Causes Of Relay Failure In Switching Energy Provides And The Treatments


In a switching energy provide, generally the relay contacts fail because of a short-circuit, however the product continues to work. In the long term, this damages the system, particularly as a result of it’s tough to seek out the failure. Relays have to be modeled appropriately, and contacts have to be chosen correctly to forestall such cases

All units require an influence supply to work effectively. Energy provides are units that convert electrical present from a supply into the voltage required to energy any equipment. An influence provide could be of two sorts—linear or switching. For the scope of this text, we can be specializing in switching energy provide and analysing sure failures which can be related to it.

Regardless of being dependable, cost-effective, and splendid for low energy purposes, linear energy provide has its disadvantages. Switching energy provide design was developed to beat these disadvantages and is relatively a current methodology. It makes use of switching units which can be turned on and off at excessive frequencies. That is why an digital change is likely one of the most basic elements of a switching energy provide circuit. The 2 mostly used switching units are the bipolar NPN transistor and the N-channel MOSFET. Moreover, whereas the switching system is in its non-conduction mode, the switching energy provide makes use of elements akin to inductors and capacitors to produce energy.

Efficiency indices for switching energy provide

Energy issue

The facility issue of {an electrical} or digital system is the ratio of the facility it attracts from the primary provide to the facility it truly consumes. A gadget with an influence issue of 1.0 consumes the entire vitality it attracts. It’s doable to attain a unity energy issue, however many units don’t.

Fig. 1: Block diagram of a switched-mode AC/DC energy provide (Credit score: www.monolithicpower.com)

Within the case of digital units, akin to PCs and chargers which can be pushed by switching energy sources, the facility issue could also be low as a result of the system pulls present in a non-sinusoidal waveform. That is generally known as the ‘distortion’ energy issue, and it is vitally vital to rectify it. If that isn’t carried out, the distorted energy issue could cause undesirable harmonics.

Energy issue correction

In an ideal system, 100% of the facility drawn from the AC mains is put to good use. Nevertheless, such a situation is simply too good to be true. As a result of low energy issue, a number of the vitality from the AC supply isn’t utilised and is misplaced. This will increase value and contributes to international warming too. Therefore, energy issue correction (PFC) is completed to extend the facility issue.

PFC could be carried out in two methods:

First, passive PFC, which improves PF by utilising passive filters to filter out harmonics. That is generally utilized in low-power purposes. Nevertheless, it’s inadequate in high-power purposes.

Second, lively PFC, the place a switching converter is used to change the distorted wave right into a sine wave. As a result of the brand new sign’s sole harmonics are on the switching frequency, these could be simply filtered away. This can be a good strategy, but it surely provides to the design complexity.

Enter inrush present/Surge present. While you activate an influence supply or electrical tools, it generates a brief present known as inrush present. That is as a result of excessive preliminary present wanted to cost the capacitors, inductors, and transformers. The inrush present is also referred to as the switch-on surge or enter surge present. It is a vital efficiency indicator for switching energy provides.

Within the case of a switched-mode energy provide (SMPS), the output capacitor, which is generally massive in worth, can maintain an enormous quantity of cost that’s similar to a short-circuit when not energised. The switches within the SMPS hyperlink the enter supply to the empty capacitor when an vitality supply is connected to the SMPS. This creates a low impedance path for the inrush present to circulate to the capacitor. The voltage throughout the capacitor rises, however slowly decreases thereafter. That being stated, the inrush present is giant sufficient to break the switches within the provide circuit, and therefore must be designed as small as doable.

To regulate the inrush present, a ‘pre-charging’ circuit could also be used. The standard design is to attach a thermistor (NTC) in collection on the reside wire on the enter of the switching energy provide. For switching energy provide with the next energy, a relay is linked in parallel to the thermistor (NTC) on the similar time to cut back system loss and enhance reliability when the product works usually.

Allow us to now give attention to analysing the causes of contact short-circuit failure after incorporating the relay.

Discovering the fault

When the PFC circuit in Fig. 2 was utilized to the product, it was discovered that the inrush present exceeded the usual worth. This consequence was fully completely different from the theoretical calculation. Based on Fig. 2, the units that have an effect on the enter inrush present are primarily the thermistor RT1 and relay K1.

Fig. 2: PFC circuit
Fig. 2: PFC circuit

After testing the thermistor and relay it was noticed that the thermistor resistance is regular, and the usually open contact of the relay is taken in (sucked) with out a energy provide. Which means the relay is the irregular system right here.

Fig. 3: Conduction waveform of relay contact (the yellow waveform is the enter voltage and the blue one is the relay contact present)

When the relay isn’t powered on and the usually open contact has been closed, the relay contact is short-circuited. There are typically three prospects accounting for this failure.

Case 1. The operation frequency of the relay is simply too excessive, and the instances have exceeded the switching instances that the relay can bear

Case 2. The encircling temperature of the relay is simply too excessive

Case 3. The surge present flowing via the relay is simply too giant

A bad contact weld
Fig. 4: A foul contact weld (Credit score: https://www.pickeringtest.com)

After analysing every of those three circumstances, it was observed that the failure was not brought on by the excessive operation frequency of the relay or the excessive surrounding temperature. Therefore, circumstances 1 and a couple of could possibly be dominated out. The failure was brought on by the big surge present flowing via the relay.

The measured contact present of relay K1 is proven in Fig. 3. From the determine, it may be discovered that relay K1 has its peak contact present inside a time frame after conduction. Because the most present the relay can stand up to is 10A (in line with the specification), and the surge present is far more than 10A, it might harm the relay contacts and result in a short-circuit.

 relay with silver contacts
Fig. 5: PT570730 relay with silver contacts (Credit score: element14)

The reason for surge present

Troubleshooting revealed that the explanation for the relay contact short-circuit was that the surge present flowing via the relay was too giant. Within the circuit in Fig. 2, what was responsible for the surge present throughout relay attraction? There are three prospects.

