An ohmmeter is an electrical instrument that measures electrical resistance, resistance to electric current. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (also a trademarked Megger device) measure large resistance values. The unit of measure for resistance is ohm (?).
Video Ohmmeter
Design evolution
The first ohmmeters are based on a kind of meter movement known as a 'ratiometer'. This is similar to the galvanometer type movement found in later instruments, but not hairsprings to supply the restoring force they use to do 'ligaments'. It does not provide a clean rotation style for movement. Also, the movement is wrapped with two coils. One is connected through a series resistor to the battery supply. The second is connected to the same battery supply through the second resistor and the resistor being tested. The indication on the meter is proportional to the ratio of the current through the two coils. This ratio is determined by the magnitude of the resistors tested. The advantages of this arrangement are twofold. First, the indication of resistance is completely independent of the battery voltage (as long as it actually produces voltage) and no zero adjustment is required. Second, although the resistance scale is not linear, the scale remains true during the full deflection range. By swapping the two coils, the second range is provided. This scale is inversely compared to the first. The feature of this instrument type is that it will continue to show a random resistance value after the test lead is disconnected (actions that release the battery from movement). Ohmmeters of this type have only ever measured resistance because they can not be easily incorporated into a multimeter design. The insulation testers that rely on hand generator turn on are operated on the same principle. This ensures that the indication is entirely independent of the resulting voltage.
The next ohmmeter design provides a small battery to apply voltage to the resistance through the galvanometer to measure the current through the resistance (batteries, galvanometers and resistance all connected in series). The galvanometer scale is marked in ohm, since the fixed voltage of the battery assures that as the resistance increases, the current through the meter (and hence the deflection) will decrease. Ohmmeters form their own circuits, therefore they can not be used in assembled circuits. This design is much simpler and cheaper than the previous design, and is easily integrated into multimeter design and is by far the most common ohmmeter analogue form. This type of ohmmeter suffers from two inherent weaknesses. First, the meter needs to be maximized by shortening the measurement points together and making adjustments for the zero ohm indication before each measurement. This is because because the battery voltage decreases with age, the circuit resistance in meters must be reduced to maintain a zero indication at full deflection. Secondly, and consequently on the first, the actual deflection for each resistor is given under the test change as the internal resistance is changed. Staying right at the center of the scale alone, that's why such ohmmeter designs always cite the accuracy of "only on a central scale".
A more accurate type of ohmmeter has an electronic circuit that passes a constant current (I) through resistance, and another circuit that measures voltage (V) across resistance. This measurement is then digitized with a digital analog converter (adc) after which the microcontroller or microprocessor makes the current and voltage divisions in accordance with Ohm's Law and then decodes this to the screen to offer the user a reading of the resistance value they are measuring at that point. Since this type of meter already measures current, voltage, and resistance at the same time, this type of circuit is often used in digital multimeters.
Maps Ohmmeter
Precision ohmmeters
For high precision measurements with very small resistance, the above meter type is inadequate. This is partly because the deflection change itself is small when the measured resistance is too small in proportion to the intrinsic resistance of the ohmmeter (which can be handled through the current division), but largely because the meter reading is the sum of the resistances. from measuring leads, contact resistance and measured resistance. To reduce this effect, the precision ohmmeter has four terminals, called Kelvin contacts. Two terminals carry the current from and to the meter, while the other two allow the meter to measure the voltage across the resistor. In this setting, the resources are connected in series with the resistance to be measured through the external terminal pair, while the second pair connects in parallel with the galvanometer measuring the voltage drop. With this type of meter, each voltage drop due to resistance of the first lead pair and their contact resistance is ignored by the meter. This four-terminal measurement technique is called Kelvin sensing, after William Thomson, Lord Kelvin, who discovered the Kelvin bridge in 1861 to measure very low resistance. Four-terminal sensing methods can also be used to perform accurate measurements of low resistance.
See also
- Megohmmeter
- Ammeter
- Multimeter
- Measurement instruments
- Electronic test equipment
- Electronics
- Electrical circuit
- Electronic topic list
- Series and parallel circuits
- Galvanometer
- Rheochord
References
External links
- Measurement Circuit DC chapter of Lesson In Electrical Vol 1 DC free ebook and Lessons in Circuit Electrical series.
Source of the article : Wikipedia
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