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eHam Forums => Amplifiers => Topic started by: W8IMO on November 15, 2006, 08:28:04 AM



Title: Plate transformer possibilities
Post by: W8IMO on November 15, 2006, 08:28:04 AM
Sunday my old, at least 16 years old, microwave bit the dust.  It was a big 1KW oven.  There were no sparks, no arcs, no thunder, no lightning, and no smoke.

Assuming the transformer is still good, has anyone ever used a microwave oven transformer to supply B+ in an amplifier?

Bob, W8IMO


Title: Plate transformer possibilities
Post by: HA5RXZ on November 15, 2006, 12:24:59 PM
Yes, old microwave transformers can be used as plate transformers but there is one problem you need to work round. Most microwaves have one side of the plate transformer secondary grounded. Previous attempts to get round this issue I have seen involved using two identical transformers with the primaries connected in antiphase, two HV rectifiers could then be used on the secondary for full wave rectification.

HA5RXZ


Title: Plate transformer possibilities
Post by: WA9SVD on November 27, 2006, 06:59:32 PM
While it IS possible, you will have to be careful, and have a HV probe to even check out the transformer voltage.  In the higher wattage ovens, the voltage can be well over 2500 Volts; in some, as much as 3KV!  You normally can't test that with your regular multimeter.
    Also be aware, that the transformers are not designed for good regulation; that's just not needed in an oven.    And again, one side of the secondary is usually connected to the frame of the transformer, often in an inaccessible place.  It usually isn't practical, as tempting as it may be.


Title: Plate transformer possibilities
Post by: RK3BU on December 11, 2006, 03:41:17 PM
The only bad thing with those transformers from microwave furneces is BIG idle curent and comparetivly
big power with small sizes.It cold be used with this big power/smal size BUT only for short period of time.It overheats!!The primery coil has small amount of turns-and a big idle curent.To decreese idle curent we need to increese amount of turns at primary....AND WE LOOSE a POWER!!


Title: RE: Plate transformer possibilities
Post by: KJ6HYC on June 12, 2011, 04:41:32 AM
The only bad thing with those transformers from microwave furneces is BIG idle curent and comparetivly
big power with small sizes.It cold be used with this big power/smal size BUT only for short period of time.It overheats!!The primery coil has small amount of turns-and a big idle curent.To decreese idle curent we need to increese amount of turns at primary....AND WE LOOSE a POWER!!

Couldn't  the idle current be addresses by doing what the microwave designers do, use a triac and a opto isolator, run the transformer only when transmitting. The transformer has a grounded secondary, use it in a half wave voltage doubler configeration.


Title: RE: Plate transformer possibilities
Post by: W8JI on June 12, 2011, 06:04:40 AM
The only bad thing with those transformers from microwave furneces is BIG idle curent and comparetivly
big power with small sizes.It cold be used with this big power/smal size BUT only for short period of time.It overheats!!The primery coil has small amount of turns-and a big idle curent.To decreese idle curent we need to increese amount of turns at primary....AND WE LOOSE a POWER!!

Couldn't  the idle current be addresses by doing what the microwave designers do, use a triac and a opto isolator, run the transformer only when transmitting. The transformer has a grounded secondary, use it in a half wave voltage doubler configeration.

People use microwave transformers. Personally, I would not.

First, they are bad for regulation. This is because they have high equivalent secondary resistance. If they get hot, it also means they are lossy. When they are lossy, it means they have bad no load to full load regulation.

Putting them in a full wave doubler makes them worse for regulation and heat, putting them in a half wave doubler would be terrible.

This isn't saying they can't be used. It just all depends on what your standards of acceptance are.