Low Cost Jacob's Ladder Made From a Salvaged Oil Transformer




No Mad Scientist or Halloweener's display is complete without a Jacob's Ladder aka the Traveling Electric Arc. These are easy to build, look really cool, and can be very dangerous if you're not careful. Anything like that is my kind of device!

This unit was made at basically zero cost. The transformer was salvaged from an old oil furnace, the power cord was cut off a discarded appliance, and the wires laying around the shop. Have fun and be safe.

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Step 1: Safety First - Extreme Electric Shock Hazard!

If you're not 100% comfortable working with electricity, either find someone who is, or get more education before attempting this build. The transformer used here puts out 10,000 volts at 23 milliamps. Most stun guns only put out about 3 milliamps. I don't know exactly what this would do to a person, and I never want to find out.

ALWAYS disconnect the power before working on it, or adjusting anything.
DO NOT touch the terminals or rods when the transformer is energized.
DO NOT put anything conductive (including you) close to the terminals or rods when the transformer is energized.
DO NOT do anything stupid.

Transformers of this type do not vibrate, make noise, light up, or let you know they are energized in any way. Just because there is no arc does not mean the unit is safe. Always check to make sure it's unplugged before handling.

Step 2: I've Got the POWER!

The heart of this project is the high voltage power supply. There are two readily available sources for this  neon sign transformers and oil furnace ignition transformers. I use the ignition transformers as they can be had for free or a few dollars. Call your local furnace shop, they may have units that were removed from service that are of no use to them. Transformers that are removed from furnaces can still have enough power for this project. These instructions are based on a furnace transformer. There are other instructables which cover the use of neon transformers, so I won't rehash that info.

Check the data tag to determine the output voltage. You need at least 10,000 volts. More is better. This tag shows that the transformer runs on 120 volts and puts out 10,000 volts, which is sufficient for our project.

Step 3: Power Connections

There should be two wires coming out of the transformer. These are the lines that will be connected to the 120 V line. It doesn't matter which one is hot or neutral. Connections can be made with crimp connectors, solder, or wire nuts.

Step 4: High Voltage Connections

The high voltage terminals are usually a threaded stud or a smooth post. If it's a threaded stud, it will be easier to connect the wire. For the wires, you want something that's stiff enough to stand up on it's own and not flop around, but still be able to bend it into shapes.

TIG welding rod, thin brass rod, or even a wire coat hanger will work for this. Really, any conductive material will work. If using a coat hanger, it can't have any paint or coating on it. A plastic coat hanger won't work for this. If you didn't know that, maybe you shouldn't be playing with electricity.

Step 5:

Bend a small loop in one end of the wire, big enough to fit over the stud. Bend a right angle app. halfway between the terminals. You need a slight gap, the wires can't touch. About a 1/4 Repeat for the opposite side. The distance between the wires at the bottom will be determined by the voltage. Higher voltage allows the wires to be further apart. The wires diverge at a slight angle as they go up. The distance and angles will need to be adjusted to get it to run correctly.

When its running, it looks like one arc running up. In the pictures, you can see multiple arcs because the shutter was open for 1/8 of a second. Since this is alternating current, the arc should be jumping 60 times per second, unless your local power is at 50Hz, in which case the arc will jump 50 times per second.

Step 6:

Stand back and energize the transformer. IF everything is set up properly, an arc will jump across the bottom and travel up the wires. If no arc is present, the bottom is too far apart. If the arc jumps across, but doesn't travel, the wires are too close at the bottom, or the wires do not diverge enough as they go up. ALWAYS disconnect the transformer from the power source (i.e. unplug it) before making any adjustments.

In this example, a piece of electrical tape holds the tops of the wires at a set distance. This isn't necessary, but it keeps the wires from bouncing around. I don't know why, but as it runs, the wires will start moving. Maybe it's some sort of induced magnetism or something.

Step 7: Gee Mr. Wizard, Why Does It Do That?

Well, I'm glad you asked. Electricity follows the path of least resistance, which is why it starts at the bottom where the wires are closest. The arc is very hot, so it heats the air above it, make it more conductive. The arc moves up to that more conductive area, heating the air above that. It does this continuously until it gets to the point where the arc to just too long. At this point, it starts over at the bottom.

If you're going to have this on display, you should build an enclosure to keep people from touching it. I didn't go into how to build an enclosure, you can figure that out yourself and write your own instructable about it.

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84 Discussions


Reply 3 years ago

Your hypothetical output current at 830KV may not kill you (although hooking up 10KV into the next transformers 120V input will burn it up) but your value of 50mA needed to kill a person is probably a little high. A lot less will kill someone!

