How To : Determine MOT Output Voltage

Here is a way to be able to figure out the output voltage of a microwave oven transformer. Basically we hook up the mains AC to the HV secondary of the MOT and measure the low voltage coming from the primary of the MOT. Dividing the two voltages gives us a ratio. Multiplying the line voltage by this number will give us the MOT output voltage.

Warning : The output of an MOT is lethal. Do not work on these unless you are experienced with electricity and are able to work safely with high voltage.

[ThenZ]

There is literally a video for everything on YouTube and this is exactly what I needed. I have a project that uses a microwave transformer and I was pretty sure my multi meter wasn’t rated high enough to test out with voltage. Now I know how to test output voltage on a microwave transformer. Thank you.

[Max_Chooch]

Dooood.. You just saved me the cost of a HV probe for my Multi-Meter. Nov I've got funds for an LCR Meter. Thank you

[jonjimihendrix]

Amazingly helpful! Thanks so much for this simple explanation for a simple audience member.

[thetriode]

Clever approach, I usually took a 6V power transformer and hooked it to the primary to get the ratio.

[glenntagaylo]

Getting the ratio and multiplying it by the current input voltage. Brilliant formula! Thanks for this very informative video.

[schaltnetzteil]

You can also do it by connecting a multimeter in uA-Range through a large resistor (at least 10 Megaohms) to the secondary and power the transformer like you would do normally. Then calculate the output voltage.

[felixboian-togyik]

Cool! I have always thought transformers have been interesting, with the number of times the primary voltage gets multiplied. Very nice educational video!

[malcytull]

That was very informative, I never knew you could do that, thank you Bill for posting this.

[shoaibakhtar]

very nicely and comprehensively demonstrated

[polarblue]

Thanks I did wonder about the Step Up step down ratio of many Transformers now I have my theories confirmed from your video thank you very much enjoyed it I will like it 4-ever!🙂

[gth]

That hot MOT looks perfect for your next bug zapper :). Thank you for sharing this tip! Welcome back, and I hope all has been well.

[KEVINNOAD]

Thank You very much... Finally someone with a common sense brain... Great information I was looking for.

[diecksl]

Interesting idea. I always use a variac at 10 % line voltage and then multiply the output of the secondary by 10. But of course, not everyone has a variac so this is a nice alternative.

[francoisguyot]

I noticed that a lot of MOTs are being scavenged to rewire the secondary as DIY spot welder for Lithium battery tabs where the primary still runs at 120 and the secondary only has up to 3 turns of hefty gauge cable to draw the large current needed. Reversing the connection as you did, equates to the same thing except that the output voltage is about twice while the current is halved. You need about 200A for a 15 mil thick tab. People are using 12VDC, 7Ah LIPO batteries rated at 50C and a control PCB that parallels MOSFETs to drive the timing within milliseconds. You got 6VAC and the microwave is rated at 1200W, so the current should be 200A max.. N Channel MOSFET works on DC, but can drive the 6VAC if one end of the coil is used as COMMON source. It will then see it as a 12V DC wave that is smoothed out using a 220uF 25VDC electrolytic capacitor. The Gate can be driven from a 9V DC battery with its negative connected to the COMMON source. Theoretically, this should works and save us the rewiring time and energy to achieve the same goal. By the way, those PCB spot welder controller only cost lees than $20.

[randysmith]

Great information - cleared up a lot of questions - thanks so much for sharing your time and expertise on this matter.

[petersmith]

Bill had the same English teacher as John

[seannye]

Pretty sweet video man... I never really had a good way to measure the voltage of a MOT I like this method thanks for sharing....

[Subgunman]

Open circuit voltage can be a bit higher than that of a secondary under a load. If you have a HV probe used by TV techs you could measure the output voltage directly. Use caution.

[Bodragon]

Hey Bill, what accent is that? It sounds like a cross between Irish and Newfoundland. Am I anywhere near?
Thanks. Awesome video by the way. Often wondered how to determine the high voltage of the secondary winding on these MOTs.

[Plasma.Prince]

Incredibly useful, thank you very much!