Instructables

The transformer in my Tesla coil fried, how can i prevent this from happening again?

I recently built a tesla coil, and after some demonstrational use in school the transformer burnt out (it wouldn't do anything, and a red light appeared inside). I have already ordered a new one, but i want to prevent this from happening again. Any tips from experienced tesla coilers? thx in advance, all help is appreciated.

                 additional details:
transformer: 6500v, 21 ma. ordered here.
the tesla coil has gone without sparking for a few seconds at a time, however resumed after turning it off and back on.

could this be related to the spark gap length or something similar?


Mr. Apol4 years ago
Most smallish transformers (like those used with neon signs) are current limited in design. This means they cut off when the voltage asked of them exceeds their limit. If your spark gap is too wide, the Tesla circuit tries to fire, but the transformer can't muster the voltage to bridge the too-wide gap. If you continue this way, you can burn through the insulation inside the transformer, or create carbon tracks (short circuit pathways made of burnt insulation) that stop your transformer.

If the transformer cuts out, shut down immediately. When things calm down it should recover and work again. Tighten your gap before starting again. If you run too long without activity at the spark gap, you may ruin the transformer.

I would not spend a lot of money on new transformers. Old style neon sign transformers and oil burner transformers are easily and cheaply found. Try eBay, or similar sites, or grab the phone book and find your nearest neon sign shop. Do not buy a modern transformer with GFI (ground fault interrupt). They will not work in a Tesla circuit. GFI units are small and light. Old fashioned non-GFI units are hefty. Ask the dealer if you are not sure.

Paul
kf7oor1 month ago

Just in response to some other comments... if your say 6000v NST has a secondary center tap to ground then it actually is +6000v and -6000v so across both it is 12000v @ say 30mA (thats what mine is) so for my tesla i have 10:800 ratio so 12000 in yields 160,000v out the top at 1.125mA so yes i can and do touch it and play around BUT this is with no top load!! the top load or toroid is a Capacitor and when emitting sparks can conduct enough current to kill. so as far as safety goes i recommend you calculate your current values with no top load to be less than 10mA if you intend to "play" with it when there is no top load. i would never touch mine with the top load on. also older NSTs are best, the one i use though does have a GFI in it and trips every 15 sec or so but i just turn it off and on again and its fine but be prepared to replace it eventually.

my guess is that you are using a nst that has a gfi I would look for nst that is older and does not have one also you might think of having a safety spark gap that helps the feedback to your nst or nst's if you phase them together you will only increase the ma not the voltage hopefully helpful
pdivakaran1 year ago
Hello there, I have been following your conversation, I want to start building a tesla coil for my science project. Well intend to buy a neon sign transformer(10kV 110V A410FL). is it safe and what are the issues that I may face.
biolethal (author)  pdivakaran1 year ago
according to others, they are current limited (I wouldn't trust that). If it puts out more than 30ma of current, it can easily kill you. yours does.

It looks like a cheaply made and flimsy unit anyways (it's probably an inverter), I would go with something beefier (like an actual transformer).
sidetrackbk4 years ago
Oh, in regards to my comment below... if your capacitor (mmc, saltwater, whatever it is) is too large or too small it will cut out as well, which I'm sure you already know. But if you don't, try pulling out or adding a string or two and see if it helps at all.
sidetrackbk4 years ago
I had a similar problem with mine and I used a little 6000v/30mA NST for my power supply. Initially, I was tuning it outside and it kept cutting out on me. It was a very hot and humid day, which would alter the break down voltage of the air (300V at STP) making it more difficult for the transformer to jump the spark gap, stationary or rotary. I moved mine into the basement and grounded the secondary to an I-Beam, and proceeded with the tuning process. When you're outside the temperature changes along with humidity which makes it very difficult to tune smaller tesla coils running on lower voltages. Give it a try in the basement or in an air-conditioned room and see if it makes a difference. I know everyone says, "You can't run them inside!!!" which is true if you have 7' arcs, but for a smaller coil with smaller arcs, you'll be fine. You'll just have the nice smell of ozone lingering in the room. I hope this helps a little, if not, let me know what it was...
framistan4 years ago
First thing i want to tell you is that transformer is not a TESLA transformer and you may have been fooling with something DEADLY !!   If you calculate the WATTS available from that transformer, you get about 136 watts!!!  just multiply 6500 volts times 0.021 amps.  When i was a small boy, i built a tesla coil and it put out about  30,000 volts... but it was almost ZERO AMPERES... so it would NOT EVEN SHOCK YOU !!!   the spark would slightly BURN your skin though, so it was still something you didnt want to touch.   If you repair or re-purchase another one like that one, you might want to use it to feed voltage INTO a tesla coil.  but the whole thing would hopefully be built into a kind of safe enclosure especially if it is used at a school.   A transformer like that is like what is used to power large commercial neon signs.  

Now... about the burning out question.  Yes,, the spark gap too-close causes an INCREASE in milliamperes used... and that could destroy the transformer.  Moisture or humidity inside the unit also could cause it. High voltage is difficult to control because EVERYTHING becomes a conductor at that voltage... even slight moisture.    When you carry the unit from a cold environment like outside ... into a warm place like a schoolroom... then condensation can build up inside and cause problems. 

Everyone reading this... please be carefull when buying high voltage transformers for use as "tesla" coils.  Just because it puts out HIGH VOLTAGE does not make it a tesla coil !!!  Neon-transformers are not safe to play with!!!   Do a lot of research before exposing yourself or children to dangerous voltages.    See some of the RELATED LINKS at the  rightside of this page for more information.
biolethal (author)  framistan4 years ago
the transformer is not the Tesla coil, it is part of one. the whole Tesla coil is enclosed in acrylic. i do not really understand what you are saying about the Tesla transformer...
the tesla coil i built basically works like this: 110V  go in to the first transformer, and are stepped up to 6500 volts at 21 ma, 60hz. then it charges a capacitor, which discharges through the spark gap when the voltage is high enough, sending the current through the primary coil of the second transformer. this creates a large magnetic field that induces a very high voltage current in the secondary coil, which charges the toroid at the top. end output is around 250000v ac, at about 800khz. this is not a dangerous current, or me, my teacher, and countless other people would be dead right now.

trust me, i built the Tesla coil, and i have done plenty of research. 
the transformer is only part of the Tesla coil, it alone is not the Tesla coil.

i don't think it was moisture, as the transformer seemed to be encased in some kind of resin.

my tesla coil makes about 8-12 in arcs out into the air, less if it arcs to a grounded discharge wand.
Did you make sure of what type of power is running at the school? I think it's older buildings that sometimes run 3 phase electrical systems which cant be used for many modern things. I could be totally wrong though

The problem with the spark lag could be low capacitance. I'm sure a couple laden jars or capacitors would help keep it continuous.
I may just be speaking out of ignorance as I have not built my own tesla coil, but wouldn't the secondary coil (if made large enough and with enough windings of a sufficiently smaller wire) significantly raise the voltage and drop the amperage rendering vaguely harmless?

I agree 6500v is a little weak, but I've seen many people use 10,000v transformers to great success. I also agree with the reasoning of moving the transformer from one sort of environment to another to be the cause of the failure.

I'm not trying to correct in some way, I just want more info on the safety aspect.