Coils inductance measurements

[nelson]

Hi to all.
Has all we know, there has been writting a lot about coil inductances.
Indeed, the tesoro golden sabre coils has some values around 5.5 mH and 6.5 mH, for tx and rx coils.
Then when i measured coil inductances with an ordinary multimeter, i got values around the one show above. But when i take a reading with the L/C Meter IIB, that is show on the picture, the readings goes about 3 mH up (8.5 and 9.5 mH). This L/C meter IIB has a reputation of been one of the good meters on the market and even when i have test it with know inductance values, the meter was correct.
So now my question is how good is the inductance values posted? Are they obteined from the coils factory or are they got from hobbiest measurements?
Sorry for all this questions, but something is not working for me, cause if i have a good working L/C meter IIB, how can i explain the diferencies obteined.
Regards
Nelson

[Qiaozhi]

Hi to all.
Has all we know, there has been writting a lot about coil inductances.
Indeed, the tesoro golden sabre coils has some values around 5.5 mH and 6.5 mH, for tx and rx coils.
Then when i measured coil inductances with an ordinary multimeter, i got values around the one show above. But when i take a reading with the L/C Meter IIB, that is show on the picture, the readings goes about 3 mH up (8.5 and 9.5 mH). This L/C meter IIB has a reputation of been one of the good meters on the market and even when i have test it with know inductance values, the meter was correct.
So now my question is how good is the inductance values posted? Are they obteined from the coils factory or are they got from hobbiest measurements?
Sorry for all this questions, but something is not working for me, cause if i have a good working L/C meter IIB, how can i explain the diferencies obteined.
Regards
Nelson

Personally I measured the inductance values of several Tesoro factory-made coils. I found the values to be approximately TX=5.9mH and RX=6.5mH. The LCR meter was from Peak Electronics, and claims an accuracy of +/- 1%.
http://www.peakelec.co.uk/acatalog/jz_lcr40.html
I'm not sure that the accuracy is quite that good for the large air-cored coils, as used in metal detectors, but even 5% accuracy should be close enough for our purposes.

[Molzar]

Hi Nelson,

I think the problem with inductance measurements is Frequency and if there measured in Parallel or Series.

I have access to quite a few different professional type LCR meters (GenRad, Fluke/Phillips). These will do measurements at different setting. When I set the instrument to measure in series the numbers for TGS coils are 5.5 ~6.5, When do the same but set for parallel is approx. 3 mH higher.

This is setting on instrument, not special setup. Cheaper meters do not have these options.

Most times meters use 1kHz as measuring frequncy, but some use other 102Hz, 850Hz, etc) This I think can also affect readings.

I also have cheap chinese meter, measures in series, 1Khz and measures a little higher/lower (0.2 mH) than pro meters.

So I think exact measurement is difficult but most times with measurements here you are in general area.

Molzar

[gwzd]

Hi Nelson,
Have you tried to measure a factory made inductor with your instrument?
You probably can find some inductances with 3 to 5% tolerances and you could use them as a reference.
Another way, you can get a good polyester capacitor with 1% tolerance or even better a film capacitor with 0.1% tolerance. Then you'll find out the resonance frequency with your inductance and calculate the value of the inductance from:
f=1/(2*PI*SQR(LC))
Keep in mind though that your coil also has some capacitance and that will affect the measurement. Ideally you would take that into consideration and subtract it from the final result.
If you have the possibility, then a network analyzer is the best option.
Regards

[nelson]

Excellent explanation about frequency and series and parallel measurements of inductance.
So i just whant to thank everybody who has take the time to explain me what could be my problem when i compered my L/C meter IIB with my ordinary multitester.
Indeed i m think i ll get one LCR meter from peakelec.com.uk, cause this shows good performance when we need to get our coils to work on correct inductance values.
About the formula, to compensate my 3 mH increment, please let me know an example of how to do that calculations.
Sorry, but has all know i m not very skilly with electronics.
In advance many thanks
Regards
Nelson

Personally I measured the inductance values of several Tesoro factory-made coils. I found the values to be approximately TX=5.9mH and RX=6.5mH. The LCR meter was from Peak Electronics, and claims an accuracy of +/- 1%.
http://www.peakelec.co.uk/acatalog/jz_lcr40.html
I'm not sure that the accuracy is quite that good for the large air-cored coils, as used in metal detectors, but even 5% accuracy should be close enough for our purposes.

[gwzd]

Hi Nelson,
without knowing what's inside your equipment it would be very hard for me to give you some advice on how to calibrate it. The majority of these budget LC meters are based on a LM311 oscillator and a reference capacitor. By measuring the shift in frequency is possible to calculate the values of the components inserted in the LC tank.
Check it out, if you have something like that then perhaps I might guide you to calibrate it. If not, then try to see if it is possible to open it and post some pictures.
regards