The days of designing for one’s local market only are now gone. Today’s world economy demands that products be developed for different global areas. Each region has its own unique set of requirements, agency approvals, energy management, and environmental issues. Engineers are faced with producing designs that can be used in various markets while keeping costs under control.

Relays are one of the main interface methods between digital controls and power switching. Producing one control board that will meet every market’s needs often is not cost effective. Currently the European market has some of the most stringent design and safety requirements in the world. A product built to these standards meets virtually every other markets’ safety requirements. Some components in Europe must now meet a glow-wire test, where a wire is heated to 850 DegC and placed next to the plastic of the component. The plastic must self-extinguish in 5 seconds. A different and more expensive grade of plastic material must be used, thus adding to the component’s cost.

Other requirements might include higher CTI (comparative tracking index) ratings, increased spacing between coil and contacts, or an elevated insulation system rating. To meet these specifications, changes in the relay’s material and structure must occur, both adding to cost.

Relay manufacturers have responded to this demanding market with enhanced products in the same footprint. Designing “one package” with numerous characteristics will aid in meeting each regional need. A trend to standardize board design for local consumption is underway. One layout helps to reduce board and assembly costs. Changes in certain cost-sensitive components are being made, depending on end market requirements. This allows designers to meet local safety and design requirements while keeping costs under control.

Coil and contacts

Enlarge this picture Fig. 1. An illustration of PWM when the steady state is applied to the coil of a 12 V relay. When the relay’s hold level is 9 VDC the amplitude would be 18 VDC, since it is on for only 50 percent of the time.

Energized relay coils generate heat. For regulatory and electrical life reasons, OEMs seek to keep their total PC-board heat below a set level. New methods to reduce the heat are always being sought. Designers are faced with the challenge of keeping their power supply costs down. Designs with a single relay don’t typically call for a reduction in coil power, but for a board with multiple relays, consideration has to be given to the accumulation of power required.

Relay input methods are varied, ranging from batteries, to regulated and unregulated power supplies, and IC drivers. The regulated power supply source and IC driver are the most reliable methods used. IC drivers and discrete semiconductors are very reliable, but present their own issues. When diode protection of the driver is implemented, an increase in relay release time occurs. The resulting slow down in release time may not break minor tack welds. Solutions to this situation include using a diode with a higher PIV, or putting a diode in series with a Zener diode. On occasion, the driver may not drop down to zero volts, thus a 3.3 VDC to 5 VDC relay may not reach its dropout voltage. When this occurs, either the relay manufacturer must shift their product to the new dropout range, or use a larger power-supply voltage and a relay with a higher coil voltage.

The main concern on rectified power sources is the ripple factor. This must be low enough to not cause relay chatter or dramatic contact resistance changes. Unregulated power supplies run the risk of not delivering the correct operate and release voltages for the relay. This issue can be resolved by early discussions with the relay supplier, and by letting them know the unregulated range. The relay can be designed to assure that the operate range will match the power supply range.

In order to address the issues of reducing coil power for energy efficiency and temperature rise, many OEMs are using PWM (Pulse Width Modulation) methods to achieve their goal. (See Fig. 1.) The typical method involves energizing the coil with a short-term over voltage to engage the relay, and then reducing the effective voltage (thus power) to a much lower level. For example, a 12 VDC nominal relay coil may be energized with 18 VDC for 100 mS, and then reduced to a nominal of 8 VDC or 9 VDC for the balance of the cycle. This cycle is repeated while the relay is activated. The 8 VDC to 9 VDC is the effective average of the PWM duty cycle. The relay does not require the full nominal level once the contacts are in their normally closed condition. As long as the relay armature is fully seated, the effective power level is sufficient.

PWM has three characteristics that must be considered:

· Frequency/duty cycle of the input signal.

· Release time of the relay.

· Proper seating of the relay armature.

By working closely with the relay suppliers, solutions can be achieved. Those include:

· Changing the PWM frequency, making it higher or lower to reduce the chance of dynamic contact resistance due to vibration of PWM waveform.

· Changing the PWM duty cycle to assure that the effective voltage is above the relay maker’s minimum hold level.

· Producing a relay with higher-than–normal, closed-contact pressure.

For some relays, the duty-cycle frequency may be low, and the contacts can chatter or create dynamic contact resistance changes, which may create heat on the contacts. The release time is affected because the coil is no longer operating at nominal power, and will not have the inductive kickback needed to maintain the quick release. If this occurs, one solution is to change the effective PWM duty cycle, moving the hold voltage higher.

Contact loads

Enlarge this picture Fig. 2. The top sine wave is the output. This shows the delay time of the relay, illustrating its design to switch close to the zero-crossing point. The design incorporates the relay’s operate time, in this case close to 1/4 cycle, as an example. The bottom zero-cross exhibits when the contact is actually made, which is as close to zero as possible.

As part of the effort to reduce the number of relay variations used, OEMs are looking to have a single relay for many applications. The loads, which a relay must switch, can vary anywhere from signal levels, to purely resistive (heating elements), solenoids, lamp loads, ballast loads, to highly inductive motor loads. No matter which load is being switched, the actual application’s conditions, as they affect the relay, must be thoroughly tested. Design engineers should discuss with relay manufacturers the true load profile of what is being switched by the relay. The load profile is an exact current vs. time trace of the load. Also, the load profile permits the relay maker to identify the best product for the application. Design engineers should share with the relay manufacturer the load profile of every application. Reducing the choices down to one relay might not be possible, but a reduction of variations is quite feasible, whenever this information is shared.

OEMs are now designing circuits to get as close as they can to zero-cross load-switching, thus effectively increasing the life of the relay. (See Fig. 2.) Most power relays, other than automotive, are typically switching AC loads, which normally consist of random firing. As a result, the contacts can switch at any point of the AC wave. At the zero-cross point, the contacts switch virtually zero current, causing little to no degradation on the relay contacts. This is accomplished by determining the relay’s average operating speed, then building a circuit that will activate the relay to have contact closure as close as possible to the zero-cross point. This technique has been used successfully for many years in industrial grade solid-state relays.

Designers must also be aware that the load interconnect must be designed correctly to avoid heat damage. Solder pads must be wide enough for the solder joints to be 100 percent covered. Proper quick-connect terminals should be specified and crimped correctly.

Environmental issues

Enlarge this picture

An important issue concerning RoHS is new solder profiles. Relay manufacturers must produce components capable of meeting higher temperatures for exposure to longer heat times.

Most of the major contact makers in the world have spent a great deal of time trying to develop materials that perform as well as silver cadmium-oxide and still meet cost targets. To date this challenge has not been met. (Directive 91/338/EEC amending Directive 76/769/EEC relating to restrictions on the marketing and use of certain dangerous substances and preparations states “cadmium plating is viewed as being permitted for electrical contacts in all the WEEE categories to which the RoHS Regulations apply.” This exemption continues through 2009.)

Relay manufacturers are still searching for the cadmium of the future, a low-cost material that helps prohibit welding on the solenoid/motor inrush, while still carrying the current without reducing electrical life.

Today’s global economy continues to challenge designers as they compete and attempt to excel in this marketplace. It is important to keep in mind that global relay manufacturers also face new technologies, laws/regulations, and expectations every day. Early in the design process, engineers should take advantage of the relay manufacturer’s experience and knowledge and share with them their load profiles, unique issues and specific needs. Those manufacturers can offer unique solutions to help a company’s products be more reliable and cost effective.

For more information email: components@omron.com