Using multiple toroids - issues

[Mikett]

I remember seeing a warning about how not to use multiple toroids as well as something about what not to use in the centre.
Anyone know of these issues?

[AndrewT]

Hi,
if you intend connecting them in parallel then they must be matched i.e. same type from the same manufacturer.
The centre bolt must not create a shorted turn. Insulate it carefully to ensure the top and bottom of the bolt can NEVER touch the metal case at the same time.

[Mikett]

That's the thing I remember. The "shorted turn" can you elaborate on this a bit? So the rubber riings are not for vibration reasons?

[officeboy]

The rubber rings are there for vibration.
The "shorted turn" comes into play when you create a circuit that wraps around the toroid much like the wires inside it do.

From http://www.plitron.com/pages/technote.htm
Shorted Turn Condition
A completed path by any conductor passing through the center of toroidal transformer, around the outside is a shorted turn (eg: the top of the mounting bolt shorted to the chassis). As with any short circuit, this condition will give rise to high circulating currents, and high heat. The transformer may be damaged beyond repair.

[Mr Evil]

Originally posted by officeboy
The rubber rings are there for vibration...

More importantly, they prevent sharp edges from piercing the insulation.

[Mikett]

I still don't get exactly what the shorted turn is. I am thinking that when mounting a toroid in an enclosure, make sure that the bolt does not contact both the top and bottom.

Is that correct?

[planet10]

Originally posted by Mikett
I still don't get exactly what the shorted turn is. I am thinking that when mounting a toroid in an enclosure, make sure that the bolt does not contact both the top and bottom.

Or any other metal bits on both sides... i use nylon bolts or zap straps just to make sure.

dave

[AndrewT]

Hi,
You are allowed to connect ONE end of the mounting bolt to the chassis. The dish usually formed in the big washer is there to keep the bolt head from projecting above the transformer insulation. This helps by preventing the case from touching the free end of the mounting bolt however the washer is still exposed to contact. The case or other chassis metalwork must NOT touch the washer or bolt head.
You could add a layer of insulation over the top of the washerplate and bolt head to prevent inadvertant shorting of bolt to chassis particularly while dismantling or assembly with power on (dangerous pastime).
A shorted turn is a complete electrical circuit from the bolt bottom to the chassis to the side panels across the top (lid) down through the bolt head back to the bolt bottom. If this circuit is completed then the turns ratio of the transformer becomes about 200 to 1 and although only a small voltage is generated, maybe 1/2 to 1 volt the resistance is very low possibly 1 to 10 milliohms the current could run to KA in parallel to the true secondary winding and will definitely overheat your transformer.

[mrlots2do]

I understand the shorting loop you described. Glade this topic came up before I secured it wrong.

However my toroid only came with one metal washer for securing toroid. What can I 1) use as insulator between washer and toroid and 2) use as insulator between toroid and chassis base.

Phil

[carlosfm]

Originally posted by officeboy
The rubber rings are there for vibration.

The best thing you can use for that purpose is not those thin rubber rings.
I use a mouse pad cut to size, under the trafo.
Mouse pads are cheap.