The True Form B Reed Switch — How’d We Do That?

Recently the release of our HSR-376R — the world’s first true Form B reed switch — has been getting a lot of attention. And a lot of people have been asking how exactly it works and how it was developed. I’ll attempt to answer some of those questions here.

In the past, we, like other manufacturers, can and have offered a Form B switch by using 2 of the 3 leads used to make a Form C switch. Customers use the common lead and the normally closed lead, and ignore the normally open portion of the switch. This method of achieving a Form B function in a reed switch is not economical because you pay for a component in the switch that is unused.

What makes the HSR-376R different is that it does not have the 3rd switch lead — there is no Normally Open component inside the reed switch. The cost savings comes in not wasting that element.

Another unique thing about this switch is how it functions. All reed switches manufactured on an attractive force inside the switch. Form A’s and Form C’s have a Normally Open contact inside them. When magnetism is presented to the switch, this Normally Open contact develops an opposite polarity across the contact area. Opposite magnetic fields attract. When the magnetic field is strong enough the contacts pull together, making the contact close. In the case of the Form C switch the attractive magnetic force causes the built-in Normally Closed contact to break continuity, and the Normally Open contact closes.

In our new HSR-376R, the internal reed blades are so aligned that they develop a similar type of polarity. Their alignment, being parallel, develop opposing force (like polarity repel), and when the magnetism is sufficient the contact pushes away from each other to break a normally closed contact. It is not subject to magnetic level. That is, once there is sufficient magnetism to open the contact, it will just open more with more magnetism. It will not be damaged by excessive magnetism.

This switch has the same type of components as our normal reed switches. Special alloy makes the reed blades and specific glass for the switch body. They have the same coefficient of expansion, so they are matched thermodynamically for performance over a wide temperature range. The hermetically sealed contacts feature our standard rhodium plated contacts protected by inert atmosphere for long life performance.

So that’s the how…but why did we do it? The short answer is that size really does matter.

The size of the switch is significant because smaller is always better when it comes to signal devices in relays, animal tracking devices, and implantable applications. In reed relays large switches require large amounts of magnetism to operate them. More magnetism means more electrical current passing through a coil to make the switch function. This translates into power, which makes heat, which equipment manufacturers do not like inside devices. Neither do they like to consume power, as it costs more to operate.

In the case of the implantable devices or animal tracking devices, obviously smaller is better. Real estate inside the implantable devices is at a premium. They need to pack in as much computing power and battery size as possible, so the signal device used to turn it on or off needs to be as small as possible.

The industry has always looked to HSI to do things with reed switches that no one else can do. We make the world’s smallest Form A switch, the world’s smallest Form C switch, the only switches with solid tungsten contacts, high voltage switches, RF switches, pressurized switches, and now the only true Form B switch.

If you have a design challenge that a custom sensing solution might solve, just contact us. We’d love to help.