Hi,
glad to hear you're awake enough to note my caution.
The rewinding of the primary will involve a very long length of enamelled wire. You will have to remove it from the spool to transfer it to the bobbin (or what else you come up with to pass the correct length through the hole in the toroid) then you have to wind the primary without kinking the wire. The modern enamels are quite flexible but kinks and repeated transfer from spool to bobbin to core may crack the insulation. have you been able to calculate how long a length to wind onto the bobbin? Too much and the bobbin may be too big to pass through the hole and too little and you need to joint your primary!
Next you have to consider the voltage between turns. Adjacent turns will only have a few tenths of a volt or so between them but you always have more turns on top of each other at the inside edges of the core. Here the voltage is not between turns but across many turns. This should still not be a problem if you can avoid those cracks in the enamel coinciding with each other.
Now you have wound about 70% (or any number) of turns and you find that you have returned to the start point. Oh dear! the voltage between the latest few turns and the beginning of the winding is going to be about 70% of the mains voltage. A short between turns here will be a disaster and this voltage mismatch will continue to occur all the way around the last 30% of the turns you have to finish the double layering on.
There is a solution, but it consumes more volume. Wind on an inter layer of insulation before beginning the second over layer.
Use 55% overlap in the inslation so that you can GUARANTEE two layers of insulation between windings with a significant potential difference. This also applies to the primary to secondary separation insulation.
Remember the 55% overlap applies at the outside of the core, the overlap at the inside is more likely to be 80% meaning you have about 4 to 5 layers of insulation (even more volume wasted) at the inside of the core.
If you are very clever/lucky you will just finsh the primary winding before you reach the beginning, but theoretically you want this beginning to end gap to be zero or as close as physically possible. The manufacturers will set up their winding machines to just achieve this with a small gap they have found does not reduce performance. The gap can be used to solder and insulate the flexible lead out wires you need to attach.
Are you beginning to appreciate the problem?
glad to hear you're awake enough to note my caution.
The rewinding of the primary will involve a very long length of enamelled wire. You will have to remove it from the spool to transfer it to the bobbin (or what else you come up with to pass the correct length through the hole in the toroid) then you have to wind the primary without kinking the wire. The modern enamels are quite flexible but kinks and repeated transfer from spool to bobbin to core may crack the insulation. have you been able to calculate how long a length to wind onto the bobbin? Too much and the bobbin may be too big to pass through the hole and too little and you need to joint your primary!
Next you have to consider the voltage between turns. Adjacent turns will only have a few tenths of a volt or so between them but you always have more turns on top of each other at the inside edges of the core. Here the voltage is not between turns but across many turns. This should still not be a problem if you can avoid those cracks in the enamel coinciding with each other.
Now you have wound about 70% (or any number) of turns and you find that you have returned to the start point. Oh dear! the voltage between the latest few turns and the beginning of the winding is going to be about 70% of the mains voltage. A short between turns here will be a disaster and this voltage mismatch will continue to occur all the way around the last 30% of the turns you have to finish the double layering on.
There is a solution, but it consumes more volume. Wind on an inter layer of insulation before beginning the second over layer.
Use 55% overlap in the inslation so that you can GUARANTEE two layers of insulation between windings with a significant potential difference. This also applies to the primary to secondary separation insulation.
Remember the 55% overlap applies at the outside of the core, the overlap at the inside is more likely to be 80% meaning you have about 4 to 5 layers of insulation (even more volume wasted) at the inside of the core.
If you are very clever/lucky you will just finsh the primary winding before you reach the beginning, but theoretically you want this beginning to end gap to be zero or as close as physically possible. The manufacturers will set up their winding machines to just achieve this with a small gap they have found does not reduce performance. The gap can be used to solder and insulate the flexible lead out wires you need to attach.
Are you beginning to appreciate the problem?
To be fair it's a pain in the *** rewinding any transformer, primary or secondary.... especially a toroid where you have to feed the wire through the middle and the spacing is critical to get it around the toroid neatly and without overlaps or huge gaps. If it's a toroid you would be easier and cheaper just getting a replacement custom made.
Hi,
Even if not a toroid , you will need a winding machine with counter.
It is possible to build a counter from calculator with magnet contactors and little magnet rotating. The contactors would short and release "+" and "1" of calculator buttons(would have to solder through a resistor).
A better way is to find a mechanical counter from old tape players.
Once you have build a machine , tranformer winding is much simplified
.
Having no ideas how toroid winding machines look like...
Regards,
Lukas.
Even if not a toroid , you will need a winding machine with counter.
It is possible to build a counter from calculator with magnet contactors and little magnet rotating. The contactors would short and release "+" and "1" of calculator buttons(would have to solder through a resistor).
A better way is to find a mechanical counter from old tape players.
Once you have build a machine , tranformer winding is much simplified
.Having no ideas how toroid winding machines look like...
Regards,
Lukas.
Hi, don't know it was so complicated to rewind a transformer. but should have add to my question that this is not a toroidal transformer, it's an ordinary iron core transformer. I tought that I only have to remove the bad wire, counting the turns and rewind it the same way it was.It is not a very big transformer, this is part of a Denon POA8300 triple mono power supply. There're 3 transformer in this power amplifier and two have no reading on the primary side.
Hi,
if a thermal breaker is fitted it will more likely be under the secondary winding.
If the windings are on separate spools, it could be under the primary insulation or buried in the primary winding.
These thermal fuses are not intended to be easily repairable/replaceable due to the damage to insulation that is likely at the high temperature that causes the thermal to eventually trip.
Another case of throw it away ideology.
if a thermal breaker is fitted it will more likely be under the secondary winding.
If the windings are on separate spools, it could be under the primary insulation or buried in the primary winding.
These thermal fuses are not intended to be easily repairable/replaceable due to the damage to insulation that is likely at the high temperature that causes the thermal to eventually trip.
Another case of throw it away ideology.
Just get a new one. You will damage some of the laminations trying to get them apart, if you manage at all. The whole thing is soaked in varnish so must be solvent dipped to clean it off you you will simply be fighting all the way and ruin it.
And if the secondary has been wound over the primary it's lot of work, you'll probably need a helper if you don't have a winding machine as well.
And if the secondary has been wound over the primary it's lot of work, you'll probably need a helper if you don't have a winding machine as well.
WOW. much more complicated than I thought. Wrote an e-mail to denon to have the cost for two brand new transformer. Will depend on the price, else will salvage the 3 amps. One transformer still good so I have one power amp at 120w. and two board which I could find an old transformer with approx. 45v 0 45v to go with the two remaining boards. As always, thanks for your help.
The machine is mosly clever.
One can build this kind of machine from a bicycle wheel rim.
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