toriods and microwave ovens...

[philba]

Well, I got a $5 microwave oven and tore it apart to find that it didn't have a toroid transformer in it. sigh. The transformer didn't look at all that easy to pull apart, either (I'd have to pull the primary off before the secondary). the good news - I didn't destroy it and it does work so my shop now has a coffee warmer.

Is there anyway to tell what kind of transformer is inside one? At $5/pop, this could add up fast, not to mention what to do with old microwave ovens... Could people that have gotten toroids (sounds like a disease...) from them tell me the make of microwave oven they got theirs out of?

Phil

[Al_The_Man]

I have never seen a Toroidal used in a microwave yet. They are mostly square lamination type. There possibly is always an exception.
Al.

[Mcgyver]

Phil, search around the site - there's some great material on building your own via rewinding microwave transformers - full tutorial courtesy of Bubba. I'm in the middle of it myself with a microwave transformer as Al describes. I'm wondering why you can't get the secondary off - are they buried underneath the primary? here are some shots I took of mine, is yours a different style?

[philba]

I didn't take pix but the pirmary seems to be overlaying the secondary. I'll cruise the garage sales this weekend, maybe I'll get a toroid or one like yours. Have you wrapped your secondary yet? I wonder if the toroids are in older units. The one I got was fairly new.

[Mcgyver]

no, haven't wrapped it yet, haven't had time for the visit to a motor repair shop to buy the magnet wire. I'm expecting that little job to be a PITA

btw, anyone reading this have a guestimate on how much and of what gauge I should buy (apologies for the hijack) or any guesses or rules of thumb on how many primary turns there might be??

[Bubba]

Mcgyver,
I have had good sucess getting my magent wire from either MPJA.com or surplussales.com (much better selection). I usually use #14 for my higher amperage supplies (much less for 5 or 12V). Do a google for wire sizing and there are charts and formulas but usually the recommendation is for 750 to 1000 CM per amp IIRC.

Usually, I find the average V/T is on the order of 1V for most of the transformers that I have diesected.
Hope this helps.

Philba,
I have never seen a toroid in a microwave. All have been square lamination.
Also, the primary and the filiment secondary share the same "hole" in the laminations. I leave that one (usually about 3V) and have used it for either a low voltage supply in some cases or for buck or boost in others.

[Mcgyver]

Originally Posted by Bubba 750 to 1000 CM per amp IIRC. .

Is was thinking 14G as well, but not following the lingo here - what;s CM?

Originally Posted by Bubba Usually, I find the average V/T is on the order of 1V for most of the transformers that I have diesected.

ok, thanks. There are motor repair places within a short drive that'll sell me the magnet wire by the pound, so for convenience that's what I'll do. Based on wanting something under 50 volts, and 1 volt/turn, 60 ft to the lb and a guestimate of 1 foot per turn, I'm figuring a pound will do. Does that match with your experience??

Any hints for the actual act of winding? because you've got to pull the entire length of wire through for every turn, seems like there'd be a lot of wear and tear on it that might damage the varnish....then again I’ve never worked with magnet wire before so don't know how resilient it is.

[Bubba]

CM is Circular Mills and by defination (found on one site) = " A circular mill is the area of a circle with a diameter of one thousandth (0.001) of an inch."

The last transformer I built was for a 50VDC (36VAC) unit and IIRC, I used about 40' +/- of wire. According to mpja.com, a .5lb spool of #14 is approximately 40' long. A lot will depend on how tightly you wind your coil. Your average of 1' per turn is not far off in my experience. If anything it is a little high but again a lot depends on how tightly you pull the turns in.
I typically put the frame in a vice and string the entire length out across the basement. I also use some insulating material or even "black electrical tape" on the outside edges of the transformer laminations to keep from scraping the insulation on the wire. This is very important because broken insulation again relates to "magic smoke escaping" Then I thread it through and try to keep the winds tight. In some cases, I have used a wooden "wedge" to hold the wire while I thread through the next side. (place the wedge snug, but not tight so you don't damage the insulation). As each layer is wound, I spray it with insulating varnish and usually wrap with a layer of glas tape to keep that layer tight. Make sure you have an insulating layer on the inner frame of the transformer steel or you will surely make "magic smoke" when you test it. (Yep, been there done that one too). When you get close to the "calculated" number of turns, test it (carefully) and you will know if you need to add or subtract turns.

[philba]

Thanks guys - it looks like the "toroids in microwave ovens" is an urban legend.

Funny coincidence. I was driving home this morning and on one street there was a big pile of trash awaiting its destiny - guess what was in the pile? When I ripped it open, the square transformer appears to be exactly like mcgyvers. So, I think I'm good.

Bubba, it sounds like a good estimate is 1 ft per volt on the secondary, though the ft/v would increase as the coil gets fatter. Your advice of protecting the insulation is sage - I don't know if I would have thought of that.

Thanks for all your help.

Phil

[vulcom1]

Phil,
To further add to the comments. I have made a few myself for different reasons(spot welder for 1) Besides wrapping the core sides with some tape I also use one of those clamps with the plastic tips for holding the wire between wraps. Looks like a large cloths pin. One wrap per volt is fairly close and when you get that many wraps on it is easy to test by energising the primary and using a ac voltmeter on the secondary. Just be very careful as most of the time it is wide open and easy to get yourself injured or killed.
John

[philba]

Thanks, vulcom1. That's good advice. I'm pretty paranoid about line voltage. I always use a GFCI outlet when experimenting. I also have built a "safety box" (for lack of a better term). It's a standard electrical junction box with a screw terminal barrier strip in it. The power cord comes in through there. I will then make solid connections to the device under test and screw the wires into the barrier strip. Not fool proof but at least the hot wires are well under control. I am thinking of expanding on this idea to include a panic cut-off (big fat push button) switch. I suppose a good addition would be a GFCI right in the safety box so I don't have to seek out a GFCI circuit (though my shop is already wired with them)..

[Old_Man]

Do not forget something , the 2 pieces of magnetic shunt which are placed between primary and secondary , the magnetic shunt limits the magnetic flux which passes from primary to secondary , also limits your secondary amperage. You must remove them.
For 50V AC at 10A you will need a 700-800W tranie which is common on microwave ovens.
Make sure you use the max gauge wire which fits the freeed transformer window, u'll have then a cool transformer running with enough power?