Project mot transformer

Jochen's High Voltage Page

Microwave oven transformersMicrowave ovens contain a very powerful high voltage transformer (MOT = microwave oventransformer), see photo. A typical output voltage is 2kVeff, at around 1000W power. This isequivalent to about 0.5Aeff output current @ 2kVeff output voltage. The short-circuit current iseven higher.

MOT. Transformer from microwave oven. Primarywinding (lower, thick wire) 230V, secondary winding(upper, thin wire) 2kV. The few turns of very thick wirewound on top of the secondary supply about 3V at afew A to the magnetron filament.

However, MOTs are not internally current limited (like OBITs are). And as an arc is pretty mucha short-circuit for the secondary winding, the output current should be externally limited for thepurpose of drawing arcs. This can be done by inserting a resistive or inductive load into theprimary or secondary circuit, see figure. When using a transformer (e.g. another MOT) as aninductive load, the secondary winding of this transformer may be short-circuited to reduce it'sinductive impedance. Without current limiting, chances are good that the mains fuse will blowwhen drawing an arc. With or without current limiting, the secondary will probably overheatwhen arcing takes place over long time.

Figure (a) shows secondary-side current limiting using asecond MOT (with shorted primary) as ballast. Figure (b)shows primary-side current limiting using a heaterelement (the bigger the better).

When using secondary-side current limiting, remember that the limiting element is atfull output voltage! In particular, when using a transformer with a grounded winding asin the figure, the transformer core is also at high voltage and must not be touched.

Jochen's High Voltage Page : Microwave oven transformers http://www.kronjaeger.com/hv/hv/src/mot/index.html

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In spite of not being noticeably current limited, with all MOTs I've tested so far I could draw anarc without the fuse blowing, but the winding heats up within seconds!

When connecting a MOT to the mains (i.e. switching it on), there may be a very high current forsome very short time. This may be enough to blow the fuse. This problem can be avoided byusing a so-called switch-on current limiting circuit, see figure. Such a circuit is used in allmicrowave ovens and should be saved as well when cannibalizing such a device. Afterswitch-on, the resistor limits the current to a sensible value for the short time the relay needs toswitch.

Simple switch-on current limiting circuit. The relay must besuitable for direct control by mains voltage, and must be capableof switching a few Amps. The resistor must be a high power (wire-wound) type capable of at least several Watts.

Correct grounding is important with MOTs. The inner end of the secondary, which is near to thecore, should be connected to the iron core. In many MOTs, this is already the case. The reasonis that the insulation between core and winding is usually insufficient to withstand the fulloutput voltage. Therefore, like OBITs, MOTs cannot be connected in series to increase outputvoltage. Only two MOTs can be used for double output voltage, when the cores are connectedand the primaries anti-parallel, see figure below. However, in principle arbitrarily many MOTsmay be paralleled for higher output current (although usually not more than two can be run onthe same mains outlet).

Circuit with two MOTs giving 4kV output voltage between the twosecondaries. Note that between each of the outputs and ground thereis still only 2kV. Each of the two blocks marked by dashed linessymbolizes one MOT, with one end of it's secondary connected tothe core. Together, the act like one transformer with a center-tappedsecondary.

MOTs are very dangerous due to their high output current. Touching the high voltageend will probably result in death, at least in very severe burns. Although the voltage isnot very high, it is high enough to jump an air gap and thus makes the "one hand in thepocket" rule useless.

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Project mot transformer