right side of the picture below). There are two terminals that connect to the primary side windings, and the third (ground, green wire) connection connects to the transformer case.

One of the pictures below shows the 120 volt input on the back side of the transformer display case. The receptacle for the power cord, the fuse holder, and the ON/OFF switch are all mounted in that area.

Transformer High Voltage Connections

The neon sign transformer has two high voltage terminals. On the transformer used here, the high voltage terminals are the two large terminals on opposite sides of the transformer. The layout of the terminals may be different on other neon sign transformers.

Use the special high voltage wiring to make the connections between the high voltage transformer terminals and the electrode mounting connections.

Electrode Mounting

The electrodes are connected to binding posts. The binding posts are mounted on a piece of polyethylene angle stock. The high voltage wires from the transformer connect to the binding posts. One of the pictures below shows the connections between the high voltage wires and the binding posts. The binding post have a threaded portion and a nut on the rear side. I used a crimp on ring terminal on each of the high voltage leads to allow connection to the binding post.

Most binding posts have small holes in them for passing wires through. I used those holes to pass the electrodes to pass through. If the holes in the binding post are too small for your electrodes you will need to enlarge them with a drill. The posts are then tightened down to hold the electrodes in position.

Electrodes

The electrodes are made out of 1/8" brass brazing rods. Bend the rods so that they angle toward each other after the binding posts, and then diverge appart after that. It will take some adjustment in order to shape and position the electrodes such that the arc travels all the way to the top.

Display Case

The display case could of course be considered optional. I wanted to display the unit without the risk of anyone accidentally contacting the dangerous parts, so I built the case.

I built it in two sections, one for the transformer portion and another for the electrode portion. The two sections then bolt together into one.

I wanted the inner workings to be clearly visible, so I made hinged Lexan doors on the transformer section. The electrode portion also has a hinged Lexan door so that I can access the electrodes for adjustment.

Step 4: Operation and Adjustment

Again, I will restate the saftety disclaimer:

THIS PROJECT USES HIGH VOLTAGE WHICH CAN ELECTROCUTE YOU.
Make sure that power is disconnected when working on or making adjustments to the unit. Unplug the unit from the wall outlet to be safest. Do not be complacent with safety on this project.

When the electrodes are spaced and angled properly, the arc should form and begin to travel upward when power is applied and the switch is turned on.

You will need to play with the spacing and angle to get the best results. Generally the angle between the two electrodes will not be very sharp, as shown in the earlier pictures.

The output voltage of your transformer will determine how close the electrodes have to be at the closesest point, to get the arc to initiate. If they are too far appart the arc will not form at all or be intermittant.

If the electrodes are angled so that they diverge too much, the arc may extinguish before reaching the top, or it may not travel upward at all.

Again, this is an area where some experimentation is required for best results. REMEMBER TO BE SAFE AND DISCONNECT POWER FOR ALL ADJUSTMENTS!!!

Step 5: Performance

The picture below is a time exposure, where the shutter was left open for several sweeps of the arc from top to bottom, making it look like the whole space between the electrodes is on fire.

Here is a link to a YouTube video of this Jacobs Ladder in Operation:

http://www.youtube.com/watch?v=ZeTTfKUB-3U

I welcome any comments, questions, and feedback.