source of high power transformers

Altaic

Member
2002-06-02 11:47 pm
I'm making a bunch of ESD 3A for a home entertainment center, and when I bought all the parts, I realized transformers are ridiculously expensive. Then it occurred to me that I had read about laser diyers scrounging for transformers. An excelent resource for lasers is a widely mirrored site called "Sam's Laser FAQ". Check out www.laserfaq.com.

Back to scrounging for transformers-- a dangerous, but good way of getting a free 500W or so transformer is to rip it off a junked microwave. I'd guess the quality might not be as good, but one can use filter caps and such. Has anyone tried this for audio?

I'm going to give it a go. I'll stick it in a hole in my yard to make sure the thing doesn't kill me when I first try it out.

Will
 

Altaic

Member
2002-06-02 11:47 pm
It's got two secondary windings.. one is ~15kV, and supposedly is also connected to the ferrite core. Quite deadly.. =)

The other winding is much thicker, and I'm not sure the usual voltage. I was thinking of just taking both secondary windings off and winding my own. It's free and just takes a couple hours to get a good .5kVA transformer to my specs.
 
okay, you need some info here.

A mic trannie is usually between 450 and 1500 watts depending on what kind of oven it came out of.

The output on the high voltage winding is not 15kv! It's only 1.5kv
or thereabouts. If it were 15kv the trannie would be potted first of all, and the wire would be thin as hair anyway. Next, that thick winding you see runs the filament for the magnetron, which is a type of radio tueb that generates microwave frequency. The thick winding is generally 3 volts but may be different and can supply up to about 15 amps. Make absolutely sure you carefully discharge the huge capacitor in there before you dare remove wires, the shock will kick you like a horse and hurt for a long time.
Also, don't even think about experimenting with the magnetron tube from the oven, if it can put out enough energy to cook food, think what it can do to you! It will make a nice display item or you may find a repair shop that wants it but who knows.

What you need to do is rewind the trannie yourself and use a turn per volt. Most of these have 110 or so turns or 220 depending on primary voltage and you should also note that the filament winding mentioned earlier will only have about 3 turns.

For project 3A, make enough turns for one rail, then make a center tap, then make the other half of the winding. DO NOT clip the winding at the center tap, just sand some shellac off of it and tightly twist the center tap lead around it, then solder carefully. Another trick is to make a small loop where you want the center tap to be and continue winding to the end. Then you have this loop of wire sticking out to connect to and you dont get a nasty lump under the windings cause of the center tap joint.

I hope this helps you, also, you don't need to dig a pit to run the trannie, just sit it on a board or table and connect an appropriate extension cord and test it from a safe distance, 1.5kv can't jump far, but use common sense since it is deadly if you touch it...

Your best bet is just to make the new winding and then test the revised transformer, then you have no need whatsoever to worry about high voltage and you can see if you wound it right.
 
VA means volts times amps which means watts basically.
A 1kW mic trannie has a special core in it that limits it's output power to a specific level. This is because the magnetron is basically a big diode tube that runs forward biased and will draw all the current it can get. The transformer limits the power to 1kw so the magnetron only outputs 1kw and doesn't blow up. In your case, you'll get 1kVA out of it if it was originally rated for 1000w output. This should be enough for two amplifiers easily. So, calculate the gauge of wire you need to get however many VA you need for that voltage. For example, if you have 25-0-25 volts, and you need 150VA, then you use ohm's law and calculate how much current will flow. current = watts/volts current = 150/25=6A. So you need a gauge of wire that's good to carry little over 6 amps, 12 or 14 gauge wire will be easily sufficient for this. I hope this helps.... Don't forget that for project 3A you need 35+/- DC, this means you need about 25-0-25 transformer to get the right output voltage.
 

Altaic

Member
2002-06-02 11:47 pm
I was wondering how good the transformer is when compared to a nice toroid or something. I don't know much about transformers, so I'm not sure if the microwave transformer is less efficient, or if it just has more noise. BTW, Duo, have you ever used a microwave transformer for audio? If you have, I was wondering if the noise level is acceptable.

For instance I could spend $20 on a nice ferrite toroid, and wind the primary and secondary myself. If the microwave transformer only has 115 windings, then I could do that myself on a toroid, which would be better. But I've heard I should have upwards of 400 windings on the primary, which the microwave transformer doesn't satisfy. I don't want to spend days winding a trannie =)
 
Actually, less windings means less noise generally, but less efficient. I'm not actually perfectly sure if the trannie has 110 windings, but I figured that since I got 1 volt per winding on my secondary. Though, transformers may vary but I doubt it. I don't see how the microwave transformer would cause noise problems, not many transformers introduce noise, in fact, they usually block it better the bigger they are. In the case of audio, I'm sure it'd work fine, only thing is, make sure you don't have an amplifier that'll draw more power than the transformer is rated to limit, if this happens, the core becomes saturated and the limiting design of the core kicks in and limits current very well. For example, if you had ESP 3A on a 1kw trannie, you could most likely run 4 or 5 amps at once and not have a problem, but I wouldn't reccomend it. Two or four amps on there would be quite good though.

Basically, I would try the mic trannie before the toroid and see how you like it, cause it's probably just fine.