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Old 13th May 2014, 03:13 PM   #1
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Default Strange transformer test results

So I pulled a pair of small transformers from the speaker level input of an old Optimus car amp. I have tested them for voltage ratio and inductance. What is strange about them is that they measure as 1:1 voltage ratio, however, the primary side where the incomming speaker connection went measured to be ~1kohm@1kHz .149H ~60ohms DC and the output side which fed into an opamp measured as ~10kohm@1kHz 1.57H ~80 ohms DC. Based on this these should step up voltage rather than being 1:1. Test method was to feed the winding with a 1kohm resistor in series and determining AC impedance of the winding based on current through the 1k resistor and the voltage dropped by the winding and using Z-R(dc)/(2*pi*Freq)= Inductance.
So the big question is am I just doing it all wrong or am I missing something obvious?
Opinions, lessons, or critics welcome

Last edited by jerluwoo; 13th May 2014 at 03:17 PM.
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Old 13th May 2014, 03:38 PM   #2
DF96 is offline DF96  England
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You can't just add/subtract resistance and inductive reactance as they act in quadrature, not in phase. Show us exactly what you did, and the raw measurements you obtained. We can then help you extract some useful information.
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Old 13th May 2014, 04:03 PM   #3
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I am feeding the winding from the output of an amp. I have the positive output connected to one side of the winding, a 1k resistor connected to the other side of the winding and ground terminal hooked to the other side of the 1k. I am feeding it .558Vrms@1kHz, confirmed with meter and soundcard scope. I then measure the voltage drop across the 1k resistor (.056Vrms), determine the AC current I=V/R, (.000056A). I then measure the voltage drop across the winding (.521Vrms), and determine its impedance X=V/I (~9303ohms). I then subtracted the winding resistance from the total impedance, 9303-80= 9223. I then use the equation L=X/2*pi*F to get the inductance of the winding.
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Old 13th May 2014, 05:57 PM   #4
sbrads is offline sbrads  United Kingdom
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I see no squared or square root terms in your calculations, see the gain equations in the wiki link. If you're familiar with -3dB points in frequency response graphs, this occurs in say a low pass RC (could be LR but rarely used) circuit when the impedance of the capacitor EQUALS the value of R, and yet -3dB equals 0.707 of the input voltage, not 0.5 as your method would give. i.e. you're expecting exact results from a flawed very rough and ready method.

Your step-up transformer may be being loaded on the secondary, dropping the voltage and making it look like 1:1. Hard to say for sure. Try it the other way round.

Even with hyper expensive LCR bridge analysers you can get funny results so I wouldn't worry too much.

RL circuit - Wikipedia, the free encyclopedia
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Old 13th May 2014, 07:35 PM   #5
Elvee is offline Elvee  Belgium
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Yes, everything is vectorial. In this case, I don't think the problem lies with the series resistance: it is too small to change something between arithmetic and vectorial calculations, but the magnetizing impedance is hugely complex, in particular if the iron is of poor quality. Even for purely linear conditions, it cannot be modelled simply with ordinary linear RLC components, because of effects similar to dispersion in dielectrics.
A vectorial LCR-meter operating at a reasonable frequency should give a good approximation of the equivalent small signal inductance.

Other methods, except a suitable bridge will be mostly out.

Note that a "vectorial meter" sounds expensive, but it isn't: most handhelds use some kind of vectorial measurement, otherwise they wouldn't be able to resolve 0.1µH at 1KHz
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Old 13th May 2014, 08:13 PM   #6
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Yea algebra classes were 20 years ago so I'll have to spend some time getting caught up on transposing to get terms the right way since all the equations I've found have the inductance known rather then impedance etc etc. The transformer however is 1:1 since the voltage measured is the same on both windings regardless of which is the driven with the other only loaded by the meter (1meg input impedance). When the 1k resistor is in series with say winding A the voltage drop is equal across the winding and resistor, and when it is in series with winding B theres a 10:1 voltage division with the winding being 10. And still no matter which way you drive it, it still puts out 1:1 voltage from the windings. Ugh think I'll put it away for a bit my head is starting to hurt.
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Old 14th May 2014, 11:00 AM   #7
DF96 is offline DF96  England
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If what you measure seems impossible then the most likely explanation is that you are not measuring what you think you are measuring. Try one or more of the following:
- 10k resistor instead of 1k (best to measure reactance with a more similar resistance)
- transformer on ground side instead of signal side (different effect of stray capacitance)
- different frequencies (200Hz, 5kHz?) (avoid a resonance?)
- try shorting the unused winding - you may then need to use a smaller resistor (checks coupling between windings)
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Old 14th May 2014, 12:48 PM   #8
ontoaba is offline ontoaba  Indonesia
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May be there is resistor connected inside for damper and flattening its frequency response. Isolation transformer usually has good mutually, it will be similar in primary and secondary if the ratio is 1:1.
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