Check integrity of the coils in transformer - Page 1
Forum section: electronics / beginners

author: Jasonbit
author_rank: Regular Contributor
timestamp: January 25, 2019, 09:45:33 am
content: hello folks,I have a simple transformer 230VAC to 12VAC (5VA). I want to test if the coil is in good condition. I take my multimeter, set it up as continuity check and put the probe in terminals primary and after I repeat the steps in secondary coil.Primary: I took this value (806) without sound;Secondary: I took this value (004) with sound;Would not I be expected to get the continuity signal on both coils? Because the transformer is working properly. I have another transformer (200VA) where I get a beep on both coils.Thanks

author: mvs
author_rank: Frequent Contributor
timestamp: January 25, 2019, 09:54:08 am
content: Smaller transformer has higher primary coil resistance, so its ok that you do not get a beep.

author: Jasonbit
author_rank: Regular Contributor
timestamp: January 25, 2019, 09:57:02 am
content: Quote from: mvs on January 25, 2019, 09:54:08 amThis means which the coil is good, because the multimeter can induces a current in the coil but it is not enough to get a beep, right? But how can I check the integrity of the coil?Thanks

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 25, 2019, 10:08:51 am
content: Most DMM's (digital multimeters) will beep around 50 ohms or less in continuity mode. Your multimeter is working correctly and you have continuity in both winding.

author: Jasonbit
author_rank: Regular Contributor
timestamp: January 25, 2019, 10:12:54 am
content: This means if I get in my multimeter value infinity (like this image:https://d32c3oe4bky4k6.cloudfront.net/articles-videos/-/media/uscamediasite/images/story-images/2017/6/resistance_1803_cropped.ashx?la=en&modified=20170726020407&hash=710E29302E7B433AAC96832A89E6E401F8884D34) The coil it is spoiled. If I get a beep with value or a value without beep my coil it is good! Right?

author: Brumby
author_rank: Unknown
timestamp: January 25, 2019, 10:20:24 am
content: Basically - yes ..... but (there's always a "but")Getting a resistance reading that is not infinity indicates there is continuity through the coil - which is good. If the resistance is low enough (higher power transformers) then the continuity buzzer will sound, but lower power transformers will have a higher resistance which might be above the buzzer threshold.The "but" is that there is still the possibility of a shorted turns fault. This is something that a simple resistance or continuity test cannot detect.

author: Jasonbit
author_rank: Regular Contributor
timestamp: January 25, 2019, 10:24:55 am
content: Quote from: Brumby on January 25, 2019, 10:20:24 amSo what tests can I perform on a transformer?Thanks

author: spec
author_rank: Frequent Contributor
timestamp: January 25, 2019, 11:16:49 am
content: Quote from: Jasonbit on January 25, 2019, 10:24:55 am.Hi Jasonbit,Firstly forget about the sounder on your digital multimeter(DMM) and make these measurements [expected result].resistance between primary and secondary (any terminals) resistance between primary and transformer body (conductive area) resistance between primary and screen terminal (if fitted) resistance between secondary and transformer body (conductive area) resistance between secondary and screen terminal (if fitted) primary resistance [see below]secondary resistance [see below]Tests 1 to 5 should be done using the highest resistance range on your DMM.The values for tests 6 and 7 depend very much on the transformer type, voltage and current ratings and frequency range, and the best bet is to refer to the data sheet for the transformer, or one that is similar.The above tests are the basic tests for a transformer and any more exhaustive testing would require some special-to-type test equipment (STTE), but it need not be elaborate or costly. For mains transformers, you can simply connect them to the mains supply via an appropriate fuse and test their output voltage and current capability. To do this safely you would need a simple piece of STTE to ensure protection of the hardware and safety of the user.You mention shorted turns: they can be easy to detect or impossible to detect, depending on where the shorted turn is and what type of transformer is involved. For example a high power toroidal transformer would hum and take excessive current off load but, on the other hand, if you had a small E and I core transformer with a 5,000 turn primary and only one turn were shorted it may be impossible to detect.A handy piece of transformer test equipment is a 100W, or higher, mains transformer with 3V, 6V, 12V, 24V, 48V secondaries, all fused and protected in an earthed metal case, of course. The five AC voltage lines can then be used to test a wide range of transformers: mains power, coupling, valve output, etc, by connecting the appropriate AC line to an appropriate winding on transformer under test (TUT) and measuring the TUT off-load and on-load characteristics. You would need a range of power resistors to do the on-load tests.Taking an example, you have a guitar valve amplifier output transformer in you spares box and you do not know if it is faulty or what its turns ratio is. Using the transformer test equipment described above you cold easily find out if the transformer is OK and what its characteristics are.On a practical point, I don't know what the position is in Brazil, but in the UK you can get stuff real cheap from discount stores, petrol stations, Alibaba, ebay, etc: analogue multimeter for £2.50, metal boxes for £2UK. My favorite is stainless steel roasting tins, but they are hellish expensive at £4.00. So making simple, STTE need not be expensive these days.