The one downside is the potential for a large switch-on surge, because the core material remembers the residual magnetism at the last switch- off, causing it to initially saturate if it's not switched on at a similar point in the mains cycle. Larger toroidals sometimes have a soft-start circuit. I have an old one which used to run many halogen lights, and I wired a NTC thermistor in series with it to prevent it dimming the other lights when it switched on. It was not unnkown for toroidal halogen transformers to trip MCBs at switch-on.
Yes. I still play vinyl on occasions, and a pukka wall wart transformer for a peripheral on the sound system was inducing hum on the pickup. Despite being a couple of feet away. Changing to a toroidal I had lying around sorted it.
Part of that problem will be compounded by the extremely low cold resistance of the halogen lamps. I'm not going to argue that that's the only reason (see [1]) but the tighter coupling in a toroidal transformer will reflect this almost short circuit load back to the mains primary circuit more effectively than the classic open frame type.
I suspect that if you were to disconnect the lamp load and repeat the turn on tests, the dimming effect will be less obvious (still present but much briefer).
I realise that even 'soft iron' will retain *some* residual magnetism but I wouldn't have expected it to be significant enough to cause switch on saturation effects even in a core without the inevitable air gaps that arise in the classic E&I or T&U stampings.
I've often wondered why there should even be such a thing as 'current inrush'[1] on an 'inductor' which, theoretically, draws zero current at switch on, unlike a capacitor which has the opposite property.
[1] I've noticed this 'puzzling' effect on larger (500VA) open frame transformers in the absence of any secondary loading but was never able to figure out a reason for it. I hadn't realised that magnetic materials, chosen specifically for their non permanent magnetisable properties, could still exhibit enough of this undesirable phenomena in an 'iron cored' inductor or transformer application to cause such a profound effect. It's something else for me to keep in mind if I ever have any dealings with sizeable toroidal transformers in the future.
That's a description for a regular transformer. A toroid is significantly worse for a couple of reasons. The steel used to make the core holds more residual magnetism after power- off, and when you power on again, you can add to this and attempt to drive the core well over saturation, so the transformer will have no inductance, just the winding resistance for first 1/2 - 1 cycle. Secondly, the winding resistance is usually lower with a toroidal transformer because the larger space for windings means thicker wire can be used to make the transformer more efficient. This double whammy gets you a much higher inrush current in worst case of applying power at opposite peak voltage where it was last turned off.
An NTC thermistor is a cheap solution, although it won't work if you turn the transformer off for only a short time as it won't have cooled down ready to limit the next inrush. (Also need to allow for the NTC thermistor failing explosively without igniting anything nearby.)
You can design a toroid not to have this problem - you just double the crossectional area of the core so it can handle twice the magnetic flux without saturating. Of course, that more than doubles the cost of the transformer, which is why it's not done.
A work colleague had one fail in a 200W (RMS) PA (guitar) amplifier the week before last with an open circuit primary winding.
After checking the fairly extensive range held by Rapid Electronics Online and finding that they had a few in the range missing the replacement of the same brand stocked by Rapid was obtained on Ebay.
No I mean sewing machine.. Weaving of course too, but that's not a rotary action like a toroid winder or a sewing machine.
The last poster is right, it does twist the two threads together but that's because the shuttle goes round and round rather than back and forth.
In some machines the shuttle is static and the upper tread is hooked around it instead.
But the mechanical problem is the same. How to get a length of wire or thread through a confined space over and over again.
This is the old style way of doing it:
formatting link
Later on this method became more popular#
formatting link
note its still called a 'shuttle hook' and that the upper thread has to pass completely around the bobbin assembly. So the bobbin assembly passes 'through' the loop in the upper thread.
I understand how the later mechanism works, but have never really got my he ad round the earlier one. Having looked a the old shuttle style sewing mach ines long ago I know they don't let go of the spool of thread at any point, which seems at odds with the above gif.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.