UL Safety Standards limit accessible current to a half of a milliamp: 0.5mA.

The output of most oil burner ignition transformers (which is what this Instructable is using) will kill you. Don't touch the electrode wires! Make sure that no one can touch the wires!

Be extremely careful with high voltage!


3 years ago

Neat cool Jacob's Ladder, but very dangerous.

I would never operate this without having the wire electrodes inside some clear container to prevent touching them. And still keep the entire unit out of reach. Touching the wire electrodes can easily kill you, or an unsuspecting person or child.

An abundance of caution is required with this type of high voltage.

Also, most oil burner ignition transformers are not made to run continuously, so only operate it for short bursts unless you can verify that the transformer is rated by the manufacturer for continuous operation.


Reply 4 years ago on Introduction

Grab one off an old oil burning furnace that someone's tossing out--with the price of fuel oil these days, everyone is switching to gas. Or call an HVAC company and ask if they have any used ones laying around. Last ditch, head to a scrapyard and see if you can find a gas furnace someone tossed out.


5 years ago

you used Oil based transformer from a furnace correct? Oil based as in minerial oil, or PCB oil. which that oil is extremely dangerous and toxic. but, on to my original question. would an actual transformer from the power company work. something that takes 13kva and steps it down to 120v work? I would just have to switch, and reverse it to get 7,690kv. these small ones I'm mentioning have only one high side bushing. for your primary and 2 bushings at the bottom for 120v , and the neutral.

1 reply

No, this is a transformer from an oil burning furnace.

A transformer works in either direction, so you could step the voltage up or down depending on whether you connect to the primary or secondary circuit. If it takes in 25,000 volts input and puts out 120 volts, connecting 120 volts to the input should yield 25,000 volts to the output.

However, my understanding is that the transformer is going to draw power as soon as you connect it. An electrical engineer can comment on that. If you have a 13kVA transformer, when connected to 120volts, it's going to draw about 108 amps. If it's more than 20% of that, you could end up blowing your circuits.


6 years ago on Introduction

Actually a microwave transformer will work, but it's worse. I made one with a single microwave transformer and it was a nice orange arc but it doesn't stretch very wide. And you can't run it continuously or it will over heat. To get a bigger arc, you can put a bank of 2 parallel microwave capcitors in series with one high voltage output or for bigger arcs you can put two transformers with secondaries in series and 2 banks of 2 parallel capacitors in series. That would definitely kill youo though and you would need either a 30 amp breaker or run it off 240 volts with the transforers' primaries also in series.

1 reply

Reply 6 years ago on Introduction

The oil transformers can heat up as well. I believe they're potted in that tar-like substance to act as a heat sink. I know there's a lot that can be done with microwave transformers, but I have yet to delve into them.


8 years ago on Step 7

insanely good article, i just found out a few days ago that i have quite a few of these sitting next to the furnace in my house. i think i'll wait till spring comes to really play with them. quick question, would it destroy them if i connected them in series I.E. connecting one transformer's HV leads to the primary of another transformer?

2 replies

Reply 8 years ago on Step 7

I don't know if it would destroy it, but the outcome would probably be bad. With a 10,000 volt output and 120volt input, that's a ration of 83:1. If you put 10,000 volts on the primary, it would be trying to put out 830,000 volts. At the same time, the amperage going in is very low, so the outgoing amperage would be reduced by a factor of 83. That's the theoretical part. In the real world, I suspect it either wouldn't have enough amperage to actually make the voltage on the secondary, or if it did, the insulation would start to fail.

I don't know for sure since I've never done it, but I wouldn't try it.


8 years ago on Step 7

i have been wanting to build one of them for years thanks for your input


10 years ago on Introduction

Sweet! im 12 and im wondering where i can get an oil burner transformer and if anyone would even giv me one. And YES I know the dangers of playing with high voltage and i have taken apart many CRT's and other high voltage devices (if disposable cameras count) : p Please reply.

5 replies

The oil furnace ignition transformers typically aren't oil filled. The one's I've seen were all solid. Usually only larger transformers, such as ones used for power transmission, are oil filled. The cancer danger you mention is from PCB (polychlorinated biphenyls) found in those older transformers. The US banned production of PCB in 1979, so MOST transformers won't have this, but when in doubt, take all precautions.



Reply 10 years ago on Introduction

Im also going to build a wooden enclosure around the transformer and a plexiglass enclosure over the electrodes. and would a 15 amp 120v switch work for the input leads?


Reply 10 years ago on Introduction

yes that switch would work for the input. make sure you keep the hv lines away from the wood its quite conductive at 10-15kv.