Hi, I have recently rewound a microwave oven transformer to make a power supply for doing a quick charge on a 150A deep cycle battery, I replaced the secondary coil with some 8 B&S wire and once it goes through the full bridge rectifier and capacitor to smooth out the ripples, I ended up with 15V (no load) and about 13.5 to 14.7V once hooked up to the battery. The primary was using 2.75A at 240V 50Hz. The capacitor is 6800uF at 200V and the full bridge rectifier is a 150A at 1000V.

It may be a dumb question but how would you guys work out what amps the transformer is capable of safely putting out? Obviously if I do a dead short or put a really heavy load on the secondary coil before it gets connected to the full bridge rectifier and the capacitor it will probably come up with a very large number of amps. However I would then anticipate that the secondary coil would probably heat up to the point of the insulation melting and then shorting out between the coils which is something that I would like to avoid.

So is there a simple way of doing something to measure its safe output? When I connected the 150A battery to it, it started off with 10A going into it for a short time which reduced to about 1.7A within 15 minutes, however the battery was not really discharged to start off so it was just a top up. I was just curious as to what it might be capable of out putting in case I use it as a power source for something else.

 

The maximum current which the secondary will handle can be calculated based on the size of the wire used. There are tables on www to help with that. There will also be a limit imposed by the size of the core and the primary winding. You can estimate that by taking the power of the original microwave and doubling it (estimate of magnetron efficiency) which will give you the maximum power which can be taken from the secondary. Then use the least of the two figures as the maximum.

 

You can estimate that by taking the power of the original microwave and doubling it

Really? I would have expected a value less than the original power rating, on safety grounds, to allow for prolonged use.

 

There are two things to consider that limit the output current:

• The thickness of the secondary winding
• The VA capacity of the transformer.

BTW: Proper engineers don't use the term "Amperage" - it's "current carrying capacity". And "wattage" is power!!

 

Thanks for the replies, if I understand this properly and if I have done my sums correctly, the transformer uses 2.75A at 240V so that equates to 660W so as the step down voltage is 15V and using the same 660W that should give me 44A minus a percentage for losses, so should I assume that 30A-35A would be a safe value, is that correct? Incidently the 8 B&S cable is rated for 50A at a car voltage.

 

Really? I would have expected a value less than the original power rating, on safety grounds, to allow for prolonged use.

I meant the cooking power = magnetron output power.

 

My estimation of a typical MOT with that core size should be around 750va to 1Kva with secondary current dependent on wire gauge.
Max.

 

and capacitor to smooth out the ripples

An output capacitor is not needed when charging a battery since the battery looks like a big capacitor and doesn't care about any current ripple.

 

The idea that there is a considerable energy budget for inefficiency in a Magnetron turned a light bulb on for me. It seems obvious because the Magnetron is so inefficient that it needs a fan to get rid of that wasted energy. Duh!

Anyway, in a transformer manufacturing shop, there was a list of cores and primary windings that went together. Each of those cores with proper primary was rated at x number of watts. The secondary can be any combination of windings that use those watts.

I would look at a 1000 watt MOT and assume I can make any secondary winding that uses up to 1000 watts because 1000 watts is actually delivered to the food. I've done the measurements. The labeled wattage is the watts delivered to the food.
Considering the waste in the Magnetron, 1000 watts is a conservative estimate of the transformer core.

 

ps, the shape of the rectifiers changes the peak to average current ratio and distorts the available power like this:

 

An output capacitor is not needed when charging a battery since the battery looks like a big capacitor and doesn't care about any current ripple.

I put a capacitor in so that I can use this setup as a power source if required plus it boosted the voltage up a bit so I didn't need to add as many windings, anyway if it outputs 30-35A safely it should be able to handle any loads that I put on it. I did install a fan over the transformer to cool it and the rectifier down. Off the top of my head I think that the microwave oven it came out of was rated about 1Kw and the transformer weighed about 8Kg.