An ohmmeter is a simple test instrument for measuring electrical resistance. A portable electronic measuring device used to measure relatively small values of electrical resistance in a circuit. The unit of resistance is the ohm. Resistance is one of the main properties of an electrical circuit and measures the resistance to current flowing through the circuit. The ohmmeter shows the approximate resistance value. You can also use an ohmmeter to roughly check the condition of the capacitor.
It is one of the basic test tools along with voltmeter and ammeter. Ohmmeters form their own circuits and cannot be used in circuit assembly. To measure circuit resistance, turn off the power before starting the measurement. Most ohmmeters are designed to cover a wide range of resistance values, from very low to very high. A standard ohmmeter consists of an ammeter, a battery source, and some type of current-limiting resistor. The battery provides the necessary constant voltage.
Ohmmeter in Details
The ohmmeter is an instrument for measuring electrical resistance , whose unit of measurement is the Ohm (Ω).
To measure resistance correctly see the photo below.
The value of a resistor inserted in a circuit cannot be measured because it is connected to other elements; nor can its value be measured if the circuit is operating, as there is a risk of damaging both the circuit and the ohmmeter. Another common use of the ohmmeter is to check the conduction of an electrical wire or a different conductor (such as a section of printed circuit for example) in the case of a suspected interruption. If there is conduction the ohmmeter gives a measurement close to zero; otherwise it goes close to infinity. Some types of ohmmeters are used for measurements of grounding resistance, insulation and continuity. Among these, the most famous is Megger , a brand of Evershed & Vignoles Ltd. London, whose patent dates back to 1903.
The ohmmeter has three parameters:
the precision class;
the flow rate;
The precision class is the ratio between the maximum absolute error detected for the entire scale, at a given flow rate, divided by the flow rate itself and multiplied by 100. The precision classes according to the CEI standards are the following:
0.05 – 0.1 – 0.2 – 0.3 – 0.5 – 1 – 1.5 – 2 – 2.5 – 3 – 5
They therefore range from high precision laboratory instruments to panel instruments.
The flow rate is the maximum value that can be measured by the instrument. In many instruments, various ranges can be chosen to have the best sensitivity, i.e. the minimum quantity that the instrument can measure. For example, if you want to measure a resistance, you start with the highest range, and then gradually choose the one that comes close to the value already measured; in this way the highest sensitivity is achieved.
Sensitivity is the minimum measurable quantity, not to be confused with accuracy which indicates the precision of the measurement. Resolution is synonymous with sensitivity, since it represents the minimum appreciable variation of the quantity under examination for the entire measurement range.
Description of some ohmmeters
Each ohmmeter has a different operating principle, based on this there is a different type of ohmmeter, for example:
inverted scale amperometric ohmmeter;
direct scale amperometric ohmmeter;
crossed coil ohmmeter;
electronic ohmmeter with display: either on scale or numerical;
digital ohmmeter for insulation measurements.
The first three ohmmeters in the list are the analog type.
Analog amperometric ohmmeter with inverted scale
We first illustrate the amperometric ohmmeter with inverted scale, which for years has been the most commonly used; we will limit ourselves to saying that the direct scale one has the ammeter in parallel with the resistance to be measured. To give a simple explanation we refer to the figure in which the principle diagram is represented.
It is made up of a battery, a microammeter, a variable protection resistor and the two test leads a and b .
The microammeter generally has a moving coil immersed in the field of a permanent magnet. The variable resistance, in addition to protecting the ammeter, also serves to position the pointer on zero by regulating the maximum current when the probes are in contact (zero Rx resistance).
When the resistance to be measured is inserted between the probes, a current passes through it (and into the microammeter) which corresponds to the value of the resistance on the scale.
The higher the value of the unknown resistance, the lower the current and the pointer moves to the left.
If there is no resistance between the tips, the circuit is interrupted and there is no current, in this case the pointer remains at the extreme left of the scale and the measured value corresponds to a theoretically infinite value.
Crossed coil ohmmeters
They are used for measurements of earthing resistance, insulation and continuity. They are called Megaohmmeters or meggers. So THEY are still used today for their cheapness, since modern digital ones are much more expensive. Furthermore, they have the characteristic of producing very high test voltages to measure a very large resistance (for example, the insulation resistance of electrical systems, or the insulation resistance of the circuits of an electric machine, or the earthing resistance, insulation and continuity).
To generate the test voltages inside them, there is a small direct current generator, with permanent magnets, controlled by a crank that the operator must rotate. The voltage reached varies depending on the type of use of these ohmmeters. There are ohmmeters with voltages of 100 V, 500 V, 1,000 V and also with voltages of over 2,000 V.