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Need help fixing a Jacob’s ladder

1375 Views 12 Replies 6 Participants Last post by  .id.
I pulled down the prop from the attic this afternoon. It’s been out of their own use for probably over twenty years. I cleaned off surface rust from the wires and fired it up. It worked, and then it didn’t.

After fiddling around a little bit, it seemed to work OK but then at some point it would arc but not climb up the wires. For a moment there were some Sparks that shouldn’t have been happening so I quickly and unplugged it and after that I got nothing.

I hope I didn’t burn out the transformer. Can someone help me diagnose this? Perhaps I could check the transformer with a multimeter, but I obviously want to maintain safe conditions. I’d be happy to describe the unit or post photos if that would help.

Thanks in advance.
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Welcome to the forum! Please post some pics and more description - that would help a lot. Also if you can expand on '...sparks that shouldn't have been happening...' Where were they coming from ?

Jacobs ladders I have built had been a bit tedious to get working 'just right'. The spacing of the electrodes has to be just right to get the spark to start, climb and blow out at the top. If the initial gap, taper of the electrodes or spacing of the top is off, it can cause issues in operation. We won't even discuss what happens when a slight draft or gust of wind comes along! 20 years in an attic and rusty can also create havoc with all sorts of high voltage terminals, insulation and windings. Sometimes a good cleaning and drying can help. Rust and any sanding/cleaning dust made from it can be conductive at multi-thousands of volts.

I guess the big key is that a standard old school Jacobs ladder may have LETHAL voltages at the terminals. So treat it with respect, make absolutely sure it's unplugged - and double checked to be unplugged - before attempting to work on it. Definitely don't rely on any switches or safety trips.

A quick check with a multimeter would be to look at resistance of the primary and secondary side of the transformer... all completely UNPLUGGED, obviously. But I'd think you'd see 10's of ohms on the primary side and 100's to 1000's of ohms on the secondary side.

Looking forward to pics!
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Wow - neat old prop! Would love to see some video when you get it working.

Those old school transformers are pretty bullet proof. They have built in current limiting and essentially run in a 'short circuit' through a neon tube, anyway. So there shouldn't be much that could go wrong. Even a dead short would be almost 'normal operating conditions'. Sec 6000 and MA 30 means it is a 6,000 volt, 30 milliamp transformer. Fairly common - a little low voltage. 12,000V would be nice and make the arc much easier to strike. But with some careful adjustment, it should work fine.

Your multimeter sounds like it was set to "2000K" ohms - or 2 megaohms full scale. You read "10" which should be 10K or 10,000 ohms. That seems a bit high, suggesting you might have a burned wire and actually measuring resistance through a carbon burn track or something. For reference, I grabbed a 9,000V transformer off the shelf and the secondary side was 780 ohms.

Typically these things are potted solid with tar in a metal box. Though a do have a few where the top opens and there was a small re-settable fuse inside. So you might look for any type of lid or screw-on panel on the transformer and see if there is anything accessible. Doubtful, but there might even be a reset somewhere on the case. The down side is that any fuse would be on the low volt side... 6000 volt fuses are big and expensive!... so even a blown/tripped fuse wouldn't explain the high ohm reading.

Some things to consider are where, exactly you took the ohm measurement. It's likely the circuit is open even closer to the transformer side than that. Also you mentioned sparks earlier - what happened there?

Also looking at your photo - not sure it is a trick of light or what exactly is going on, but there appears to be a gap between one of the terminals and one of the jacobs ladder legs. Not exactly sure how those connections are made, but might not hurt to double check them. Ultimately, the two wires of the ladder should show the same resistance as the leads of the transformer which would confirm everything is hooked up solid.

Let us know what the additional searching turns up!
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A pro at a shop should be able to take a look at it and test it out.

"Won't climb" typically means the wires spread apart too far, at too large of an angle. The main thing which powers the arc up is that the air above it is just a tiny bit warmer and ionized, so the arc wants to move slightly upward...this heats more air above and lets it rise a bit more. If the 'ladder' wires are at too much of an angle the extra distance creates more resistance than the warm/ionized air removed.

Actually the photos above look like the ladder is pretty wide - wider than I would have expected for 6KV. So if you get it working and it doesn't climb, you might bend the wires to be closer together. Though on the positive side, it looks like the box also has a slot for a sheet of glass or acrylic on the front. So that might be something to consider installing / reinstalling as well. It will stop any outside drafts from disrupting the ladder and also should help concentrate some ozone inside the box which will be good for arc length. You'd probably notice the ladder start working better after it 'warms up'.

For a few other issues:

"Won't strike / won't start" - this typically means the wires are just too far apart at the base. For 6KV, I wouldn't expect that to jump more than about 1/4 inch (or less) to get started. This can be a bit tricky to set up because if you tie or crimp the wire on, then the ties/bundles might be fatter than the ladder wires and the arc can jump between them instead of the ladder wires.

"Won't blow out / hangs at the top" this typically means the wires aren't far enough apart at the top. This can sometimes be remedied by adding a little extra curve, but I always think it looks neat to see the arc slip right off the top.

"Blows out half way up" - you could just be running out of power/voltage in the transformer. At some point, you just don't have enough volts to sustain the arc over the gap. Can usually be remedied by making the angles of the ladder steeper so they separate more slowly toward the top.

Let us know how it goes. Looks like there is an exact replacement for that transformer up for sale on ebay right now. Though if you have the option, more volts and the same/more amps would be better!
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