rewinding transformers - HomeBrew Forum at eHam.net

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rewinding transformers	Reply
by VKNOCALL on August 23, 2006	Mail this to a friend!
g'day brewers! Im going to homebrew most of my station .my first project was a little 1.5a variable power supply.works well.next on the agenda is a 5amp or higher 13.8v.i have sourced a few old tv transformers for experimentation.they all look to be 240v primary 110-112v secondary.now the foundation licence in oz allows 10w pep ,im thinking 5a supply now ,larger later after licence upgrade.my q uestion is this;i have some household electical cabling (3 core stranded )i think its current rating is 20 a.does this mean if i rewind the transformer secondaries with it the peak current draw will be 20 amp from the thing or will stranded wire affect transformer performance.i havent found too many good sites on transformer winding so any suggestions will be greatly appreciated.the arrl handbook is some help but i find it very hard to read and understand it at times.ive read it cover to cover once and understand about 50 % of the content,am i stupid or do other begginners have the same opinion.thanks for reading .brendan from australia

RE: rewinding transformers	Reply
by N3OX on August 23, 2006	Mail this to a friend!
The primary has a current rating too. Let's say you've got a 240V to 120V transformer, 0.5A rating on the secondary. The rated maximum power out is 120V*0.5A = 60W... so the power in is slightly more than that, but let's call it 60W as most transformers are pretty efficient. This is a current in the primary of 0.25A If you rewind the secondary to give you 15V that you'd then regulate down to 13.8, then you can likely draw 60W/15V = 4A of current without stressing the primary more than it was in the original configuration. If you've got the iron and the primaries lying abou the shack, though, you might as well throw some turns on there and see what happens. You'll need a way to figure out how many turns are on the primary so you know how many turns you need on the new secondary... 73, Dan

RE: rewinding transformers	Reply
by N3OX on August 23, 2006	Mail this to a friend!
By the way, you can't find transformer rewinding info because no one rewinds their own transformers! You've really got the ham spirit! The only amateur radio power transformer winding project I ever saw was in the ARRL handbook in a big-current homebrew switching supply. Good luck... Dan

RE: rewinding transformers	Reply
by VKNOCALL on August 25, 2006	Mail this to a friend!
thanks for the reply dan.while i was waiting for a reply i went ahead and rewound one.115v on the secondary ,460 turns 460/115 =4 turns per volt.found some heavier enameled wire from an old motor .wound 60 turns on .put the thing back together and what do you know 14.95 v,amazing.total time spent 4hours.now time to rewind the big sucker.

RE: rewinding transformers	Reply
by VKNOCALL on August 25, 2006	Mail this to a friend!
total cost $0 & i have4 others to do.a 5a transformer in oz costs about $30 thats $30 towards 1st rig.I like this home brew idea.always liked to pull things apart. 73 brendan

RE: rewinding transformers	Reply
by WB6BYU on August 26, 2006	Mail this to a friend!
The limitations are the current ratings of the windings (based on the wrire size) and the size of the core. Basically the core size limits the maximum power that the transformer can handle. I don't have the formulas handy but there was a good article on rewinding transformers in QST, probably in the early 1970's (since that was about the time I rewound a filament transformer for my first SSB rig.) As long as you don't try to pull more power from the transformer than the original rating, you should be fine. A big mistake I made was reducing the number of turns on the primary so I could use fewer turns of thick wire for the secondary. NOT a good idea - the transformer overheated and made a real mess.

RE: rewinding transformers	Reply
by VKNOCALL on August 28, 2006	Mail this to a friend!
thanks for the reply champion.the primaries stay put,ok.the 130 watt transformer is the next victim,any tips for getting those first couple of plates out easily aaaaaaaaaaaaaaargh.

RE: rewinding transformers	Reply
by RADIOFREEISRAEL on October 21, 2009	Mail this to a friend!
Greetings, I have worked on or around pretty complex things over the years as a senior engineer with Uncle Sam military avionics, but trying to work on ferroresonant regulators used on older test equipments, stymied me. I need to rewind a Boonton FR regulator to change it from 60 Hz to 50 Hz application. There is not point on trying to re tune the tank circuit by changing capacitors as the frequency difference is huge. The Boonton OEM regulator accepted 90-120 VAC at 60 Hz and provided an output of 127 to 130 VAC, onto a load of about 90 watts. I presume that the core would be at least marginally reusable. Does anyone know of a rewinding facility or has the equipment to do this? All help appreciated Cordially Sam

RE: rewinding transformers	Reply
by K4DPK on October 21, 2009	Mail this to a friend!
This information may be useful. I have wound quite a few transformers over the years, and it has served me well. You can determine the approximate number of turns per volt by Turns/Volt = 10^8 / (222000 FA) F is the line frequency, A is the cross-sectional core area inside the winding, in square inches. For 60 Hz, this is simplified to Turns per Volt = 7.5 / A This works well when starting with an empty core. If you are rewinding an old transformer, and the primary is intact, you can simply wrap a known number of turns around the spool and measure. Then pro-rate for the desired voltage. Of course, the wire size must be adequate for the amperage required. As mentioned earlier, core area is crucial to wattage rating. With old cores, this can be estimated based on what the transformer did in its previous life. Phil C. Sr. k4dpk

RE: rewinding transformers	Reply
by VK2HHS on October 21, 2009	Mail this to a friend!
VKNOCALL, Got into this business some years ago with the same idea as you; build my own station. To that end, I have made some progress, including building my own power supplies. I would recomend this PS without hesitation! http://www.users.on.net/~endsodds/ps20.htm Mine works extremely well, does 20A peak with ease and I've heatsoak tested it for several hours at over 200W output into halogen lights. There's plenty of info in this article on rewinding transformers too. Don't hesitate to contact me offline. de VK2HHS.

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