Sam Davis Banner
Part 4: Power Applications
Chapter 14: Circuit Protection Devices (Preview)

Several types of devices are employed in electronic systems to protect against voltage transients, power surges, and excessive voltage or current. Varistors is one of those devices that protect against excessive transient voltages by shunting the current created by the excessive voltage away from sensitive components.

A varistor’s resistance varies with the applied voltage. It has a nonlinear, non-ohmic current–voltage characteristic similar to a diode, except that it has the same characteristic for both directions of traversing current. At low voltage it has a high electrical resistance that decreases as the voltage is raised. The most common type of varistor is the metal-oxide varistor (MOV). It is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs.

When a small or moderate voltage is applied across an MOV’s electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

Go back to view more!

A varistor remains non-conductive as a shunt-mode device during normal operation when the voltage across it remains well below its "clamping voltage", thus varistors are typically used for suppressing line voltage surges.

Varistors will eventually fail from not successfully limiting a very large surge from an event like a lightning strike, where the energy involved is many orders of magnitude greater than the varistor can handle. Follow-through current resulting from a strike may melt, burn, or even vaporize the varistor. This thermal runaway leads to the failure of dominant current paths under thermal stress when the energy in a transient pulse exceeds the manufacture's "Absolute Maximum Ratings". You can reduce the probability of catastrophic failure by increasing the rating, either by using a single varistor of higher rating or by connecting more devices in parallel.

MOV
The most common type of varistor is the metal-oxide varistor (MOV) that contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides sandwiched between two electrodes). The boundary between each grain and its neighbor forms a diode-like junction, which allows current to flow in only one direction. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages. Fig. 14-1 shows the current vs. voltage of a typical MOV compared with an SiC diode.

As shown in Fig. 14-1, the varistor has symmetrical bi-directional characteristics. It operates in both directions (quadrant Ι and ΙΙΙ) of a sinusoidal waveform in a manner similar to two zener diodes connected back-to-back. When not conducting, the I-V curve has a linear relationship as the current flowing through the varistor remains constant and low at only a few microamperes of “leakage.” This occurs because its high resistance acts as an open circuit and remains constant until the voltage across the varistor (either polarity) reaches its particular rated voltage.

An incorrectly specified MOV may allow frequent lower power swells to degrade its capacity. In this condition the varistor is not visibly damaged and outwardly appears functional, but no longer offers protection. Eventually, it proceeds into a shorted circuit condition as the energy discharges create a conductive channel through the oxides.

The main parameter affecting varistor life expectancy is its energy (Joule) rating. Increasing the energy rating raises the number of (defined maximum size) transient pulses that it can accommodate exponentially as well as the cumulative sum of energy from clamping lesser pulses. As these pulses occur, the "clamping voltage" it provides during each event decreases, and a varistor is typically deemed to be functionally degraded when its "clamping voltage" has changed by 10%. Manufacturer's life-expectancy charts relate current, severity and number of transients to make failure predictions based on the total energy dissipated over the life of the part.

Let-through voltage specifies what spike voltage will cause the protective components inside a surge protector to divert unwanted energy from the protected line. A lower clamping voltage indicates better protection, but can sometimes result in a shorter life expectancy for the overall protective system. The lowest three levels of protection defined in the UL rating are 330 V, 400 V and 500 V. The standard let-through voltage for 120 V AC devices is 330 V.

To continue reading, click here to download the complete book for free.

Featured Power Applications Assets
Sponsored by:
eBook: Analog Vol. 1
By: Bob Pease
DOWNLOAD THE BOOK
PFC Engineering Essentials
By: Sam Davis
VIEW NOW
eBook: Alternative Energy
By: Sam Davis
DOWNLOAD THE BOOK

Brought to you by: