Chapter 11 - Transient Suppression

Transient voltage is a sudden or momentary over voltage condition. Transient suppression should be provided for all critical electronic and electromechanical controls as well as for electronic sensors which extend outdoors. Several things can be done to minimize transients that might get into a control system. One of the first things is to provide suppression at the breaker box or at the front end of a controller to protect against surges on the AC supply. Devices for this include varistors, gas diodes, transorbs, and combinations of these. The energy rating of these devices, usually given in joules, indicates how much energy they can absorb or shunt to the ground. For a given price, a higher rating would be preferred. Some devices are designed for installation in a breaker box, others may be mounted in a control box, and others are designed to be mounted on or near electronic circuit boards. It is also helpful to keep low-voltage sensor cables and control wires separated by several inches from AC power lines to avoid transients being induced into the low-voltage wires by motor starts, solenoid operation, etc.

The key points where protection is needed are:

• On the power line before it reaches the controller,
• On cables coming from sensors which extend to the outside of the building, and
• On communication lines going into or out of a building to a controller or input/output board in a building.

Many electronic controllers have built-in transient suppression for external sensors, incoming and outgoing communications lives, and their power supplies. Packaged transient suppression devices are readily available at reasonable cost for both power line and low-voltage applications. The cost and effect of protecting controllers is well worth the effort compared to dealing with random and unexpected damage that comes without protection.

Some key definitions

A varistor is an electronic component used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. The most common type of varistor is the Metal Oxide Varistor (MOV). MOVs react very quickly to electrical surges but have the tendency to allow the voltage between power lines to get further apart.

A gas diode is also known as a 'glow lamp' which is a glass envelop filled with gas which contains two or more electrodes. They are cold-cathode devices which require no heater or filament. Glow lamps produce a very interesting, mild, beautiful colorful glow with the color of the glow dependent on the gas combination used. Pure neon produces an orange/red glow while pure argon produces a purple glow. Diodes are used to prevent reverse current flow in circuitry.

Transorbs are components that carry current after a certain voltage is exceeded. This is called the trip voltage. The transorb keeps the voltage between the two lines at a set voltage and won't allow it to go any higher. This prevents your appliances from being damaged by the application of too great a voltage at their inputs. Transorbs can absorb a lot of current but turn on more slowly than MOVs.