Question : Profiling an unknown power inductor or power transformer - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: November 20, 2013, 03:38:38 am
content: I think we've seen too many threads asking identification for an unknown inductor or transformer that people salvaged or found. This makes me curious if there is a way, although limited, to get a certain usable specification out of it using non destructive method ?Non destructive here means without the need of cracking the core which sometimes is fully epoxied or potted, or need to unwind the wires, all data gathered only through it's termination points and external physical inspections.Assuming we have these pre-conditions before we start the probing :Tools :- A decent LCR meter like those popular handhelds such as MS5308, DE-500 or similar.- An inductor saturation tester for finding the current saturation point, like this example ->HERE.- A scope and/or sig-gen, maybe ?Inductor/transformer physical informations :- Ability to measure or at least take an approximation of the wire diameter size from the termination points.- Winding count and tapped count if any.- Core's shape/size/volume.- Winding topology if they're visible from outside like overlapped, stacked windings and etc ?- Anything else that I missed regarding the needed info from the physical inspection on the inductor/transformer ?Understand that there are limits on inductor specifications that we can gathered for example like the important factor isolation and creepage limit and etc.Questions :Aware the outcome definitely will not 100% accurate, is this doable ?Can we somehow get "good enough" specifications to re-use the inductor/transformer later for one off project if needed someday ?Of course the user also required to be aware of the risks involved like lacking of the isolation limitation info and etc. For example its not wise to use in a thousands volt circuit, or for welding (high current) using a small iron core transformer salvaged from a cheap a wall wart AC to DC converter, or a power ferrite inductor from those cheap car DC->USB cellphone converter/charger to be used in 1000 watt power switching circuit.PS :Please see this question in the context of enthusiast/hobbyist perspective, understand that these activity probably will be useless in commercial/pro environment, since it will be much more economical to buy an off shelf ready product which fit in as required by the circuit. But for pack rat like me that love to salvage used inductors/transformers, I will be more happy if I can identify and use them even though not 100% suitable but still acceptable, compared to buying a new inductor even its cheap. Hoping I'm not alone here.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 21, 2013, 12:55:46 pm
content: Hi,With an LCR meter and method for determining saturation can you can reverse engineering a transformer.Turns ratio(s)Step 1Locate all the windings using an ohm meter.Lets say we found two windings.Measure the inductance of each winding using the LCR meter.Example:Winding 1: 22uHWinding 2: 2.44uHSince L=N x N x Al (N = number of turns, Al = inductance factor)Since the inductance ratio = 9:1The turns ratio = 3:1We also know that you can not have fractions of a turn, the turns are always integers.So the transformer must have3 turns : 1 turn, 6 Turns : 2 turns, 9 Turns to 3 turns etc...We can get an idea of the number of turns using the inductor saturation tester.Faraday's Law of Induction: V=N dPhi /dtPhi = total flux = B x A (Where B = Flux density , A = cross section of the core)V= N x A x dB/dtFor Ferrite we can assume that the saturation flux density is 0.35 Tesla. (Most power grade ferrites are similar)The area is the area of the center limb of the core. You can measure this with calipers.With the Inductor saturation tester you can measure how many volt microseconds it takes to saturate the core. We will measure the 22uH winding.Let say it takes 200 Vus. 200E-6Let say the area is 25mm2(5mm x 5mm) 25E-6m2From V=NA dB/dtN = Vt /A x dB = 200E-6 / 25E-6 x 0.35 = 22.8 turnsBut from the turns ratio measurement we know that this number is an integer and a multiple of 3.So the number of turns is either 21:7 or 24:8 (You can not really tell)If the construction of the transformer allows you add a winding, then you can measure ratio between the winding that you know and the unknown windings.You now have the turns ratio, the saturation current, the primary and secondary inductance. You can now look at some datasheets from Pulse Electronics or Wurth to find a transformer that is similar to determine the power rating.Remember in some circuits like Flyback power supplies, you need to identify the phasing, the starts and finishes of the winding)This best way is to experiment with a transformer of known characteristics.I hope that this helps.Jay_Diddy_B

author: bingo600
author_rank: Super Contributor
timestamp: November 21, 2013, 03:10:33 pm
content: @JDBThat was a neat explanationEven i could understand it/Bingo

