Rewinding a toroidal transformer?  - Page 1
Forum section: electronics / repair

author: natalietea102
author_rank: Newbie
timestamp: August 06, 2020, 04:15:51 am
content: My friend handed me off some lighting equipment, and some of which were working fine and some of which immediately blew their fuses. I believe I've narrowed it down to the toroidal transformer inside - I'd disassembled the unit down to just AC -> Fuse -> Switch -> Transformer, and the fuse immediately blew. I compared the resistance between the transformer from the working unit and dead unit, and found similar secondary resistances, but a resistance of 52Ohms on the good unit, and 9 Ohms on the dead unit.I'd love to fit in a new transformer, but unfortunately, these don't appear to be easily available - 27W 115VAC input and 11VAC and 7VAC outputs (although board is marked for 12VAC and 7.5VAC, which corresponds to the transformer's output at maximum main's voltage (125VAC). A custom unit from China would cost $60 each, which is the majority of the unit's worth. Would The following procedure work?Unwind Secondary, then primary.Use calipers to measure gauge of wire, and buy the appropriate wire gaugeMeasure length of primary and secondary wires. Since the lengths are identical, the turns should be identical for the most part. Replace the secondary winding preemptively so don't have to worry about the 12 year old coating dying on it too.It just seems... too simple. I'm a mechanical engineer, not electrical, but is that really all there is to it?EDIT: Thanks y'all, after thinking about my (new) options, I think I'm going to buy a new 7V transformer from antek for $11 and transplant the 11V secondary windings from the old transformer to the new one. Sound like a plan?

author: TheMG
author_rank: Frequent Contributor
timestamp: August 06, 2020, 05:13:21 am
content: It's possible and yes matching the wire gauge and the number of turns will work.However... be prepared for a VERY tedious rewinding process which will undoubtedly take many hours of time, unless you happen to have access to a toroid winding machine. The main challenge with toroids is that you need to pass the entire wire through the core for every turn.Also the primary on most toroidal mains transformers I've encountered is underneath the secondary, meaning you will need to completely unwind the secondary regardless, in order to get at the primary.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: August 06, 2020, 05:48:29 am
content: Quote from: natalietea102 on August 06, 2020, 04:15:51 amYep. When young (and mad) I once added an additional 48 V low amp secondary to a toroid. Takes a while but still works.

author: natalietea102
author_rank: Newbie
timestamp: August 06, 2020, 06:25:17 am
content: Thanks y'all, after thinking about my (new) options, I think I'm going to buy a new 7V transformer from antek for $11 and transplant the 11V secondary windings from the old transformer to the new one. Sound like a plan?

author: Ian.M
author_rank: Super Contributor
timestamp: August 06, 2020, 06:39:11 am
content: Nope. sounds like a disaster. Reusing magnet wire from a toroidal transformer is insane, as you'll inevitably kink it to death.OTOH adding a 4V overwind using new magnet wire to put in series with one of the secondaries of a 2x 7V transformer to bring it up to 11V would be reasonably practical and would need the least hand-winding. You can get the wire gauge from one of the failed transformers.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: August 06, 2020, 07:14:56 am
content: It is quite tedious for sure, but not impossible. Have a look at this:https://youtu.be/Ip0P3p3vHTY

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2020, 08:23:50 am
content: If there's room, then why not buy two transformers?Do you know the power rating of each secondary coil?My advice is either buy the new custom transformer, or count your losses and dump it. Your time could be better spent doing something else. You won't learn much by doing this, other than it's very boring and you'll never do it again.

author: natalietea102
author_rank: Newbie
timestamp: August 06, 2020, 10:07:52 pm
content: Quote from: Ian.M on August 06, 2020, 06:39:11 amI'll get some fresh wire thenIf we do go 2x transformers, wouldn't it be easier to buy a 12V and just unwind it until we get 11V? Also, the secondaries just feed into bridge rectifiers, so I could potentially add a resistor to down it down to 11V.Quote from: Zero999 on August 06, 2020, 08:23:50 amThere is room, but it requires me to drill holes and reconfigure the chassis to make room for everything. I figure doing one boring repair would be preferable to making a bunch of modifications to fit it in.I don't know the power rating of each secondary, but I can say the 11V line is overbuilt. It only connects to a driver for a 10mW laser diode.

author: shakalnokturn
author_rank: Super Contributor
timestamp: August 07, 2020, 10:55:21 pm
content: Wouldn't being a mechanical engineer mean building the toroid winding machine first?It all depends how involved you want to get with a repair...

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2020, 09:34:22 am
content: Quote from: natalietea102 on August 06, 2020, 10:07:52 pmI think the modifications for two transformers, would be easier, than rewinding a toroidal transformer.QuotePresumably it's rectified to DC. How much current does the laser use?

author: natalietea102
author_rank: Newbie
timestamp: August 11, 2020, 04:24:24 am
content: Quote from: Zero999 on August 08, 2020, 09:34:22 amI made a mistake, here are the proper specs from my testing:The 12VAC (1.7A inrush, 1.5A continuous) line feeds a directly into a RS307 3A bridge rectifier, which powers the control board, steppers, and integrated laser diode. I noticed a unused power out terminal right after the bridge rectifier.The 7.5VAC (0.2A inrush / 0.43A continuous) line feeds directly into a RS307 3A bridge rectifier, which powers a separate laser driver. There is a LM317 linear regulator down the line - the 7.5VAC line would vary between 6.5VAC-7.5VAC, depending on the mains voltage. I'm not sure why they decided to use a whole separate 7.5VAC line - but I presume it's to reduce heat on the voltage regulator.It seems the easiest (and possibly cheapest) way may be to unwind a 12V transformer into a 11V transformer, then tap the power out terminal to feed an prebuilt LM2596 DC-DC buck module from China, which goes plugs straight into the 11VAC laser driver input. Unlike another transformer, that module is small enough that I wouldn't have to rearrange anything.

author: ceoxrad
author_rank: Regular Contributor
timestamp: August 11, 2020, 06:25:45 am
content: Quote from: feedback.loop on August 06, 2020, 07:14:56 amThe effort this guy puts in is really inspiring, but using electrical tape in a transformer is a big no no! The stuff will melt and ruin your day in the long run.Transformers should be only rewond using either kapton tape or yellow transformer grade tape.