Metal Oxide Varistor (MOV)

Was checking about transient suppressors, whether it be spikes or surges and one device that came to mind was TVS diode. The other alternatives are spark gaps and MOV. Before going other let us get clarified what basically a transient is. A transient is nothing but a high voltage compared to normal operating voltage of a circuit which can be termed spike or surge based on the time period it is present. Termed spike if it is for lesser time. A surge if it exists for a long duration.

When a transient hits a circuit, it may lead to catastrophic events like electronics component damage and in some cases burning. So, as a designer it is must to take care in design phase itself to include protection for transients. Protection can be done either by using MOV, spark gap or a TVS diode.

What is the basic difference between TVS diode, MOV and spark gap? 

TVS diode has low clamping voltages. So, it is suitable for a circuit and helps circuit not to get damaged by over voltages. MOV has higher clamping voltage and is able to carry more current than a TVS diode. We have discussed about spark gaps previously in the blog. Just reminding again, a spark gap has very high breakdown voltage compared to MOV and TVS diode.

What is MOV?

MOV, short form for Metal oxide varistor is a nonlinear, voltage dependent device. Varistor sounds like a variation of resistance but in reality it is non ohmic variable resistor which is different from potentiometer (ohmic variation). MOV acts as transient voltage suppressor and is connected in parallel in a circuit at the input. Under normal input voltage conditions, MOV does not conduct. When transient occurs and input voltage reaches above breakdown voltage of MOV, it clamps voltage to safe level and shunts down the current. The power dissipated as heat in the device. MOV provides high resistance at a low voltage and low resistance at high voltage.

Symbol of Varistor:

MOV view:

How to increase the power handling capability of MOV?

MOVs must be connected in parallel to increase the power handling capacity. To increase the voltage handling capability, MOV must be connected in series. 

What is the internal configuration of MOV?

The configuration is like metal oxides between electrodes. These metal oxides from a back-to-back diodes and internally there are number of back-to-back diodes in parallel. 

Ratings of MOV:

• Energy measured in Joules
• Breakdown/Clamping Voltage
• Capacitance Response time

Disadvantages of MOV:

• Cannot handle high energies like lightning
• Too much energy burns MOVs
• Too many spikes degrades the MOV operation
• Cannot control over current condition in a circuit
• Higher clamping voltage, so, because of this lower over voltage conditions cannot be protected. For this, separate protection mechanism must be used which proves costly

Selection of MOV:

Take the case of a power line operating at 230 V AC. In this case, if you want to use a MOV, the best value if to use MOV which has breakdown between 260-275 V. Keep in mind that a power lines sometimes can have spike in the range of kV.

Advantages of MOV:

• Cheap solution for protection
• Easy integration to circuit
• Can conduct more current compared to TVS diodes
• Higher clamping voltage
• Response time is less compared to Spark Gap