author: BravoV
author_rank: Super Contributor
timestamp: November 22, 2013, 01:03:47 am
content: Thank you so much Jay_Diddy_B !I know this is basic, but for me, this is a really helpful knowledge, printed and stickied in front of my workbench now !Quote from: Jay_Diddy_B on November 21, 2013, 12:55:46 pmDoes this mean I can get an accurate turn counts on the unknown windings if I can managed to sneak in there my own winding with few turns ? Sorry, example of exact steps if you don't mind.Quote from: Jay_Diddy_B on November 21, 2013, 12:55:46 pmPlease explain further about this, I'm not sure I can have a strong grip in this matter.Again, really appreciate the help given.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 22, 2013, 03:47:25 am
content: Hi,I had a few minutes so I took a transformer that I have in my junk box. I added a winding of 5 turns of 30awg wire-wrapping wire like this:I now have a winding with a known number of turns.I now measured the inductance of all the windings:5 Turn reference winding 3.83uHPrimary 233uHSecondary 25.9uHAux 4.58uHThe Al value of the core can be calculated from the 5 Turn windingAl = L/N2= 3.88 /25 = 0.155uH per turn2For the other windings we have:Turns = SQRT (L / Al)Primary Turns = SQRT (233/0.155) = 38.7 (has to be 39)Secondary Turns = SQRT (25.9/0.155) = 13Aux Turns = SQRT (4.58 / 0.155) = 5.4 (5 turns)Turns ratio is Primary : Secondary : Aux = 3 : 1 : 0.4The phasing is indicated on a transformer with 'spots' like this:The spots indicate polarity. If the one spot is positive, the other spots will also be positive. This is important in a flyback supply. The transformer is arranged so that when the primary switch is closed the rectifier diode will be reverse biased.Jay_Diddy_B

author: Odysseus
author_rank: Regular Contributor
timestamp: November 22, 2013, 07:08:55 am
content: And don't forget leakage inductance or coupling coefficient, which is very important for simulating switching converters. It's found by first measuring inductance of one winding while the other winding is shorted. Once Lshortis found, then the coupling coefficient, K, is calculated as sqrt(1-Lshort/L). Then the uncoupled portion of the winding, Lleak, is (1-k)*L while the coupled portion of the winding, Lmutualis k*L.Seehttp://en.wikipedia.org/wiki/Leakage_inductancefor more details. It also gets more complicated for multiple winding transformers.

author: The Electrician
author_rank: Frequent Contributor
timestamp: November 23, 2013, 05:52:36 pm
content: The responses so far have dealt with ferrite cored switcher transformers.Several years ago there was a thread on another forum considering this problem with typical small silicon steel transformers:http://forum.allaboutcircuits.com/showthread.php?t=38273

author: BravoV
author_rank: Super Contributor
timestamp: November 24, 2013, 08:22:04 am
content: Jay_Diddy_B, that is exactly I wanted, great & thank you Sir !Quote from: Jay_Diddy_B on November 22, 2013, 03:47:25 amCan we use sort of low voltage pulse/signal at the primary while observing the secondary result using scope for identifying the phase ?Quote from: Odysseus on November 22, 2013, 07:08:55 amGreat to know, thank you !Quote from: The Electrician on November 23, 2013, 05:52:36 pmWow ... huge thread, bookmarked, that will take sometimes to read and digest, many thanks !Yeah, I think with current trend that switching power is dominating the world, while iron core transformer is going to extinct I guess, I think most of the cases here on the salvaged inductors, majority will be ferrite cored type.

author: Neilm
author_rank: Super Contributor
timestamp: November 24, 2013, 11:59:45 am
content: Quote from: BravoV on November 24, 2013, 08:22:04 amIron core transformers will still have their place, but not in small / home applications. They all have "small" written into the design spec and the only way to do that is go for high frequency and that means ferrite.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 24, 2013, 12:57:51 pm
content: Quote from: BravoV on November 24, 2013, 08:22:04 amYes, you can use a pulse / function generator and a scope to identify the phasing.You can also use the inductor saturation tester as the signal generator, and measure the other windings with a scope.A third method is to use the LCR meter. Put two winding series, if the inductance goes up they are connected in phase, if the inductance goes down they are out of phase. Think of positive and negative turns.If I take the example that I measured, which had a turns ratio of 3:1Primary: 238uHSecondary: 22uHIn series the inductance will either be 238uH x ((3+1)2/ 32) = 423uH (in phase)or238uH x ((3-1)2/ 32) = 105uH (out of phase)Jay_Diddy_B

author: M0BSW
author_rank: Frequent Contributor
timestamp: November 24, 2013, 01:21:41 pm
content: Quote from: Jay_Diddy_B on November 21, 2013, 12:55:46 pmI'm worried , I even understood that explanation

author: BravoV
author_rank: Super Contributor
timestamp: November 25, 2013, 01:58:53 am
content: Quote from: Neilm on November 24, 2013, 11:59:45 amAgree, while most iron cores for home appliances, 99.99% are easily identified since their primary as mostly for mains while secondary can be easily spotted by looking at the load circuit or simply measure the output.

author: BravoV
author_rank: Super Contributor
timestamp: November 25, 2013, 02:03:42 am
content: Quote from: Jay_Diddy_B on November 24, 2013, 12:57:51 pmSimple enough, currently I only have the el-cheapo <$30 L and C meter, still I managed to identify one grabbed from my junk box, great.I guess its time to look for a decent LCR meter.

author: BravoV
author_rank: Super Contributor
timestamp: November 25, 2013, 02:27:36 am
content: About current saturation point, as in the saturation tester, if the "knee" bend is easily distinguishable, then its easy, but what if there is no sharp bend and it shows only curve like slope ?How can we safely decide or take a smart guess at which saturation point is the one rated for the tested winding ?Example like these below.