Is This Transformer's Primary Winding Shorted? - Page 1
Forum section: electronics / beginners

author: old-jo
author_rank: Contributor
timestamp: April 16, 2021, 12:16:19 pm
content: I bought a used transformer.The resistance of the primary winding is 0.2Ω.The inductance of the primary winding is 9.73mH. (Varies between measurement, but always near 10mH.)When I connected the primary winding to mains with no load on the secondary winding, the house's circuit breaker tripped.Does that mean the primary winding is shorted?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 16, 2021, 12:24:43 pm
content: Yes, it's shorted.

author: old-jo
author_rank: Contributor
timestamp: April 16, 2021, 12:48:51 pm
content: Quote from: RoGeorge on April 16, 2021, 12:24:43 pmThank you. I suspected that. I was going to test for high inrush current using the popular incandescent lamp trick, but I will need to buy the lamp and fixture first.Now that a regular in this forum has confirmed my suspicion, that's money saved.By the way is did you concluded it was shorted because the resistance and inductance is so low?(Edited because one previous poster removed their reply)

author: CaptDon
author_rank: Super Contributor
timestamp: April 16, 2021, 01:34:07 pm
content: Are you sure that is the primary winding?Usually the primary wires are black!!! Also the primary wires often have connections that seem to lead to the winding closest to the core. I swear it looks like you are connected to the secondary.

author: old-jo
author_rank: Contributor
timestamp: April 16, 2021, 02:01:32 pm
content: Quote from: CaptDon on April 16, 2021, 01:34:07 pmThe other winding has ticker wires, color coded black, blue, and red.Black to blue is 48.8Ω, 1.12H.Blue to red is 109.7Ω, 1.16H.Black to red is 156.8Ω, 8.43H.When I connected black and red to mains, the circuit breaker didn't trip.However the transformer quickly got hot, and began to smell, so I disconnected it.That's why I thought the yellow wires (from the first post in this topic) are from the primary winding.

author: wraper
author_rank: Unknown
timestamp: April 16, 2021, 02:14:42 pm
content: Yellow wires is certainly a secondary winding. Wasn't it obvious by resistance and inductance? Are you sure that transformer is rated for 230V, not 120V?

author: old-jo
author_rank: Contributor
timestamp: April 16, 2021, 02:25:49 pm
content: Quote from: wraper on April 16, 2021, 02:14:42 pmNow that two people mentioned it, I am not sure at all.There is this marking on top of the bracket:Code:EI-57A-9110/220VFUSE 115°CNow I am confused.If the yellow wires are from the secondary winding, is it normal for it to have thinner wires?Also, I haven't checked the output voltage in regards to yellow secondary, because of the heating problem I described above.Is the heating normal?Last I checked it stays warm for several minutes after disconnected.

author: perieanuo
author_rank: Frequent Contributor
timestamp: April 16, 2021, 02:41:27 pm
content: i'd guess, yellow isprimarysecondarylooks like the other ones are 110/220V of course, consistent with resistance measurementsyour tester is not ok for measuring inductances, at least not on all firmware variants, also transistor detection is not 100% ok, I have a good BJT detected ok with older fw and as unijunction with newer fw, developer tells my 20 years in electronics and r&s is <null>just take off some isolation tape and trace the most thin enameled wires, of course that's primary coilchinese don't respect any colour codei saw also italian industrial machines with blue as phase, so don't asume nothing

author: wraper
author_rank: Unknown
timestamp: April 16, 2021, 03:03:48 pm
content: Quote from: old-jo on April 16, 2021, 02:25:49 pmThey do not look thicker. I see them crimped into thin ferrules. So wire itself should have quite small cross section. Don't look on outer thickness. Wires on primary side should have thicker insulation obviously.QuoteDepends how hot. Small Chinese transformers often run quite hot even without any load. If it for example runs 50oC without any load, most likely this as intended.

author: old-jo
author_rank: Contributor
timestamp: April 16, 2021, 04:24:05 pm
content: So I decided to test again, this time with the assumption that:Black-Blue-Red as the primary (0-110VAC-220VAC)Yellow-Yellow as the secondaryTurns out CaptDon, wraper, and perieanuo are right.I connected black and red to mains (218VAC), with no load on secondary. Just like before, the circuit breaker didn't trip.This is from 5 minutes test:The primary (black-red) uses 26.34mA to 28.15mA of current.The output from secondary winding is 10.14VAC to 10.33VAC.And no smell.I disconnected the primary from mains, and touched the transformer: not hot, not even warm.Then I repeated the test, the result is similar:Primary uses 25.97mA to 28.21mA.Secondary output 10.07VAC to 10.32VAC.Still no smell.I disconnected the primary again and touched the transformer, this time it get just a little bit warm.(Sorry about the "hot" and "warm", I only have two multimeters, and both are being used in this test, so I cannot use the temperature probe.)I still don't understand how the result could be different from when I first tested it (heat up and smelled), but now it seemes that everything is fine.The yellow wires are certainly the secondary.And no windings are shorted.Thank you for helping out, everyone.

author: wraper
author_rank: Unknown
timestamp: April 16, 2021, 04:49:16 pm
content: Quote from: old-jo on April 16, 2021, 04:24:05 pmEither you connected blue-red or black-blue to the mains or some wires were shorted together.

author: Jwillis
author_rank: Super Contributor
timestamp: April 16, 2021, 10:38:52 pm
content: You should build yourself a current limiter for testing transformers off of mains . Not so much a concern about saftey all be it there are always risks . It would save you from tripping the breakers on your panel . These breakers have a limited number of trips before they become weak or fail .This type of current limiter can be used to test primary and secondary windings without potential damaging the transformer under test. With the light in series you can test voltages much easier .

author: amyk
author_rank: Super Contributor
timestamp: April 17, 2021, 04:31:49 am
content: If you connect a dual-voltage primary wrong, the inductance can cancel out in the two halves and the result looks like a short across the mains.

author: wraper
author_rank: Unknown
timestamp: April 17, 2021, 01:56:52 pm
content: Quote from: amyk on April 17, 2021, 04:31:49 amNot possible in this case, since winding has a center tap, not 2 separate windings.