Electronics > Beginners

MOT (microwave oven transformer) cooling.

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Ian.M:
Personally if I needed a clean 13.8V supply with 30A peak load currents and a moderate to low duty cycle, I'd float a Lead acid battery.  As RF is involved, using a linear charger (or full cycle line frequency TRIAC controlled on the secondary side) would be advisable.  10A max current from the charger would handle any reasonable TX duty cycle for non-broadcast applications, and at about 350VA with a well insulated extra winding to buck 10% of the line voltage, any MOT should run cool enough without additional heatsinking.

Berni:
Heat tends to be the limiting factor for power in a transformer. You can easily run transformers at 500% rated load, but not for long before it overheats, so it makes such operation reasonable only in cases where the duty cycle is very short.

Most of the heat comes from the copper wingdings so adding heatinsking to the core likely won't do a whole lot. It will certainly make the transformer feel like its running cooler since the core will certainly be cooler to the touch but deep inside the layers of copper coils there will still be a significant amount of heat. It does help, but not a ton. Having a powerful fan blow over the transformer is likely plenty enough cooling for how much heat can propagate out of deep inside the wingdings.

The problem is that its quite hard to tell when a transformer is running comfortably without having a temperature probe deep inside the wingdings. At a certain temperature the insulation on the wire starts to slowly degrade, this can cause a failure after a few years. If its getting hot enough that you can smell it then its really getting too hot and will probably start breaking down after days of such operation. Such breakdowns can be quite spectacular sometimes as they include lots of fire, sparks and smoke as turns short themselves and get hot causing the failure to propagate trough the wingdings until the whole thing is fried and the fuse blows.

When space and efficiency is a big factor its impossible to beat switchmode supplies.

soldar:

--- Quote from: Ian.M on January 04, 2019, 08:34:54 pm --- Personally if I needed a clean 13.8V supply with 30A peak load currents and a moderate to low duty cycle, I'd float a Lead acid battery. 
--- End quote ---
Yup. I used to do that. Here you could get used car batteries for free. They were not good for starting cars but still plenty good for my ham radio rig. Nowadays used lead batteries are so tightly controlled you'd think they were heroin.

james_s:
Lead acid batteries are easy enough to get in the USA but I don't like keeping them in my house. They inevitably produce corrosive vapor, they release flammable hydrogen when charging, they will often leak if knocked over, they can supply vast currents for short periods of time which can cause a fire, they are heavy and bulky. Many of these issues are not too hard to mitigate but it's still not something I really enjoy messing with.

Zero999:

--- Quote from: langwadt on January 04, 2019, 05:10:22 pm ---
--- Quote from: james_s on January 04, 2019, 04:55:12 pm ---It would be silly to try building a SMPS like that. You can get surplus hot-swap server PSUs for not much more than the cost of shipping. These are mostly 12V at 30-60A but many can be tweaked to increase the output voltage slightly.

--- End quote ---

or just get one of these in 12V or 15V (adjustable +/- ~15%) https://www.meanwell.com/Upload/PDF/LRS-350/LRS-350-SPEC.PDF

$36.42 from Digikey

--- End quote ---
Yes, that's what I'd do.

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