Reworking a microwave oven transformer

[davelectronic]

Hi all
I am looking for ideas from members that might have reworked a microwave oven transformer ( MOT ) my use of one of these units would be for an air cooled 13.80 volts high current regulated power supply.
I have seen some projects reworking these units, i am not interested in high voltage, but want to know if there are any members that have built a psu using a MOT reworked. Dave.

 

[KMoffett]

About MOT's:

As it is intended to drive a capacitive load, the leakage inductance of the tranformer is deliberately increased by adding a small magnetic shunt between the primary and secondary coils. The inductance is roughly equal and opposite to the doubler capacitance, and so reduces the output impedance of the doubler. This specified leakage inductance classifies the transformer as non-ideal.

The transformer is designed to be as cheap to manufacture as possible, with no regard for efficiency. This is because it is the manufacturer who pays for the copper and iron, but the user who pays for the energy consumed. Thus the iron area is minimised which results in the core being taken well into saturation with result high core losses. The copper area is also minimised, resulting in high copper losses. The heat that these generate is handled by forced air cooling, usually by the same fan that is required to cool the magnetron. The core saturation is not part of the non-ideal classification, it is merely as a result of the economics of manufacture.

Not a good choice for your application.

Ken

 

[climatex]

I don't know about american MOTs, so I'll be speaking for our 230 volt variants.

Basically, if you want to get it all prepared, take a huge flathead screwdriver and a hammer, or a drill/chisel/sickle/angle grinder/dynamite and chop off the secondary winding + all of its remains, inside-out. You have to be careful not to hit the primary when doing this, if you make a short between windings, it will turn into a smoke grenade, with some fireworks included.

Most mediocre sized MOTs will give you 0.8-0.9 volts /unloaded/ per turn, those bigger are going for 1 or 1.1V. If I were you, I would try to add as much turns as it is possible to the original primary winding to decrease core saturation and the overall heating a bit. By all means you mustn't punch out magnetic shunts from the MOT, or else the exact opposite will happen, and in even worse scenario.

Despite the primary coil "mod" I told you about, I would use a good fan to dissipate heat, the one from your microwave oven should be sufficient per single MOT (these transformers aren't for long term use, are they) and I'd use a wire for the secondary with a good enamel or insulation that would withstand higher temperatures than standard PVC, if something went wrong.

If you've converted at least two or three MOTs this way with their secondaries in parallel, you would be even able to start a car without its battery /combined with a high current rectifier mounted on a monstrous heatsink, of course./

//One more thing: you can also connect 230v to the secondary and have ~22 volts output on the original primary, but don't expect much: 50-100 VAs max.

 

[davenn]

Hi all
I am looking for ideas from members that might have reworked a microwave oven transformer ( MOT ) my use of one of these units would be for an air cooled 13.80 volts high current regulated power supply.
Dave.

as others have said ... because of their construction they are not a good choice.
For 13.8V hi current ( sounds like you are maybe in to amateur radio etc) linear PSU's (transformer ones) are pretty much history now. There are many excellent choices with switchmode PSU's with easy current capabilities to at least 40Amps.
they are only a tiny fraction of the size and weight!!

cheers
Dave
VK2TDN
HF to 24GHz

 

[davelectronic]

RE microwave oven transformer reworked

Thanks for your feed back and replys, although less power i might try the reverse option, 230 volts into the origanal secondry, and 22 volts or so output from the origanal primary, i am guessing this would run fairly cool, but i will try the high current option as well, the secondry re wound, its a try in and see for me, as i am curious to see, thanks for your feed back and replys. Dave.