  1. If the DM inductor of the π filter is saturated, it is going to lose the suppression of present
  2. If the capacitance of filter capacitor C1 is giant, the distortion within the present pulse can even be giant
  3. If the present sampling resistance of PFC is small, the enter present reaches the clamp level when the PFC begins working

    Fig. 6: Finder 60 series general-purpose relay with AgNi contacts (Credit: Finder)
    Fig. 6: Finder 60 collection general-purpose relay with AgNi contacts (Credit score: Finder)

Other than the surge present, there are different the reason why a relay would possibly fail, and it is very important handle them too. A few of these causes embody redundant objects contained in the relay, grime on the contact materials, improper course of construction, improper welding, contact ablation, adhesion, silver ion migration, and reed displacement brought on by exterior software.

Most of those failures are brought on by improper management of the relay manufacturing course of. Subsequently, it’s important for relay producers to enhance the manufacturing atmosphere, high quality management, and inspection techniques to forestall frequent relay failures.

 Hongfa relay with AgSnO2 contacts
Fig. 7: HF46F-G/012-HS1T Hongfa relay with AgSnO2 contacts (Credit score: Hongfa)

How you can overcome these issues?

Designers have to be extraordinarily cautious whereas designing PFC circuits and should be sure to optimise the parameters given above. For instance, choosing the DM inductor with a bigger saturation present and lowering the capacitance of the primary capacitor within the π filter can be helpful. Growing the PFC present sampling resistance and lowering the PFC clamping present would additionally assist.

Along with these, the choice of the relay can be important. Relay contacts are made of various materials, every suited to completely different software. Designers should first select the relay in line with the precise use necessities, after which fastidiously decide the required useful and bodily traits whereas choosing the relay to keep away from failure throughout use.

general-purpose relay with AgCdO contacts
Fig. 8: T92S7A12-240 general-purpose relay with AgCdO contacts (Credit score: element14)

Relay contact supplies

Listed below are some completely different relay contact supplies and their properties.

Wonderful silver. Amongst all metals, effective silver has the perfect electrical and thermal traits and so is the perfect all-around materials out there. When utilized in relays, it will probably deal with low to medium masses, AC and DC, resistive and inductive masses like solenoid valves, followers, and heaters, however not giant inrush currents.

The PT570730 relay is a miniature industrial relay for general-purpose purposes. It has silver contacts and is utilized in management and automation purposes. Different purposes of this relay embody shopper electronics, and automotive and energy administration purposes.

Silver nickel (AgNi). It’s usually used within the context of resistive load and present stability. The modest quantity of nickel (10%) is especially used to mechanically harden the silver and increase the resistance of the contact, making it extra sturdy underneath larger electrical masses. It’s applicable for resistive masses on the contact’s full nominal present score in addition to different masses with decrease load currents.

Silver nickel is an economical and high-performing, general-purpose materials that’s often used as the usual for a lot of energy relays. When silver nickel contacts are gold plated, they’re appropriate for dry masses, measuring and switching circuits.

The Finder 60 collection general-purpose relay is an efficient instance of relays with AgNi contacts. It has gold-plated AgNi contacts and is appropriate for industrial and shipyard purposes, and highway and tunnel lighting. It provides a lockable check button with a mechanical flag indicator.

relay with silver tin indium oxide contacts
Fig. 9: T9AS1D14-24 relay with silver tin indium oxide contacts (Credit score: element14)

Silver tin oxide (AgSnO2). AgSnO2 is utilized in purposes that may produce surge present, akin to purposes with a capacitive load, inductive load, motor load, and so on. It’s properly suited to larger masses, notably for prime inrush currents with quick rise instances. Silver tin oxide is extensively used to exchange the extra hazardous silver cadmium oxide as a relay contact materials.

Hongfa relay HF46F-G collection is a good instance of a relay that introduces a distinction in its contact materials in several purposes. The relay contact materials given within the specification is split into two supplies: AgSnO2 and AgNi. That’s, the contact materials of HF46F-G/XXT (with T) is AgSnO2, whereas the contact materials of HF46F-G/XX (with out T) is AgNi.

Silver cadmium oxide (AgCdO). Silver cadmium oxide contacts have lengthy been utilised to change high-energy, arc-producing masses. They’re much less electrically conductive than effective silver contacts, however they’ve higher materials switch and arcing resistance. They do have a considerably larger contact meeting warmth and the next interface resistance between mated contacts.

Important considerationAn instance of a relay that makes use of AgCdO contacts can be the T92S7A12-240 general-purpose relay. It’s a high-power relay with a wash-tight, tape-sealed plastic field enclosure. The T92-series is designed to regulate compressor masses and has a printed circuit board mount, printed circuit board terminal, non-latching coil, and 240V AC coil voltage. It may be utilized in HVAC purposes.

Silver tin indium oxide. Though silver tin indium oxide connections are tougher to return by, they’re extra proof against arc erosion and welding than silver cadmium oxide contacts. They’re, nevertheless, much less electrically conductive and tougher to work with than silver cadmium oxide connections. Additionally, since they’re dearer than silver cadmium oxide for the time being, many relay customers prohibit their use to incandescent lamp masses and capacitors with excessive inrush present throughout contact bounce.

The T9AS1D14-24 relay, which is properly suited to energy administration and HVAC purposes, has silver tin indium oxide contacts. It has an possibility for load connections by way of 6.35mm terminals.


This writeup is impressed by an article authored by the content material staff at Mornsun. The co-author, Aaryaa Padhyegurjar, is an Trade 4.0 fanatic at EFY with a eager curiosity in innovation and analysis.