The OHMMETER is an instrument which measures resistance of a quantity. Resistance in the electrical sense means the opposition offered by a substance to the current flow in the device. Every device has a resistance, it may be large or small and it increases with temperature for conductors, however for semiconducting devices the reverse is true.
There are many types of ohmmeters available such as
- Series ohmmeter.
- Shunt ohmmeter.
- Multi range ohmmeter.
Working Principle of Ohmmeter
The instrument is connected with a battery, a series adjustable resistor and an instrument which gives the reading. The resistance to be measured is connected at terminal ob. When the circuit is completed by connecting output resistance, the circuit current flows and so the deflection is measured.
When the resistance to be measured is very high then current in the circuit will be very small and the reading of that instrument is assumed to be maximum resistance to be measured.
When resistance to be measured is zero then the instrument reading is set to zero position which gives zero resistance.
This type of movement is used in DC measuring instruments. The main principle in these types of instruments is that when a current carrying coil which is placed in a magnetic field, it feels a force and that force can deflect the pointer of a meter and we get the reading in the instrument.
This type of instruments consists of a permanent magnet and a coil which carries current and is placed in between them. The coil may be of rectangular or circular in shape. The iron core is used to provide flux of low reluctance so it produces high intensity magnetic field. Due to high intensity magnetic fields the deflecting torque produced is of large value due to which sensitivity of the meter is also increased. Current which entered comes out of two control springs, one in the upper side and one in the lower side.
If the direction of current is reversed in these types of instruments, then torque direction will also be reversed so these types of instruments are applicable in DC measurements only. The deflecting torque is directly proportional to the deflection angle hence these types of instruments have the linear scale. To limit the deflection of the pointer we have to use damping which provides an equal and opposite force to the deflecting torque and hence the pointer comes to rest at a certain value.
The indication of the breeding is given by a mirror in which a beam of light is reflected on to the scale and hence deflection can be measured.
There are many advantages due to which we use D’Arsonval type instrument. They are-
- They have uniform scale.
- Effective eddy current damping.
- Low power consumption.
- No hysteresis loss.
- They are not affected by stray fields.
Owing to posses those major advantages we can use this type of instrument. However they suffer from drawbacks such as-
- It cannot be used in AC.
- Costlier compared to MI instruments.
- There may be error due to ageing of springs by which we may not get accurate result.
However in case of resistance measurement we go for DC measurement because of the advantages offered by PMMC instruments and we multiply that resistance by 1.6 to find out AC resistance, so these instruments are much widely used due to their advantages. The disadvantages offered by it are dominated by the advantages so they are used.
Series type Ohmmeter
The series type ohmmeter consists of a current limiting resistor R1, Zero adjusting resistor R2, EMF source E, Internal resistance of D’Arsonval movement Rm and the resistance to be measured R.
When there is no resistance to be measured, current drawn by the circuit will be maximum and the meter will show a deflection. By adjusting R2 the meter is adjusted to a full scale current value since the resistance will be zero at that time. The co-responding pointer indication is marked as zero. Again when the terminal AB is opened it provides very high resistance and hence almost zero current will flow through the circuit. In that case the pointer deflection is zero which is marked at very high value for resistance measurement. So a resistance between zeros to a very high value is marked and hence can be measured. So, when a resistance is to be measured, the current value will be somewhat less than the maximum and the deflection is recorded and accordingly resistance is measured. This method is good but it posses certain limitations such as the decrease in potential of the battery with its use so adjustment must be made for every use. The meter may not read zero when terminals are shorted, these types of problem may arise which is counteracted by the adjustable resistance connected in series with the battery.
Shunt type Ohmmeter
In this type of meters we have a battery source and an adjustable resistor is connected in series with the source. We have connected the meter in parallel to the resistance which is to be measured. There is a switch by the use of which we can on or off the circuit. The switch is opened when it is not in use. When the resistance to be measured is zero, the terminals A and F are shorted so the current through the meter will be zero. The zero position of the meter denotes the resistance to be zero. When the resistance connected is very high, then a small current will flow the terminal AF and hence full scale current is allowed to flow through the meter by adjusting the series resistance connected with the battery. So, full scale deflection measures very high resistance. When the resistance to be measured is connected between A and F, The pointer shows a deflection by which we can measure the resistance values.
In this case also, the battery problem may arise which can be counteracted by adjusting the resistance. The meter may have some error due to its repeated use also.
Multi range Ohmmeter
This instrument provides the reading up to a very wide range. In this case we have to select the range switch according to our requirement. An adjuster is provided so that we can adjust the initial reading to be zero. The resistance to be measured is connected in parallel to the meter. The meter is adjusted so that it shows full scale deflection when the terminals in which the resistance connected is full scale range through the range switch. When the resistance is zero or short circuit, there is no current flow through the meter and hence no deflection. Suppose we have to measure a resistance under 1 ohm, then the range switch is selected at 1 ohm range at first. Then that resistance is connected in parallel and the corresponding meter deflection is noted. For 1 ohm resistance it shows full scale deflection but for the resistance other than 1 ohm it shows a deflection which is less than the full load value and hence resistance can be measured. This is the most suitable method of all the ohmmeters as we can get accurate reading in this type of meters. So this meter is most widely used now days.