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Mounting considerations for a toroidal power transformer

thorpej

thorpej

AK Subscriber
Subscriber
I picked up a 400VA toroidal PT for my current project and after doing a few experiments with it last evening, will almost certainly be using it instead of the silly voltage quadrupler scheme I was using before.

So now the fun part -- how to mount it. Toroidal transformers are "easy" to mount because of the single center bolt... except it's not quite that easy. The turns of a toroidal transformer loop through the donut hole, just like the bolt does, so if the bolt attaches to something conductive, you have the chance of a shorted turn (which can really ruin your day).

The particular transformer I have includes a shielding layer that's brought out on a purple wire to be grounded. That's an instant shorted turn right there if the mounting bolt is not isolated from the chassis.

It gets better... even if I don't connect the shield to ground, I will have a perforated cage over the top of the chassis. That also has the potential to create unwanted currents (although probably not a catastrophic as with the shield). It's only one turn, mind you, so there's probably no shock hazard... but if it doesn't melt the transformer or blow the fuse, it's at the very least going to waste energy as heat.

I think my best bet is going to be to ensure that the mounting bolt is electrically isolated from the chassis ... drill a larger hole, fill it with a rubber grommet, run the bolt through that and use nylon washers on the other side...

Curious to know what other precautions others have taken when using toroidal transformers in their own projects?
 
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A shorted turn would require a complete path through the center of the transformer through which current can pass. The usual way of mounting a toroid is with the bolt and washer-plate, which does not form a shorted turn because there is no complete loop.
 
rothwellaudio said:
A shorted turn would require a complete path through the center of the transformer through which current can pass. The usual way of mounting a toroid is with the bolt and washer-plate, which does not form a shorted turn because there is no complete loop.
Click to expand...

Yup. The killer would be some type of "C" channel mount (or the chassis itself) if the bolt was in contact with both sides of the C or chassis.

As well, I have a toroid with the electrostatic shield wire. That wire I connected to chassis/earth ground; no shorted turn there either.

Here is a picture of the guts of my DIY "universal" power conditioner.

The outputs available are:

1. Filtered mains
2. Filtered, balanced/technical power (60-0-60) from the larger toroid
3. Filtered, reduced voltage (bucked) power from the smaller toroid.

20151222_144656.jpg
 
Last edited:
rothwellaudio said:
A shorted turn would require a complete path through the center of the transformer through which current can pass. The usual way of mounting a toroid is with the bolt and washer-plate, which does not form a shorted turn because there is no complete loop.
Click to expand...

Hm, does the proximity of any metal enclosure that's electrically connected to the mounting bolt factor in? Consider the layout diagram attached -- there will be a perforated steel cage around the chassis, and the toroid will nestle right up to it on two sides. It's fine because the bolt / mounting washer doesn't actually make contact with the cage?

(Sorry for the additional questions -- just want to make sure I have it right.)
 

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  • 6L6GC UL layout - Toroid.jpg
    6L6GC UL layout - Toroid.jpg
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you should have the mounting kit for the transformer . it should include 2 rubber insulators like in the pic .
500va_Hardware.JPG
 
Note that the rubber disks are for protection of the transformer WINDINGS from hard surfaces that could cause the outer plastic wrapping to wear through and short out the windings (don't ask me how I know this, too many repairs on cheap powered speakers!)
Unlike "frame" style transformers, toroidal transformers windings are on the "outside" of the core and are too susceptible to damage. :thumbsdown:

Mark T. :music:
 
Hi Guys,
I plan to use a rubber coated small metal pipe fixed to the bottom of the metal cabinet. The inside hole of the toroidal transformer will snugly fit outside the coated metal pipe. The height of the pipe will be half of the height of the transformer. Will there be any magnetic issues? Kind regards from India.
 
sleepwalker said:
Hi Guys,
I plan to use a rubber coated small metal pipe fixed to the bottom of the metal cabinet. The inside hole of the toroidal transformer will snugly fit outside the coated metal pipe. The height of the pipe will be half of the height of the transformer. Will there be any magnetic issues? Kind regards from India.
Click to expand...

That should be OK -- not that different from the mounting bolt. The key is to make sure there isn't a full loop around the core.
 
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