Every electronic designer in the land wants their new design to be as reliable and trouble free as possible, but choosing the right components to make this so, is far from easy.
Why is it that some components seem to last longer than others even though the datasheets appear to be the same? How can you tell which one’s are likely to be better in your particular application without extensive empirical testing?
The answer, more often than not, is that the designer is obliged to gain an understanding of the component construction and manufacturing methods in order to understand some of the key component differentiators.
This article sets out to lift the lid on the humble life of a reed relay to help inform those designers out there, considering a reed relay design in.
The moment of conception
Reed relays come into the world when a manufacturer decides to mount a reed switch onto a lead frame, surround it with a magnetising coil and encapsulate it in a conveniently sized package. How this is done is a matter of choice, but as with most apparently simple components, the story soon becomes complicated and has a direct influence on the component performance and lifecycle.
Crucial to any Reed Relay is the choice of reed switch. A reed switch generally comprises two ferro- magnetic blades encapsulated in a sealed glass surround. Each blade has a contact surface which combined with connections to the outside world via the lead frame, determines the switch ON resistance. As with any mechanical switch, the quality of the contact surface strongly influences the durability and consistency of the device over multiple operations and time as does the quality and consistency of the manufacturing process that brings the parts together.
Unlike electro-mechanical relays, the sealed nature of the reed switch means that oxidisation and material ingress is not of concern. That said, an encapsulated glass switch with blades separated by a few hundred microns is a sensitive component, so it is important that it is handled with care and mounted within the package without any stresses or strains that might otherwise undermine the glass seal integrity over time and environmental changes.
As with any mechanical switch, the small gap between the reed relay blades effectively determines the standoff voltage i.e. How much voltage can safely be applied across the contacts before the switch breaks down and, hold on to your hats, this is where things start to get more interesting..
Reed switches come in various shapes and sizes but for any given shape/size, reed switch manufacturers are generally obliged to grade their switches by what is known as the AT band, or Ampere Turns band to be more precise. Essentially the AT band grades the switch by measuring how much magnetic flux is needed to operate it properly. Since it is impossible to manufacture all switches with exactly the same gap, they are graded and supplied within certain operating AT bands.
Reed switch AT grading is an important feature in reed relay design, since the reed relay manufacturer is required to publish operating parameters. Reed switches graded with a higher AT band will by definition, have a larger gap between the contact surfaces compared to those graded with a lower AT band.
Higher AT banded devices are therefore LESS sensitive to an energising magnetic field and will naturally exhibit HIGHER standoff voltages compared to their more sensitive brethren. There is a further fundamentally more important benefit in higher AT graded switches..
Life expectancy and performance over time
Given that it takes more magnetic energy to activate a higher AT rated reed switch, it stands to reason that when power is removed, there is more mechanical energy available to open the switch. i.e. The spring energy in the blades. The bigger gap means the blades have to deflect further when opening or closing and since the blades are fixed with no mechanical joints, the restoration energy when opening will have a direct influence on the durability and quality of the opening switch action.
Reed relays are commonly used in ATE switching matrix arrangements which ultimately operate under software control with a variety of real-world signals being switched. As is often the case, accidental short duration overloads occur, real world spikes and unanticipated stray energy sources all contribute to put the reed relay switch contacts through their paces, often generating small micro arcs at the contact surface which can cause microscopic degradation to the material coatings.
Usually these occurrences are rare and the natural opening & closing of the switch over time will tend to self-heal such damage. That said, in demanding applications where system designers are pushing the switching capability envelopes, the ability of the reed switch to open properly under duress will be a function of the AT band. The higher it is, the more reliable it will be.
A small micro arc at the contact surface has the potential to micro weld the contacts together rendering the device useless unless sufficient switching force is available to overcome the micro weld. This is why higher AT rated reed switches have been proven to last several orders of time longer than their more sensitive counterparts when applied in demanding applications.
Not a Ctrl-Alt-Delete moment, rather a designer’s need to know and/or calculate how a reed relay might perform under certain conditions. A further advantage of higher AT rated, less sensitive reed switches is that their relative operating point is better defined. If we compare a switch rated in the band 15-20AT compared with one rated 20-25AT, the higher rated device has a more defined operating point and will be less sensitive to any stray magnetic fields (22.5AT +/- 2.5AT) compared to (17.5AT +/- 2.5AT). This feature essentially implies that operationally, the higher AT rated devices will perform in a more consistent fashion from batch to batch, which in some applications may be important.
Einstein’s influence on reed relay design
Having explained the importance of switching energy and it’s relationship to product longevity, eco- friendly designers might be concerned about the downsides in needing more magnetic energy to activate a higher AT reed switch; the energy needs to come from somewhere and energy can often mean unwanted heat.
This is where the reed relay manufacturer can ply their trade as best, they can. The magnetic energy is generated by a solenoid coil around the reed switch. The closer the coil is coupled to the reed switch, the greater the magnetic energy transfer efficiency, hence performance trade-offs can be gained through choosing whether or not to use a formerless coil or a coil mounted on a bobbin.
Formerless coils are wound in air and mounted directly on the reed switch providing the best possible ampere turn efficiency, albeit at the expense of efficiency in manufacturing. By contrast, bobbin mounted coils inherently introduce an additional gap between coil and switch, lowering the magnetic efficiency, but gaining in manufacturing efficiency through ease of handling.
As with most components, there are cost, benefit and performance trade-offs to be had when designing and manufacturing reed relays and it is important for product designers using reed relays to have an appreciation of what these might be and how they may affect their designs.
You rarely get something for nothing and when it comes to energy, it’s best to spend it wisely!
Further cost / performance options are open to the reed relay manufacturer including choices for magnetic screening, thermal junction balancing, package type, operating voltage and drive protection diodes. As with reed switches, these options can vary in terms of quality and consistency across manufacturers and the reader/designer is advised to understand the full implications of each option.
These days, datasheets and electronic data discovery via the internet is the convenient default option for most designers to gain a qualified insight into making a component selection.
As everyone knows, component manufacturers tend to standardise on datasheet parameters, which has the tendency to narrow initial selection criteria toward price and availability once parametric data has been compared and checked.
Ironically, most good designers also know that different manufacturers have subtle unpublished (or unclear) nuances in their processes that differentiate their product from the competition.
So, if you’re genuinely interested in good design, perhaps the most important figure on the datasheet is the manufacturers phone number?
Useful reed relay facts and figures
- Fast operate and release times : 80µS.. faster than many PhotoMOS optocouplers
- Exceptional Insulation resistance > 10 TΩ
- Low duty, high pulse overload handling without degradation
- Consistent Contact Performance –Quality switches provide well defined contact resistance over specification and time.
- Extended operational life to 1 billion operations: 31 years @ 1Hz
- Hermetically sealed : Do not suffer from oxidization or contamination