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Transformer or choke sealed with metal case

CLS

Member
2005-06-17 6:58 am
Taiwan
How are they positioned inside?

In tight mounting, for properly aligning their coils at right angle, I'd like to know how they were put in the cases.

They are in various shapes, some are tall, some are almost cubic, some are cylindrical... Are there any clues by the appearances? Or we'll never know unless tearing them apart?

Or, it doesn't matter how they align since the sheilding of metal cases are good enough for the magnetic stray?
 

kevinkr

Administrator
Paid Member
You could energize the primary and measure the flux leaking out of the case with a hall effect device, a home made coil probe or similar..(Use a sensitive scope or multi-meter) This in some instances will give you enough of a clue to determine the orientation with some probability of being correct, and if it doesn't then it probably doesn't matter. You'll need to read up on magnetics in order to determine the probable orientation of the core based on your readings. (I've unfortunately forgotten and am too busy to refresh at this point.) You could also take some unpotted transformers and see if you can correlate the orientation of the core to the field intensity as a learning tool.

This works if the case is not a highly effective magnetic shield which in some cases it definitely is, and in which case you don't care.

Long ago I used sensitive hall effect probes to map the magnetic field geometry and strength around a custom EI power transformer in order to determine how to orient it for minimum induction into adjoining circuitry and speaker wiring. This worked quite well as I wasn't interested in core orientation per se, (any mounting position/orientation was acceptable) just the potential orientations to minimize coupling.

UTC, Triad, Stancor, Thordarson and others all made very nice potted transformers back in the day. Not sure I totally agree with SY's case comment, the few I have unpotted for example Heathkit, and a few of the aforementioned brands have all occupied just about all of the available internal volume i.e. >90%. Cases were an expensive option for most transformers, and were IMLE reserved for critical aerospace, and military applications as well as premium grade commercial stuff. Small cored stuff tended to be in smaller cases as well. Watch out for 400Hz potted military aviation power transformers though - they are quite small and will not last long on 50/60Hz..
 
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The biggest problem I have with them...

is they don't ventilate well since they are potted. So you probably shouldn't operate them at maximum specification. They usually aren't servicable where open frame transformers are- to an extent. And lastly remember this, the more connections you have (some transformers have terminals instead of leads) the more unreliable they are.
 
is they don't ventilate well since they are potted. So you probably shouldn't operate them at maximum specification. They usually aren't servicable where open frame transformers are- to an extent. And lastly remember this, the more connections you have (some transformers have terminals instead of leads) the more unreliable they are.

Good ones are sometimes potted in thermally conductive potting compound, or often in the old days tar. (UGH) The design in a good quality unit will be uprated in order to deal with the increased thermal resistance of the potting. In 25yrs with the exception of a MAC transformer and a few Heathkits (all tar potted btw) I have never had one from a major maker fail. I actually strongly prefer them, and so do a lot of others which is why they often go for ridiculous money.
 
Not sure I totally agree with SY's case comment, the few I have unpotted for example Heathkit, and a few of the aforementioned brands have all occupied just about all of the available internal volume i.e. >90%.

Try some of the more modern ones, especially the fancy looking ones from China or used in a certain expensive French amplifier... Tim de Paravicini vividly described the unpotted transformers as looking like "drowned rats."
 
Try some of the more modern ones, especially the fancy looking ones from China or used in a certain expensive French amplifier... Tim de Paravicini vividly described the unpotted transformers as looking like "drowned rats."

I don't doubt that at all, most anything modern seems suspect these days, sadly.. :eek: I should have prefaced my comments to mean vintage and certain reputable current brands like James, Tamura, Tango and Bartolucci - there are of course others. My inexpensive HK sourced OPTs and PTs were quite good as well, but we're talking 10+ yrs ago..
 

CLS

Member
2005-06-17 6:58 am
Taiwan
Thanks for all your inputs:)

I have an idea. How about the weight? Sealing compound is obvious lighter than metals (less density). So, for a particular size, heavier one should be better, I suppose.

What? Lead shot? NO!

.....

I'm asking this partially because I plan to redesign the layout of my current amp. I use 4 potted chokes on it - a pair of Chicago and another pair of UTC. Both are heavy in their sizes:D Kind of old, should be good:D

In this amp, there're other open type EI core chokes used in filament supplies, and very close to the potted chokes mentioned above (used in HT). With limited mounting options, those open EI chokes are just pointing the coils to the potted ones. Except for the metal pots, the only "barrier" between them is 18mm thick plywood. I don't think it's a proper 'barrier'. :eek:

However they're just doing fine. Overall hum is VERY low. I tried different orientations for the open EI chokes but no detectable difference in hum (or other performance). So it seems fine for the ones I have.