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Old 22nd December 2014, 03:33 AM   #1
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Default determining a transformer's quality

I get plenty of locally made transformers here, both EI and toroidals. How can I distinguish a good one from a bad one, assuming the voltage/current ratings etc are fine. What are the things that I can check (using the usual tools like dmm, lcr meter, scope etc). I basically want to be able to pick a good transformer from a few similarly rated ones.
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Old 22nd December 2014, 04:20 AM   #2
DUG is offline DUG  Canada
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Regulation: Unloaded to loaded voltage output.

Temperature rise unloaded, temperature rise loaded.

Noise (mechanical hum)...should not be too much.

IMHO

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Old 22nd December 2014, 04:24 AM   #3
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Weight can be a fairly simple indicator of quality, relative to the claimed VA rating. You might also want to take a sample apart to examine the quality of the mains insulation on the primary side.
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Old 22nd December 2014, 11:08 AM   #4
DF96 is offline DF96  England
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Quality of wire insulation? This, together with care in winding, can determine how long it lasts before you get a shorted turn.
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Old 22nd December 2014, 11:27 AM   #5
Elvee is offline Elvee  Belgium
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The no-load current is also an important indicator, both for the quality of construction and design margins
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Old 22nd December 2014, 07:13 PM   #6
AndrewT is offline AndrewT  Scotland
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I agree that the no-load current is a good indicator.

Do you have typical values to be expected for "good" transformers of various VA and in EI as well as Toroid types?
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Old 22nd December 2014, 09:59 PM   #7
sbrads is offline sbrads  United Kingdom
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Leakage inductance being high can be a sign of scruffy windings with poor coupling, cracked core (ferrite) or loose laminations (iron). Difficult though to be precise as to what is a high value without a good t/f to compare against; I know good ferrite t/fs with primary inductances of a few mH usually measure leakage in the region of 1uH but haven't tried it with iron mains t/fs with high inductances. To measure leakage, measure the primary inductance then short a secondary to get a much lower reading, that's the leakage.
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Old 26th December 2014, 11:51 AM   #8
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Compare two from the same manufacturer. If they are far apart in specs, throw all away.

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Old 26th December 2014, 11:55 AM   #9
AJT is offline AJT  Philippines
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depending on your priority....

a good regulation traffo means low dc resistances, this does not mean it has low
no load magnetizing current....

so make up your mind choose one that you think best suits your intended application...
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