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Old 9th February 2005, 11:30 PM   #1
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Default transformer case grounding

Do you ground the outside case of chokes ,fill xformers and OPT
and If so do you only ground one corner or screw. i have seen one design were one end of the fill xformer is isolated with an insulator and the other connects to the chasis. I am about to start my 2a3 project but am trying to learn more about proper layout and grounding before I do.



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Old 10th February 2005, 03:34 AM   #2
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Not grounding the case or core of a transformer may be something which you can get away with, but you certainly don't want to spend three weeks searching for a source of hum to find out it was caused by not grounding the transformer. It's sort of in the "can't hurt - so just do it" basket.
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Old 10th February 2005, 06:08 AM   #3
garbage is offline garbage  Singapore
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food for thought...
what if all 4 corners of the case is grounded unintentionally?

this would cause some ground loops to occur huh?
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Old 10th February 2005, 01:15 PM   #4
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Default No hummmm

Hi,
In my experience I got the least hum when the transformer is connected to ground just at one point and use insulation washers for the other screws.
Much better is to put the tranfo in a shielding box, again do the same one point grounding with the box.
This worked fine for a very sensitive riaa amp with in-box supply.

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Old 10th February 2005, 01:22 PM   #5
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Default interesting

This would explain the design I saw with only one side grounded ,the other isolated
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Old 10th February 2005, 01:51 PM   #6
kmtang is offline kmtang  Canada
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I don't think it is necesary to ground the transformer casing. Some power transformer has a wire for shielding which can be used for grounding purpose. Therefore, I don't see it is needed.


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Old 10th February 2005, 02:51 PM   #7
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Hi thzere.....
When I was building my amp psu, I did a test with the 650VA transformer with shoulds fitted with insulat washers and found 1 volt AC across each of the end shrouds...implying a relatively few turns/volt. This means if the studs were simply bolted in direct contact with each end then a sort of <sloppy eddy current shorted turn> exists via shrouds and to chassis mounting. Okay this isn't directly in the magnetic field path of the transformer but I kept the isolating washers to avoid any poss heating effect and isolated one end shroud from the chassis.
However in saying this, there must have been loads of amps mass-produced in 50's generation that had no isolated screws. A well designed transformer should be fitted with isolated screws to avoid eddy current heating. Daresay with Low VA transformers the situation ain't so important except in preamps.

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