COMPARISON OF TEMPERATURE COEFFICIENT OF STANDARD INDUCTOR BY MEASURINGCHANGE IN INDUCTANCE AND RESISTANCE

M. A. Ansari, Jyotsana and Anil Kishore SaxenaNational Physical Laboratory

(Council of Scientific and Industrial Research)LF & HF Impedance Standards,

Dr. K.S. Krishnan Marg, New Delhi 110012, Indiaaksaxena@nplindia.org

Abstract

Standard inductors manufactured by General Radio(GR) are widely used by NMIs. The temperaturecoefficient for these standard inductors is 30 ± 5ppmfOC, as per manufacturer. NPL India (NPLI)had earlier determined the temperature coefficientof standard inductors by measuring change ininductance. Now by measuring change inresistance, temperature coefficient of inductors isre-determined. Temperature coefficientsdetermined from the two methods are reported here.

Introduction

Temperature coefficient of standards plays a majorrole in precision measurements. NPLI had earlierdetermined the temperature coefficient of standardinductors of value 1 mH (GR 1482, three numbers)by measuring change in inductance using PC basedsetup [1, 2]. The temperature coefficient of theseinductors in the temperature range of 22°C to 27°Cwas between 42 to 47 ppmfOC with deviation of ± 5ppm. These results were in agreement with theresult reported in [3] and [4].

As per specification of Inductors, the temperaturecoefficient of inductors can be determined bymeasuring DC resistance of inductors. It ismentioned that 1% increase in resistance, producedby temperature increase of 2.54 °c corresponds to0.0076% increase in inductance.

Therefore it was decided to verify the previousresults by measuring change in dc resistance ofinductors.

Methodology

NPLI had earlier used the setup based on differencevoltage measurement (Method I) to measure changein inductance and thus determine temperaturecoefficient of inductors [2] at 1 kHz. Change ininductance of test inductor Lx is measured withrespect to auxiliary inductors LA. The LA is 10 mHinductor of GR make. Initially both the test

inductor and auxiliary inductor were kept atconstant temperature for more than 24 hours inseparate air baths, developed at NPLI, with atemperature controlled within ±0.2 0c. Temperatureof LA was kept constant throughout the experiment.Then the temperature of enclosure of test inductorwas changed by 1°C. The software controls theLock-in Amplifier and records the differencevoltage at an interval of 30 minutes for 24 hours.The software calculates change in inductance frommeasured difference voltage and also records thechanges in inductance values. Measurements wererepeated for the temperature range from 22°C to 27°C. Experiment was repeated with other two 1 mHinductors.

NPLI has currently determining the temperaturecoefficient of these inductors by measuring thechange in resistance with respect to temperature(Method II). These inductors were kept in acommercial air bath, whose temperature stability is± 0.05 °C. The air bath temperature was set at aparticular temperature for about 24 hours andresistance of the inductor was measured by usingFluke make 8846 A, 6Y2 digits precisionmultimeter. Measurements were taken at an intervalof 30 minutes. Process was repeated for 1 °C and2.54 °C temperature change.

Graph and results obtained by two methods areshown in table 1 and fig. 1 to fig.5.

Temperature Coefficient (in ppm)Sr. No. with deviation

Method I Method II18892 46±5 26±518577 42±5 29±519570 47±5 31±5

Table 1. Temperature Coefficient of ImH, GR 1482standard inductors

Temperature change from 24°C to 25°CInductor:lmH, Make:GR, Sr. No. 18577

$' 0.000040.e-""::> 0.000030,s"::l'" 0.000020.::.5

"OJ) 0.000010::>...cU

0.000000 -I'----,-------;-,-----,------,-----,-,-----,-----,-----r---l

Inductor:lmH, Make:GR, Sr. No. 18892

0.00008

$' 0.00006

.e- Method ((0.00004

" ~ 23°C to 25.54°C"= 0.00002..ti ~ 25.54°C to HOC::l 0'".:: 17 21 25 29 33 37 41 45.5 -0.00002

"OJ) -0.00004=...c

-0.00006U

5 9 13 17 21 25 29 33 37 41 45 -0.00008 -'-------------------"

Number of Readings after 30 minutes

Fig.1 Change in Inductance for 1°C temperature change

Number of Readings after 30 minutes

Fig.S Change in Inductance for 2.S4°C temperature change

ConclusionTemperature change from 24°C to 25°CInductor:lmH, Make:GR, Sr. No. 18892

0.000050

$'.e- 0.000040

""=.. 0.000030ti::l

'".:: 0.000020.5"OJ)

= 0.000010...cU

0.000000

It is clear from both the methods that inductors take10 to 12 hours to stabilize against change III

temperature and the temperature coefficient isdifferent for different inductors of same value.

The temperature coefficient obtained by method I isagreed with results reported in [3], [4]. It isdifferent from temperature coefficient determinedby method II.

5 9 13 17 21 25 29 33 37 41 45

Number of Readings after 30 minutes Acknowledgement

O-F-----,---,-----;----,------,----,-----r-,---------,-J

1 5 9 13 17 21 25 29 33 37 41 45

Fig.2 Change in Inductance for 1°C temperature change

References

A.K. Saxena and Mohd. Saleem "AnAutomatic Bridge for Calibration ofInductance based on Difference VoltageMeasurements" CPEM 1998 Conf. Digest,pp 500-501, held at Washington, USA fromJuly 6-8, 1998

[1]

The authors are grateful to Prof. R. C. Budhani,Director NPL for permission to publish this paperand Dr. P. Banerjee, Head Electrical and Electronicstandards for his constant encouragement.

Method ((

0.00001

Temperature change from 24°C to 25°CInductor:lmH, Make:GR, Sr. No. 18577, 18892,19570

0.000035 ,-------------------,

0.00003

"OJ)

=] 0.000005U

1l 0.000025=..~ 0.00002

'".:: 0.000015.5

[3] A. Fiebiger and K. Droge" Bestimmung desTemperature Koefficient von Induktivitats­normallen des Type GR 1482" PTBMitteilungen, vol. 94, pp 10-14, 1984.

Number of Readings after 30 minutes

Fig.3 Change in Inductance for 1°C temperature change

Inductor:lmH, Make:GR, Sr. No. 18577

0.00008

$' 0.00006

.e- Method II

"0.00004

" ~ HOC to 25.54°C= 0.00002£ ~ 25.54°C to 23°C::l 0'".:: 17 21 25 29 33.5 -0.00002

"OJ)-0.00004=..

.cU -0.00006

-0.00008

Number of Readings after 30 minutes

Fig.4 Change in Inductance for 2.S4°C temperature change

[2]

[4]

Shally Sharma, A.K. Saxena and Mohd.Saleem "A PC Based setup to DetermineTemperature Coefficient of StandardInductors Using Difference VoltageMeasurement Technique" CPEM 2008 Conf.Digest, pp 570-571 held at Colorado, USA,from June 08 to Junel3, 2008

Kewal Krishan, R.N. Dhar and S.L. Dahake"Determination of Temperature Coefficientof Inductance Standards" MAPAN- Journalof Metrology Society of India, vol 5, no.2,pp. 107 -110,1990.