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wytco0 16th July 2005 08:15 AM

Encapsulated power Supply Toroids - Are they better?
Having given up on my search for a 42-0-42 Transformer in the UK for my dual UCD400 power amp and settled for a 40-0-40.

RS have a couple of transformers that look OK but I notice that one of these is encapsulated (part 223-8926) I have never used and encapsulated Power supply transformer.

The question is what are their advantages, if any? are they worth the extra cost?

I have tried to search here and elsewhere for more info but without success.

SY 16th July 2005 12:01 PM

There are two main advantages: environmental resistance and looks. The former is generally not an issue with diy home equipment. The latter definitely is, and some people prefer the look of a potted transformer.

Brion55 16th July 2005 02:51 PM

In the case of a power transformer, they can be less noisy. (Mechinical noise - buzz)

Sigurd Ruschkow 17th July 2005 11:39 AM

A potted transformer can also have magnetic shielding buil-in in the potting stuff. Amplimo in the Netherlands made me two very nice toroid transformers that are magneticically and electrically shielded with potting :)


Vidalgo 24th July 2005 01:29 PM

With non-potted traffo you always have an option to add/change windings. Potted traffo is one-time thing, like traffo with welded E-core.

It is true, potting helps against buzzing. But adding to primary 10-20% extra windings kill buzzing at all (and lower idle current, and keep core away from saturation).

AndrewT 24th July 2005 05:04 PM

Hi Vidalgo,
did you say extra primary windings?
How do you do this without removing ALL the secondary windings?
According to an earlier thread extra copper on both windings will increase the VA. Can your experiments confirm this?

Vidalgo 25th July 2005 12:37 PM

Hi AndrewT,
of course, secondaries must be removed to give access to primary. Adding extra winding (for primary) and place it over secondaries - very bad idea, at least safety issues go up.
Why I build traffos. I'm not a manufacturer, and make some custom big transformer, once per year-two, is not big deal.
I use relatively cheap toroids for halogen lamps. Each of have about 13Vac sec., slightly buzzing, relatively high idle current.
But most important is another thing. When traffo is used for power amplifier (assuming voltages is OK) with big capacitors after rectifier, the current drawn from secondary look like narrow pulses. It's easy to observe current waveform with scope, by adding series R 0.1 ohm or so before rectifier and stay with probe on it. High current spikes (with overall moderate average current) brings core to saturate region (depends of core material and other factors). When core is saturated, primary current lost it's sine form and becomes to radiate HF noise to all around. Also, efficiency of saturated transformer drops drasty and heating issues. With pure resistive load all work fine.
To prevent saturation, need to move "operating point" to linear magnetic region. For this, number of primary turns is rised, adding about 10-20% extra turns solve listed problems.
For example, my last rebuilded toroid, 250VA: buzzing lowered to neglible level, idle current drop from 70mA to 6.5mA, no visible distortions in primary current (when works with capacitors 2x10,000uF). Tradeoffs: more weight, handwork, materials etc.
As for power ratings, I don't make measurements, sorry. Better way (to me) is get bigger traffo.

Another reason for me to rebuild traffos, is that the most of "standard" models has center-taped secondary. For poweramp transformer, I need splitted secondaries. But this is another story.

NOTE. Because there are safety issues, playing with primary winding of power transformer IS NOT RECOMMENDED for diyers without appropriate skills.

AndrewT 25th July 2005 06:18 PM

Hi Vidalgo,
thanks for that comprehensive reply.
Sounds like a lot of work but a 90% reduction in idle current and curing those other problems seems to make it worthwhile.

wytco0 30th July 2005 04:08 PM

How to fix Transformer to case?
Thanks to all who advised me on this. I bought the encapsulated version and It looks very good and solid.

Can anyone advise the best way to fix the transformer to the casing I am using. The transformer has a M8 hole but didn't not come with any fixing.

In the past I have reused old cases and managed to fix new transformers using old fixings, but in this case the transformer is much heavier and the case is made of fairly thin aluminium. Is there an standard way to fix it? if so what fixings do I need? I had thought of fixing a more substantial plate to the bottom of the case with some standoffs and them mounting the transformer on that, has anyone tried that?

On a slightly different matter is does anyone know of a web site with information about fitting boards in cases? what sort of standoffs to use, how to route wiring, what sort of connectors? etc.


tekman 1st August 2005 10:46 AM

@ wyttco0:

Use an 8 mm maschine srew to fix the tranni in the center. A support sheet of metall (2 to 4 mm Al - 2mm for a 50 VA, 4 mm for a 300VA tranni) below your thin alumininum case helps to reduce the force on the thin aluminum (more surfece == less pressure..
A layer of rubber 1 mm to 2mm isolates vibration -- put it between tranni and aluminum.

This worked in my projects, so


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