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Old 13th January 2012, 11:54 PM   #1
empaist is offline empaist  United States
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Default Linear PS CT transformer question

I have a transformer I bought off a guy from an abandoned kit. It measures 17-0-17 on the CT winding (& 6.3-6.3) but the resistances on the CT seem funny as I get 2.99 Ohms on the 17-17 and 2.23 on either the 17-0 or 0-17 when expecting 1/2 of the 3 Ohm full winding value. All I can think of is that there is an about .75 Ohm resistor in series with the CT.

Has anyone else seen this? Is this difference typical for CT transformers? If I unpeel the wrapping will I find a resistor hidden in the works? TIA

Carl
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Old 14th January 2012, 01:16 AM   #2
! is offline !  United States
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What if you measure resistance with both probes from the same end of one winding? I'm wondering if your meter and/or probe contact is adding some resistance to the measurements.
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Old 14th January 2012, 06:38 AM   #3
empaist is offline empaist  United States
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Quote:
Originally Posted by ! View Post
What if you measure resistance with both probes from the same end of one winding? I'm wondering if your meter and/or probe contact is adding some resistance to the measurements.
Good point, I cal'ed the 3468 last week but didn't recheck the short circuit probe resistance when I made the measurements above, tho I think from last week it was around .1 Ohm. I'll check & report back. If you're right I should see my mysterious extra .75 Ohms on the leads and connects.
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Old 14th January 2012, 03:24 PM   #4
AndrewT is offline AndrewT  Scotland
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Before you can measure anything you need to learn how to use your measuring instruments.
This applies to anything, from a 12" ruler to an MRI machine.
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Old 14th January 2012, 03:40 PM   #5
DF96 is offline DF96  England
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For measuring low resistances you really need the 4-point method. That avoids measuring the contact resistance too.
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Old 14th January 2012, 03:59 PM   #6
AndrewT is offline AndrewT  Scotland
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Is this the Kelvin connection?
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Old 14th January 2012, 04:16 PM   #7
DF96 is offline DF96  England
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Kelvin sensing?
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