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Surge Resistors - are you keeping your finger on the pulse..?

What is a surge resistor?

Surge withstanding resistors (also known as pulse withstanding / overload resistors) are very useful when considering circuit applications which will be exposed to a single pulse, continuous pulses or must withstand lightning strikes.

Also, the overload capability of a resistor can be an important consideration for designs which typically require a higher current demand during start up.

Why are surge resistors useful?

When designing a circuit which may be subject to a transient event, which exceeds the continuous power or voltage rating of the resistor, it’s possible to calculate whether the resistor will be able to withstand the event.  

This is particularly useful, as designers can utilise resistors which are purposely built, smaller in physical size and thus more cost-effective. 

Many resistor technologies including wirewound, thick film, thin film, carbon film & composition lend themselves well to surge / pulse requirements and various packaging options are available for example; through-hole, SMD, MELF & Chassis Mount.

Picking the right surge resistor

An infinite range of user specific demands are conceivable when dealing with surge / pulse / overload requirements and therefore the most important aspects when selecting the correct device are; defining the pulse itself, understanding environmental conditions, considering space constraints and preferred mounting configuration.

Manufacturers will take these core requirements and use the information to calculate the resistive / substrate material, process and trimming methods to prevent current crowding in the device and eliminate hot-spots in the resistor, ensuring it meets the user’s needs.

This is one of the few examples, when a custom (optimised) solution, can be far more cost-effective than a standard device, as the manufacturer doesn’t vastly exceed the requirements by default.

An added complexity can be presented when the user requires both a resistor to withstand a pulse / surge requirement, but also to fuse safely (open circuit, without emitting any debris), under a fault condition, i.e. mains voltage, potentially removing the need for an additional fuse. 

To validate calculations, samples are suggested to be tested before determining suitability.  DMTL’s manufacturers understand the importance of this and will, normally, issue samples free of charge.

What can DMTL offer?

DMTL have helped users on numerous occasions to develop such devices. With decades of experience, DMTL are on hand to guide you through the selection process and offer design assistance, to ensure you find the right product. 

To further help in this process, DMTL’s manufacturers, Viking, Firstohm & HTR, have produced a cross reference guide for the other major manufacturers of these types of resistor - Vishay, KOA, IRC, Welwyn, Ohmite, etc., which highlights DMTL’s breadth of offering for surge withstanding resistors.

More info 

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