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astrouk1 28th July 2012 03:29 PM

Ventilating Transformers
I have two transformers, one is 120VA, the other 7VA. I want to encase them in aluminium covers I've made, which give about 4mm clearance around each of the transformers. My plan was to cover the cases in MuMetal shielding.

Is it advisable to drill ventilation holes in the base and top? Will this affect the shielding properties of the MuMetal?

The transformers will not be highly loaded, I'm thinking this will be like an encapsulated transformer. Hopefully the experts can give me advice on this one. I'd rather not drill them unless it's advisable to do so.

Many thanks for any advice.

KatieandDad 28th July 2012 03:39 PM

If they are lightly loaded you should not need the ventillation holes.

Try them. See if they get too warm.

DF96 28th July 2012 04:22 PM

Any working of mu metal can reduce its shielding properties, which in any case is only good for fairly low fields. Put mu metal around the things which need to be protected from the field, not the source of the field. For a power transformer a belly band is probably a better way to reduce any external magnetic field.

astrouk1 28th July 2012 04:28 PM

Both the transformers have a GOSS Band and an Electrostatic shield.

I'm building the power supply, once I finish the Beta 22 unit, I'll see how hot they get at full load.

barrymagrec 29th July 2012 10:06 AM

Like DF96 says. mu metal is the wrong material for power transformer screening, if you really need to, use stalloy or similar materials.

Jay 29th July 2012 12:21 PM


Originally Posted by barrymagrec (
mu metal is the wrong material for power transformer screening..

What is a mu metal? That grey alloy plate? It has been used for ages to screen transformers, as well as line-level amplifiers. In transformers by transformer's manufacturer, and around sensitive amp (not around transformer) by device manufacturer. Rarely I have seen mu-metal used to "screen" transformer.

In RF devices, I only found aluminum block (no mu-metal), may be mixed with silver. Once I found gold for shielding :eek:

barrymagrec 29th July 2012 01:02 PM

Mu metal is a specific alloy containing nickel, iron, copper and chromium. It is specifically designed for low level screening, for instance tape heads and microphone transformers and is very effective. It is little use for power transformers since it easily saturates at higher flux densities and becomes inefective. It also requires heat treatment after any bending or machining which makes it difficult for the DIYer. Since it is a greyish alloy it is not easy to identify visually but it is a fair bet that any screening can round a power transformer is not mu metal.
RF screening is an entirely different problem and skin effect makes the use of copper, aluminium and even silver and gold effective.

jacco vermeulen 29th July 2012 02:28 PM

1 Attachment(s)

Originally Posted by barrymagrec (
it is not easy to identify visually

barrymagrec 29th July 2012 03:12 PM

Exactly - that`s a screening can made of ?????

jacco vermeulen 29th July 2012 03:31 PM

Rolf Gemein has been using Mu covers on toroidal transformers for 25 years.
Difference is that these have 1mm wall thickness and are ready made (require a clamp to hold them to the case)

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