Instruments used to measure resistance, inductance and capacitance

26-May-2016 11:50 am 0

An LCR meter measures the impedance of a circuit. An ohmmeter measures the resistance of any component. This article explains the working principle, provides what one needs to be mindful of while buying and while operating the instrument.

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Meco 7272 Digital Milli-Ohmmeter

Milli/Micro Ohm Meters– It measures the resistance of any entity by passing a small amount of current through it. It comprises of an internal voltage (which in turn controls the current through a feedback loop) and internal resistance. Through feedback, the voltage adjusts to a value such that resultant current doesn’t affect the external circuit. The price of an industrial grade Ohmmeter varies from ₹2500 to ₹95000 according to the robustness, range and accuracy of the instrument.

While measuring through an Ohmmeter one needs to be mindful of

  1. Appliance whose resistance is to be measured is to be disconnected from  the circuit so that there is no current.
  2. Selecting Ohmmeter of the correct range- Micro-Ohmmeter or Milli-Ohmmeter.
  3. Checking the battery.

Factors to consider before buying an Ohmmeter

  1. Maximum test current– It depends on the kind of appliances you normally test the resistances of. if they can withstand high currents then you should buy an Ohmmeter with highest test current for a more accurate reading.
  2. Manual or AutoRange– Normally, an ohmmeter comes with 4-5 range sets, between which it can automatically adjust to show you the most meaningful reading in the best resolution. For some low priced Ohmmeters, you have to adjust the range manually.
  3. Memory– If you work in a laboratory where comparing or measurement of the reading is important and not just the testing, then select an instrument which can store atleast 1000-2000 values.

Multimeter and Ohmmeter both can measure the resistance but the latter gives more accuracy and a higher range than a multimeter.

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Meco 999a LCR meter

LCR Meter– LCR meters measure inductance, capacitance and resistance. Simpler models of LCR meters provide indications of the impedance from which an approximate value of capacitance and inductance is calculated and shown.

Better designs are able to measure the true inductance or capacitance, and also the equivalent series resistance of capacitors and the Q factor of inductive components. This makes them valuable for assessing the overall performance or quality of the component.

LCR meters can be of two kinds- Either it can work on a balance Wheatstone bridge principle or by measuring the current and voltage both and using those values, the impedance(contains the values of both capacitance and inductance) can be derived. Thus, it sometimes also works as an inductance meter.

An Industrial grade LCR meter price varies from ₹1500 to ₹16000.

Factors to consider before buying an LCR meter

  1. Measurement accuracy and speed- Accuracy for a laboratory grade instrument should be around 0.1%. For Industrial use 1% accuracy is sufficient. The number of times the instrument takes the readings is important in the case of highly fluctuating values of inductance and capacitance. In such cases 10 measurement/sec is advisable.
  2. Normal operating Range-Knowledge of the current and potential range in which the component normally operates is essential before buying the LCR meter suitable for your purpose.
  3. Datahold- Another function of use in case of LCR meters is datahold which as name suggests, can hold previous readings in its memory and perform computations with the new reading whenever required.

LCR meters are not very easy to use.

Things to keep in mind while using an LCR meter-

  1. Measurement Frequency- To measure small capacitors(less than 1 µF) and inductors(below 1 mH), frequencies of 1kHz or higher should be used. Larger capacitors and Inductors can be measured at the line frequency itself.
  2. Circuit- An additional resistance is used for better readings. The connection of that resistance is of immense importance.  For high Capacitance, the series resistance may be negligible. So, parallel connections are preferable before taking the reading. In case of High Inductances lean towards using a series connection.
  3. Ensure that the capacitors are discharged before measuring.
Summary
Article Name
Instruments used to measure resistance, inductance and capacitance
Description
A brief description about the working principle, provides what one needs to be mindful of while buying and operating instruments which are used to measure resistance, inductance and capacitance.
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