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Transient Protection

At some point, all circuit designers need to consider transient protection for a circuit.

Transients can appear almost anywhere, and can be found in homes, automobiles, and industrial plants. They can easily be conducted through power lines as a result of arcing contacts in switches, relays, motors, and circuit breakers.

A transient is usually caused by an inductive load, so when a mechanical contact is opened, the inductive energy seeks a path to continue flowing and will create that path by arcing and sparking. Voltage spikes may also be induced by a rapid buildup or decay of a magnetic field.

A transient can be considered an energy spike rather than a voltage spike, so that a transient response can be defined by the product of voltage, current, and time. For any protective device used, the longer its response time, the longer the connected equipment will be exposed to the surge. However, the response time for most protective devices is fast enough because it is in the order of a few nanoseconds, while voltage surges usually take around a few microseconds to reach their peak voltage. This should be enough to suppress the most damaging portion of the spike.

There are many solutions available that would address the problem of high voltage spikes, transients, and surges.

The basic types include shielded cable and twisted wires, crowbar-type devices, filters, and Transient voltage Suppressors (TVS). Each one has limitations and various advantages and disadvantages in speed, power rating, and ability to handle voltage spikes and surges. As an embedded circuit designer would be mostly interested in simplicity of design and minimizing cost while providing the basic protection, only the TVS and the crowbar devices need to be considered here.

Transient voltage Suppressors (TVS) are semiconductor devices which provide protection against voltage and current transients by operating in the Avalanche mode and essentially clamps a voltage to a set threshold value. They usually have a large junction area to absorb large transients. The types of TVS devices are: Metal Oxide Varistor (MOV), the Zener Diode, and the TVS Diode.

TVS devices are noted for their fast response time and low impedance, and can suppress transients in one or both directions. A designer should consider the failure characteristics for these devices: A Metal Oxide Varistor (MOV) will usually short when it fails, while the zener diode and TVS diode would typically open.

The MOV and zener diodes are lower in cost, but they have a slower response times as compared to TVS diodes. TVS diodes are more expensive, but offer much more reliability and predictable behavior. Since these devices must absorb most of the energy in a voltage transient or surge instead of diverting it somewhere else, the power rating is very limited on these devices, and one might be better off considering a crowbar device, where the device would turn on like a switch when a certain threshold voltage is exceeded. Since a switch absorbs very little energy, it is then diverted somewhere else. These devices go back the OFF state when the voltage and/or current of the transient has passed or has been reduced.

The three types of crowbar devices are: spark gaps, gas discharge tubes, and thyristors (SCRs).

A designer can minimize the cost here again while maintaining a fast response time by implementing a spark gap solution. They can either be a stand-alone device or fabricated as part of the conductor pattern of the PCB, which would be a really cheap solution. However there are drawbacks to this, in that performance can degrade over time and there is no accurate control over breakdown characteristics.

Thyristors have several advantages with their four-layer solid-state construction in that they are reliable and won’t degrade over time. However, they have a few problems too, because they can be difficult to turn off, and have a slower response time as compared to TVS devices. In many applications, a combination of a crowbar and a TVS device might work out the best. The TVS device would provide the reliable low-voltage protection needed with a fast response time while the crowbar device would handle the high voltage or high currents in surges with their ability to dissipate energy.