# How can I safely determine the output voltage and current of a transformer?

So I am building a Tesla coil with my partner for a senior design project.

We safely ripped out a transformer from a microwave oven. However, after a good amount of research, we were unable to find the maximum output values from the transformer. We know the input values, the standard 120 V and 60 Hz from a US outlet. We need to know the max/peak kV to figure out the capacitance we will need for our primary coil that we need to construct.

Here is what it looks like:

• We safely ripped out a transformer from a microwave oven Sure but the safety ends at the very moment when you connect the 120 V AC input to mains voltage when you want to use this transformer. Microwave transformers are designed to deliver a high voltage at significant current levels. Because that's the power going into the Magnetron tube. If you have to ask the rating of this transformer I seriously advise you to first get more experience with (high voltage) electronics before you continue. Commented May 15, 2019 at 18:52
• To be clear: without knowing exactly what you're doing you should not mess with the transformer from any microwave oven. They're LETHAL when connected to AC mains. Are you an electrical engineer? No, then leave these transformers alone. Even electrical engineers don't mess with these. Commented May 15, 2019 at 18:57
• well we were going to make a faraday cage and enclose the coil inside it before plugging in to the outlet. we can also just make a small-scale tesla coil without having to deal with these big transformers, a student from a previous year did it and we have been using it as a reference but we're not exactly sure how it is getting the job done with only the 120 V input thats connected straight to what looks like the primary winding. Commented May 15, 2019 at 19:24
• Besides being extremely dangerous, IMO this is a poor senior design project, I am surprised that a professor would approve this. There is a lot of "black magic" involved, since the parasitics are difficult to estimate, you can't use always us rigorous engineering math. IMO a senior design project should involve requirements and math/simulations to back-up the design besides successful execution of the design. What are your requirements, lots of cool sparks? Commented May 15, 2019 at 23:04
• Have a friend do the measurements for you. Commented May 16, 2019 at 0:31

No. Do not use a Microwave Oven Transformer (MOT) for a Tesla coil.

It is the wrong impedance. The output voltage is too low (around 2kV), spark gaps will not fire reliably. The output current is too high (500mA), if it bites you, you will probably die.

The correct transformer to use is an old style (iron core) Neon Sign Transformer (NST). The output voltage of 15kV is plenty to make very forgiving spark gaps. The output current in the low tens of mA may well be survivable if it bites you.

• For clarification, so a microwave transformer is not high enough voltage to easily make spark gaps, but at the same time high enough voltage to produce hazardous sparks and high enough power to have lethal levels of current behind those sparks? Commented May 15, 2019 at 19:00
• @Toor There's a lot of technology to Tesla Coils, technology = knowledge of how to make them work. 2kV will jump very short gaps in air. However, making a reliable gap that size with big copper electrodes to dissipate the heat is hard, it's not something experienced coilers attempt, somebody with no experience is doooomed to failure. It is enough to jump through dry clothing, mains will not do that, which is why mains is relatively safe, and MOTs are lethal. Once an arc has started, that 500mA will start fires (1kW!) and is 10x the level needed to stop a heart. Commented May 15, 2019 at 19:05
• Toor, 2 kV with current capability of 500 mA will easily kill you, and not by "sparks". It's volts that jolt, but mils that kill. Commented May 15, 2019 at 19:06
• I can buy those newer NSTs but they have GFCI protection in them making them useless on a Tesla coil, although some have figured out a way to bypass that. iron core NSTs are far too expensive for our budget, at least the ones we've looked at. I can't find ones under \$100 Commented May 15, 2019 at 19:08
• @KrutikShah these are the games people get into when they try to make a MOT-powered TC, once you're up at 4 MOTs, the spark gaps become reasonable. The power level is totally unreasonable. Commented May 15, 2019 at 19:19

6170W1D012G is an LG part number. Primary 120v. HT secondary is 2210v 500 mA. If you have to ask, as has been said, you do not know enough to do this safely.

Consider exciting the high voltage secondary (at 60 Hz) and measuring primary voltage to get an idea of the turns ratio. It should be pretty simple math past that.

I agree with other comments that you might want more experience and/or meaningful oversight from your professors for this project.

• Sure but that does not tell us anything about the power rating of this transformer. Commented May 15, 2019 at 18:54
• What was the power rating of the microwave? You probably can't establish a rating without input from the manufacturer, nameplate information, or testing and knowledge of the construction (insulation temp ratings mostly). Commented May 15, 2019 at 18:55

To answer the general question, to suss out the basic specs of a power transformer drive it backwards. This is how we did it in engineering school with pole transformers.

You apply 120vac to the HV secondary of the transformer, then measure the voltage on the primary. (Note: Be sure to fuse the 120v supply.) This will let you calculate the ratio of primary to secondary voltage. There was also a way to roughly calculate the current capacity while doing this, but it's been nearly 50 years, so I've forgotten that detail.

• Note: This is a microwave oven transformer. It has an HT secondary winding, and also a 3.15v secondary winding for the magnetron cathode heater filament. If attempting a backward measurement, then identifying the right secondary is important, as applying 120v to the 3.15v winding is likely to cause hazardous voltages to appear at the primary, and/or damage the transformer. Commented May 16, 2019 at 9:24

The voltage of a microwave oven transformer & the voltage across the capacitor is about 2.1KV The voltage rating should be printed on the capacitor. But like the other guys said: Don't use it for a Tesla Coil. The voltage is too low & the current too high, and it is too dangerous. Use a neon sign transformer. In case you are wondering why such a perfect square wave across the magnetron: The transformer acts like it has an inductor in series. This happens with anything that behaves like a zener and is fed through an inductor, like a fluorescent light, or from a high leakage transformer, like a neon sign tube. The inductor (or transformer secondary) stores energy, then feeds it out into the next 1/2 cycle. When the energy runs out, the potential drops across to the strike voltage of the tube, load voltage (as in a magnetron), or the applied voltage (whichever is less).

Image source: My own experiments - Peter R. McMahon

• I got the waveforms, using a battery operated CRO with 99 megs of resistance in series. Commented Aug 30, 2020 at 1:25
• I got the voltage rating from an old microwave oven with the voltage rating written on the transformer, and I measrured to open cct voltage of one at about 2.4KV o/c. Commented Aug 30, 2020 at 2:07

To measure the voltage, while operating, I would suggest a string of 9 10MΩ resistors in series with a digital multimeter with a 10MΩ input impedance, or an oscilloscope with a x10 probe. Multiply the reading by 10 for the meter, or 100 for the oscilloscope. NOTE: The x10 probe may not handle 1/10 of the voltage across the magnetron before the filament heats up. Use an extra 9MΩ of resistance instead.

To look at the waveform across the capacitor, where both sides are live, the scope can be set to differential mode like a powerscope, using 2 strings of resistors & both channels, set in A+B mode, with B channel inverted. The trace will be the difference between A & B, with B as the reference (ground or otherwise). To measure the current, you can connect the DMM in series, but make sure it is insulated from ground, & KEEP FINGERS AWAY. There's big bities in a microwave oven power circuit.