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Microwave oven, filament transformer source




 
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Author Topic: Microwave oven, filament transformer source  (Read 665 times)
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KB5MD
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« on: March 24, 2018, 11:38:45 AM »

For anyone having trouble finding a filament transformer at the proper voltage for your favorite project,  you may want to consider this; Take a microwave oven transformer and cut away the hv secondary and wind a few turns of heavy
insulated wire in place of the hv winding to get the  proper voltage that  you require. This is relatively easy as the turns ratio in the microwave oven transformer is usually around 1 turn per volt for the secondary, thus the new winding would be small in size compared to the old hv winding (plenty of room for the new wire. I have used this method several times and thought someone else might find it useful.  #12 or #14 vinyl covered copper house wire works great and don't forget the center tap.

I have had one of these running a pair of 813's for a couple of  years with no problem.
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PA0NVD
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« Reply #1 on: March 24, 2018, 02:21:59 PM »

Very good idea.
In addition, the "leak" cores prevent the very high surge current during start-up which enhances the life or the tube. Professional filament transformers have 'leak' cores as well.
I used to modify these transformers to spot welding transformers. a LOT of copper and 2 turns sec without the leak cores. Gives up to 4000 Amps and make nice spot welders up to 1 mm plate iron.
When you only remove the leak cores, they make nice HV transformers as well I did power a linear with two TB2.5-300 tubes
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w4bfs
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« Reply #2 on: March 24, 2018, 04:27:39 PM »

I have serious doubts about these transformers with the primary energized in standby such as a filament transformer ... just measure the unloaded transformer primary current and see how much is required just to magnetize the core .... I tested 4 or so different ones and found that current to be in excess of 4 Amps .... this means you have a very hot running wasteful transformer without some modifications .... to help you get a handle on what is happening, use a varriac on the xfmr and you will see that the current draw increases sharply at about 90 Volts on a 120 Volt primary .... this tells me that the transformer is only used intermittently (which is the case in the microwave oven)

the fix is easy particularly if the magnetic shunts and the hv seconday are removed ... just wind on 30 or so turns if wire of the same guage as the existing primary (1 volt per turn) and put it in phase with it.   now check the magnetizing current at 120 Volts and it should be much lower.   100 mA is reasonable and will cause little heating.

of course, if you like the ham practice of variacs on transformers then just engineer your filament winding to be good at lower primary Voltage say 80 Volts and you are golden ... just dont go above 90 Volts ad nauseaum
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Patrick J. / KD5OEI
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« Reply #3 on: March 24, 2018, 10:56:16 PM »

I second the removal of the shunts. They are too wasteful for filament conversions use. In the microwave relatively low power is wasted in them because of the heavy load on the transformer.

Cut off the secondary and wind what you like!
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