Basics of Varistor
A varistor/voltage dependent resistor
(VDR) is a component which has a voltage – current characteristics that
is very much similar to that of a diode. This component is used to
protect electrical devices from high transient voltages. They are
planted in the devices in such a manner that it will short itself when a
high current is produced due to the high voltage. Thus the current
dependent components in the device will remain safe from the sudden
surge.
Metal Oxide Varistor – Basics
MOV is the most commonly used type of
varistor. It is called so as the component is made from a mixture of
zinc oxide and other metal oxides like cobalt, manganese and so on and
is kept intact between two electrodes which are basically metal plates.
MOV’s are the most used component to protect heavy devices from
transient voltages. A diode junction is formed between each border of
the grain and its immediate neighbour. Thus an MOV is basically a huge
number of diodes that are connected parallel to each other. They are
designed to be in the parallel mode as it will have better energy
handling ability. But, if the component is meant for providing better
voltage rating, it is better to connect them in series.
A reverse leakage current appears across
the diode junctions of each border when an external tiny voltage is
applied across the electrodes. The current produced will also be very
small. But, when a large voltage is applied across the electrodes, the
diode border junction breaks down as a result of the combination of
electron tunnelling and avalanche breakdown. Thus the device is said to
show a high level of non-linear voltage – current characteristics. From
the characteristics, it should also be noted that the component will
have low amount of resistance at high voltages and high resistance at
low voltages.
The only problem with this component is
that they cannot withstand the transient voltage more than the exceeded
rating. They tend to deteriorate after a certain level. If so, they will
have to be replaced at times. When they absorb the transient voltage
they tend to dissipate it as heat. When this process continues
repetitively for some time, the device begins to wear out due to the
excessive heat.
They can be connected in parallel for
increased energy-handling capabilities. MOVs can also be connected in
series to provide higher voltage ratings or to provide voltage rating
between the standard increments.
MOV Specifications
- Maximum working voltage is the maximum steady-state, DC voltage. In this case, the value of the typical leakage current will be lesser than a specified value.
- Varistor voltage
- Maximum clamping voltage is obtained when a certain pulse current is applied to the component to obtain a maximum peak voltage.
- Surge current
- Surge shift refers to the variation in voltage after a surge current is given.
- Energy absorption refers to the maximum energy that is dissipated for a certain waveform without many problems.
- Capacitance
- Leakage current
- Response time
- Maximum AC RMS voltage refers to the maximum amount of RMS voltage that can be delivered to the component.
Working of Metal Oxide Varistor (MOV)
- Working of Metal Oxide Varistor (MOV)
The working of a MOV is shown in the figure above.
The resistance of the MOV is very high.
First, let us consider the component to have an open-circuit as shown in
figure 1(a). The component starts conducting as soon as the voltage
across it reaches the threshold voltage. When it exceeds the threshold
voltage, the resistance in the MOV makes a huge drop and reaches zero.
This is shown in the figure 1(b). As the device has very small impedance
at this time due to the heavy voltage across it, all the current will
pass through the metal oxide varistor itself. The component has to be
connected in parallel to the load. The maximum voltage that will pass
through the load will be the sum of the voltage that appears across the
wiring and disconnect given for the device. The clamp voltage across the
MOV will also be added. After the transient voltage passes through the
component, the MOV will again wait for the next transient voltage. This
is shown in the figure 1(c).
MOV Performance
The varistor is mainly used to perform
as a line voltage surge suppressor. The device does not conduct when the
voltage across it is below the clamping voltage. But, if a high surge
(lighting) that is higher in rate that a varistor can handle is passed
through it, the component will not perform. The resulting current will
be so high that it will damage the MOV.
The performance of the varistor will
slow down with time even if small surges pass through it. The life of a
MOV will be explained through the manufacturers chart. The chart will
have graphs and readings between the current, time and also the number
of transient pulses that passes through the varistor.
Another main reason that affects the
performance of a MOV is the energy rating. When there is an increase in
the energy rating, there will be an exponential change in the life of
the varistor. Thus, there will be a change in the transient pulses that
the device can manage. This increases the clamping voltage when each
transient breaks down.
The performance can be increased by connecting more varistors in parallel. An increase in rating will also help in the process.
One of the best features of the MOV is
its response time. The spikes are shorted through the device within
nanoseconds. But the response time can be affected by the mounting
design method and inductance of component leads.
