Ohmmeter

An ohmmeter is an instrument with which the electrical resistance is measured. The colloquial term ohmmeter is misleading due to the same unit ohmmeter. Moreover, the unit is referred to as Ohm and not the physical quantity that is actually measured.

The resistance of an electrically conductive component at DC resulted primarily from its geometric shape and material properties ( electrical resistivity ) and leads to the resistor or DC resistance, in some cases also to a differential resistance.

When operating on AC voltage in addition inductance and capacitance of the device carrying an alternating current component ( reactance ) in the resistance. DC and AC component are combined into a complex impedance or electrical resistance.

A resistance meter is usually designed to measure the ohmic resistance, for example, of an unknown component or an electrical load. Simplified device for detecting an electrical connection are called continuity tester. On the device for further resistance measurement tasks referenced below.

Preferred methods of measurement

Current and voltage measuring

Digital Meters

In a digital multimeter to measure resistance, a constant current source installed, calibrated to a smooth power of ten value in mA or uA, so that the numerical value of the measured voltage exceeds the value of the resistance follows directly, without measuring the current. Only the point position and the unit symbol ( Ω, kOhm ) are switched to match the range in the device. The error limits are, depending on the manufacturer with ≤ 1% of measured value 1 ‰ of full scale.

Usually have a digital multimeter auto range and turn the current of the constant current source automatically to the appropriate measurement range. For use as continuity tester, they often have a switchable beep.

Analog Meters

Analog resistance measuring instruments consist of a pointer instrument with moving coil movement, an adjustable resistor and an auxiliary voltage source (battery); they have a highly non-linear scale on which the resistance value can be read directly in ohms or kilohms.

Because of the large, over all possible values ​​ranging measurement range 0 ... ∞ a reasonably accurate reading is possible only in the middle range of the scale. Some ohmmeter have a switchable measuring range, in order to read preferably in the mid-scale each others values.

Because of the power of a non-stabilized voltage source ( battery voltage due to aging of sinking ) is to calibrate the instrument before the measurement. For this purpose, in case of short circuit between the measuring terminals of an external accessible potentiometer ( part of the series resistor ) is adjusted so that = 0 is displayed. However, this change of principle and scaling. Even in the middle part of the scale, therefore, the typical error limits at about 10 % of the measured value.

Due to the disadvantages such as the complicated application, the larger measurement errors, more mechanically sensitive analog resistance measuring instruments have been almost completely replaced by digital multimeter to measure resistance in practical laboratory work.

Without special measuring device

In many cases, resistors without special device can be determined. On the DUT, the voltage drop and current are measured. From these two values ​​, the resistance is calculated according to Ohm's law. This method is generally not free from systematic error. The retroactivity deviations ( circuit influence error ) by the internal resistance of the ammeter or voltmeter can, however, exclude by a more sophisticated approach:

  • In the upper circuit, the voltage measured by the voltage drop across the flow meter is greater than the voltage across the resistor,
  • In the lower circuit, the current is measured by the current consumption of the voltage meter is greater than the current through the resistor,

Bridge circuit

There is a voltage compensation method, in which the voltage drop across the DUT is compared with a further drop across a known resistor. Measuring one of the voltages is adapted to the other; refer Wheatstone bridge.

Low Resistance Measurements

When measuring low resistances ( value < 1 Ω ) make contact resistance in the terminals as measured deviation noticeable; refer measuring resistor. This influence can be avoided by connecting four-wire (Kelvin connection) with terminals for the power supply and brought out separately and attached separately terminals for voltage measurement. Under the following conditions systematic errors are excluded:

If the current is negligibly small by the voltmeter

And the voltage loss in the terminals for the leads is negligible,

Results in the resistance value of

When power is supplied from a constant current source, the voltage drop has to the power terminals not control, and shall not be the cause of an error of measurement in appearance.

For an older analog measurement procedure, see below Thomson bridge.

A digital measurement method for measuring small resistances operates as follows:

Digital voltmeter form a display by comparing the measured voltage with a built-in reference voltage; see Digital measurement technology. For example, the dual-slope method, a message is generated in accordance with a voltage to be measured

This is a device constant.

The exact knowledge of can be waived if the reference voltage is also formed with a built- in four-wire technology reference resistor, see circuit.

The display is too proportional. The method is realized with a measuring range of 200 μΩ at a smallest increment 1 nw.

Measuring large resistances

When measuring large resistances ( reference value> 20 M ), the small size of the still current flowing at the usual small measured voltages to the problem. The measured voltage must be increased, but which is often only possible with insulation measuring instruments. These provide switchable measuring voltages from about 100 V. The voltage is limited by the dielectric strength or specified by testing requirements.

The main requirement of measuring very high resistances (giga - to tera ohms range) is the measurement on insulating materials (plastics, cable, film, etc.). At these bodies is to distinguish between

  • The surface resistance of the current flow along the surface and
  • The volume resistivity or volume resistivity across the surface, that is through with current flow through the body.

Because of the very small, due to external interference and leakage currents slightly verfälschbaren current levels ( to <1 pA) a shield technology ( Guard ) technique is required, which requires a third connection between the DUT and the instrument. This additional connection is ground or earth potential, and provides a consistent reference potential of the shielding, without being included in the current measurement ( for example, the guard rings ). Such guard rings surround, for example, the sockets for the test object, or the leads of the operational amplifier used for power amplification on the circuit board of the instrument.

To measure the insulation resistance of a planar object to be measured one works with

  • For measuring the volume resistivity ( Figure → ) is the measured by the body via the inner electrode to the reference potential flowing current. The counter electrode is located at the test voltage. The ring electrode is connected to reference potential, in this example ( Guard ); Therefore, there is no voltage to the inner electrode and there can be no current flow surface. In this context one also speaks of a guard ring capacitor.
  • To measure the surface resistivity along the surface of the ring to the inner electrode flowing current is measured. The ring electrode is so, at the test voltage. The counter electrode is connected to reference potential, or ground; Therefore, there is no voltage to the inner electrode, and no current can flow through the volume.

The protective resistor makes the arrangement of short-circuit proof.

The method is applicable to a range 1 TΩ at a relative accuracy of 1 % and a measuring range of 100 TΩ at a margin of error of 10%.

Measurement of differential resistances

In a rectangular diagram with linear split-axis, in which plotted, resulting in a component with ohmic behavior is a straight line through the origin; whose increase is clearly the resistance. Some components, especially semiconductor devices, as well as incandescent lamps, have a non-linear behavior: instead of a straight line results in a bent curve. The ratio here is at each current or voltage of each other; it is not a component tag more. We consider the change in voltage is due to a small change of current, and referred to their relationship as a differential resistance:

He is in the said chart the rise of the characteristic curve ( tangent to the curve ) in a given point (the operating point of the component or another determined amperage ).

The differential resistance is an important parameter of semiconductor diodes ( rectifiers, Zener diodes, light emitting diodes, laser diodes).

The circuit shown on the right is working with an oscilloscope in XY mode and is suitable for the direct determination of the differential resistance, without having to record a characteristic: With a constant current source an operating point is set on the measurement object. This DC current is superposed on an AC voltage source a small alternating current. The capacitor blocks from DC, but allows alternating current through. The coil shuts off AC power, has for DC but only about the wire resistance. The alternating voltage on the measured object is used for the Y deflection of the oscilloscope, the AC voltage as a measure of the alternating current to the X- deflection. One sees on the screen almost straight small portion of the characteristic curve around the operating point. The rise in the differential resistance determined.

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