Jacob's Ladder help

[guitarguy89]

Alrighty,

This will be my first AC project, until now all my electronic projects have been DC. Before a long thread is begun about the dangers of a Jacob's Ladder you can rest assured. Though my escapades in the world of alternating current is limited, I have some knowledge of it and huge respect for its capabilities. Most importantly those that will kill you. I have a glass box constructed to house my Jacob's Ladder.

I will break down my confusion into two simple questions.

1.) I'm working off a few different designs from the internet and salvaged a transformer from a microwave. The plans I'm working from give a voltage rating of 9kv to 12kv, this one is only 5kv. Will this be enough to get the desired effect?

2.) The transformer I am using has an extra wire aside from the primary and secondary coils. It coils a few times around the core and exits. Is this for an auxiliary part of the microwave and therefore can be ignored in this application? (picture attached to better explain what I mean.)

 

[hevans1944]

The "safest" way to power a Jacob's Ladder is with a neon sign transformer, which is constructed to be current-limited when the secondary is shorted. The transformer from a microwave oven has no such current-limiting because it is designed to deliver a kilowatt or so to the magnetron oscillator. A Jacob's Ladder is essentially a short-circuit across the transformer when the arc is running, so the transformer needs to be current-limited in its output.

Yes, 5 kV is plenty of voltage to produce a Jacob's Ladder arc from a current-limited transformer. Once the arc is struck at the bottom of the "V" wires, it will maintain a conductive plasma in air (essentially short-circuiting the transformer secondary) and rise, because of the difference in air density between the hot plasma and the surrounding cooler air, continuing upward until the separation between the two wires is insufficient to maintain the arc. The arc then extinguishes at the top and is re-established at the bottom of the "V" since the potential of the now-unloaded transformer rises back up to 5 kV. If the arc does not extinguish at the top of the "V" the tips are too close together: spread the "V" apart by bending the wires outward at the base of the "V"

The smaller number of turns on the microwave oven transformer was probably used to provide low-voltage to power the oven electronics. The high-voltage winding usually is tapped near one end of the high-voltage winding to provide low-voltage AC to the filament of the magnetron because the magnetron cathode generally operates at a negative high voltage, the magnetron cavity (anode) being grounded to the chassis and the other end of the high voltage winding.

 

[guitarguy89]

The "safest" way to power a Jacob's Ladder is with a neon sign transformer, which is constructed to be current-limited when the secondary is shorted. The transformer from a microwave oven has no such current-limiting because it is designed to deliver a kilowatt or so to the magnetron oscillator. A Jacob's Ladder is essentially a short-circuit across the transformer when the arc is running, so the transformer needs to be current-limited in its output.

Yes, 5 kV is plenty of voltage to produce a Jacob's Ladder arc from a current-limited transformer. Once the arc is struck at the bottom of the "V" wires, it will maintain a conductive plasma in air (essentially short-circuiting the transformer secondary) and rise, because of the difference in air density between the hot plasma and the surrounding cooler air, continuing upward until the separation between the two wires is insufficient to maintain the arc. The arc then extinguishes at the top and is re-established at the bottom of the "V" since the potential of the now-unloaded transformer rises back up to 5 kV. If the arc does not extinguish at the top of the "V" the tips are too close together: spread the "V" apart by bending the wires outward at the base of the "V"

The smaller number of turns on the microwave oven transformer was probably used to provide low-voltage to power the oven electronics. The high-voltage winding usually is tapped near one end of the high-voltage winding to provide low-voltage AC to the filament of the magnetron because the magnetron cathode generally operates at a negative high voltage, the magnetron cavity (anode) being grounded to the chassis and the other end of the high voltage winding.

Let me start off by saying thank you for taking time to help me with my problem.

Are you saying that this will not work with this transformer? or that it just wont be the safest choice?

I only ask because this video is one of the resources I'm working off of.

 

[Colin Mitchell]

"Are you saying that this will not work with this transformer? or that it just wont be the safest choice?"
You will find 1kW at the arc will blow your project up.

 

[hevans1944]

Well of course it will "work," as the video demonstrated. It just doesn't work very well. Is it "safe"? I would keep my eye and nose tuned for smoke from the transformer.

In the video, there does not appear to be enough voltage in the "open circuit" state to strike the arc. An ordinary automobile high-performance ignition coil, driven with a power transistor relaxation oscillator in a capacitive-discharge ignition circuit, will probably create a higher voltage and a stronger arc, albeit perhaps a little "stringy" because of the limited current and energy storage capability of the ignition coil. It would depend on the coil, the capacitor size that is used to discharge into the primary, and the voltage the capacitor is charged to. Notice that once the microwave transformer arc IS struck, it appears to carry a LOT of current. That's good for appearance but it may also lead to early failure of the transformer. But go ahead and try it out. I would remove that extra winding if you can easily do so; otherwise just insulate the ends with a strip of plastic electrical tape to keep them from shorting together..

If you are disappointed in the poor results demonstrated in the video, look around for a center-tapped 15 to 17 kV neon sign transformer rated at a few hundred milliamperes for the secondary current. The center-tap is wired internally to case ground (to satisfy NEC requirements), and the case should also be connected to the power line "green wire" ground by you when you power it up. The bare copper wires for the Jacob's Ladder (a steel coat hanger works better and won't sag) are attached to each end of the neon sign transformer secondary via a pair of ceramic insulators with protruding bolts on each end of the transformer. Keep these insulators clean. You should use high-tension insulated copper ignition wire (not the kind with a carbon core!) to connect the coat hanger wires to the neon sign transformer. Or set the Jacob's Ladder on top of the transformer and connect the "V" wires directly to the bolts protruding from the ceramic insulators on opposite ends of the neon sign transformer. A 15 kV transformer will form an initial arc with less than one inch of wire separation, and you can extend the "V" to about two or three inches across the top end. Shield it from drafts of room air with a glass or plastic enclosure (which will also keep curious fingers away) and it will be quite impressive... and noisy! Buzzzzsht! Bizzzzsht! Buzzzzsht! Love that sound!

Neon sign transformers are often junked when older commercial establishments with neon signs are razed. I found the two I have on separate visits to a dump near Phoenixville, Pennsylvania, when I was still a teenager many years ago. They also make excellent Tesla coil exciters if you take care to low-pass filter the power line to the transformer: Tesla coils with mechanical spark interrupters generate a LOT of high frequency energy that WILL find its way back into the power lines if you don't suppress it... but that's another project.

Have fun with your microwave power transformer. Thanks for your post! You have brought back many fond memories of my early days working (not always safely!) with high voltage.

Hop