Surges are an everyday occurrence in the real world and can have major negative effects on electronic devices. These effects range from data corruption, permanent damage to a device and in some cases even fire. Surges can occur for a variety of reasons but the most common causes of electrical surges are:
- Electrical switching of appliances such as refrigerators, heaters and air conditioners
- Faulty wiring and short circuits
- Lightning strikes
There are many components and devices on the market that are designed to protect devices against electrical surges that occur along either power or signal lines. These devices are collectively known as surge protection devices (or surge suppressors/arrestors) and are designed to limit the voltage supplied to an electrical device by either blocking or shorting to ground any unwanted voltages above a safe threshold. This is known as the clamping voltage but it is not the only characteristic to consider when selecting a surge protection device for your product.
This 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 will give protection but can sometimes result in a shorter life expectancy for the device.
Maximum Continuous Operating Voltage (MCOV)
This is the maximum RMS voltage that may be applied continuously between terminals of a surge protector.
Maximum Voltage Rating
As the name suggests, this refers to the absolute maximum voltage spike a surge protection device can withstand before a complete failure takes place.
Many different surge protection devices differ in the above characteristics and are thus more suitable for certain applications. Below is a brief description of some of the more common surge protection devices.
Transient Voltage Suppression Diode (TVS) or Trans orb
Transient voltage suppression diodes are also known as silicon avalanche diodes (SAD). They are a type of Zener diode that can limit voltage spikes. TVS diodes have a fast limiting action but have a relatively low energy absorbing capacity so are more commonly used in high-speed but low power circuits (such as in data communications). The life expectancy of a transient suppression diode is exceptionally long provided that impulses remain within the device’s rating.
Metal Oxide Varistor (MOV)
A metal oxide varistor is essentially a variable resistor. A MOV can conduct large currents when presented with a surge above its rated voltage, typically 3 to 4 times the normal circuit voltage. MOV’s have a finite life expectancy and degrade when exposed to large transients, or many smaller transients. As degradation occurs, a metal oxide varistor’s triggering voltage continues to fall. MOV’s are commonly connected in series with a thermal fuse so that the fuse disconnects before catastrophic failure can occur.
Gas Discharge Tube (GDT) and Spark Gaps
Gas discharge tubes and other spark gap devices act as transient suppression devices by conducting electrical currents to the ground, effectively creating a short circuit. In the case of a high-voltage spike, the normally non-conductive gasses (or air in the case of a simple spark gap with exposed electrodes) become ionized allowing for electrical current to be conducted through the gap between the terminals of the device. GDTs take a relatively long time to trigger compared to other transient voltage suppressors. It is not uncommon for a GDT or sparks gap to allow pulses of 500V or more to pass through unsuppressed before the current is conducted to the ground via the ionized gas/air between electrodes. Gas Discharge Tubes are more commonly used in slower rise time surge transients such as an AC mains surge.
Thyristor Surge Protection Device (TSPD)
Thyristor surge protection devices (sometimes referred to as Silicon Controlled Rectifier or SCR) are related to transient voltage suppression diodes though they can be viewed as having characteristics similar to a spark gap or gas discharge tube but can operate must faster. After triggering, the low clamping voltage allows large currents to surge to flow while limiting heat dissipation in the device.
When designing a product it is important to choose the right surge protection device. In many cases, several components will be required to be used in conjunction with each other to ensure your device is protected against electrical surge phenomena.
A good place to start looking for surge components and other EMC related useful components is Wurth Elektronik.