Reed Sensors Versus Hall Effect Sensors

Introduction

Since their introduction several years ago, the Hall effect sensor has captured the imagination of design engineers. Generally, it was thought that if it’s solid-state then it’s a more reliable approach, particularly when comparing it to electromechanical devices. However, several remarkably interesting advantages are observed when comparing Reed sensor technology with Hall effect technology.

But first, let’s take a closer look at Reed sensor technology. The key component in the reed sensor is the Reed switch, invented by Western Electric back in the 1930s. The other major component is the magnet or electromagnet used to open or close the Reed switch. Over the last seventy years the reed switch has undergone several improvements, making it more reliable, improving its quality and reducing its cost. Because of these dramatic improvements to Reed switches, they have become the design-in choice in several critical applications where quality, reliability and safety are paramount.

Perhaps the most dramatic application and testament to the Reed’s quality and reliability is its use in Automatic Test Equipment (ATE). Here this technology is used exclusively.

Reed switches are used in Reed relays, switching in the various test configurations for integrated circuits, ASICs, wafer testing and functional printed circuit board testing. For these applications up to 20,000 Reed relays may be used in one system. Here one relay failure constitutes a 50-ppm failure rate. Therefore, to meet this requirement, the reed relays need to have quality levels much better than 50 ppm. Heretofore, it was unheard of to have an electromechanical device with this quality level. Similarly, the same holds true for several semiconductor devices as well. Once beyond the initial operational quality testing, the reed relays then need to perform well over their lifetime. Here they have been proven to outperform all other switching devices. Because, in many cases, the automatic test equipment is operated 24 hours a day and 7 days a week to fully utilise its high capital expense; and therefore, billions of operations may be required during the reed relay’s lifetime.

Another example of its favoured use is in airbag sensors, where they have passed the test of time in a crucial safety application. Reed sensors are currently used in many critical automotive safety equipment applications (brake fluid level sensing, etc.), along with many medical applications including defibrillators, cauterising equipment, pacemakers and medical electronics where they isolate small leakage currents.

In both technologies, the sizes are shrinking. However, when comparing the reed sensor with a Hall effect sensor we see several advantages:

Cost-effective

Generally, the cost of the Hall effect device is low, but it requires power and circuitry to operate. Also, its signal output is so low it often requires amplification circuitry
as well. The net result is that the Hall effect sensor can be considerably more expensive than the reed sensor.

High Isolation

The Reed switch has superior isolation from input to output and across the switch up to 10¹⁵ Ohms. This reduces leakage currents to femtoamps (10¹⁵ amps) levels. On the other hand, Hall effect devices have sub-microamp leakage levels. In medical electronic devices inserted into the human body as probes (invasive use) or pacemakers it’s very important not to have any leakage current near the heart, where microamp and sub-microamp currents can alter the heart’s key electrical activity.

Hermetically Sealed

The reed is hermetically sealed and can therefore operate in almost any environment.

Low Contact Resistance

The Reed has very low on resistance, typically as low as 50 milliohms, whereas the Hall effect can be in the hundreds of ohms.

Switching Power

The Reed can directly switch a host of loads ranging from nanovolts to kilovolts, femtoamps to amps, and DC to 6 GHz. Hall effect devices have very limited ranges
of outputs.

High Magnetic Sensitivity

The Reed sensor has a large range of magnetic sensitivities to offer.

Easy Mounting

Reed sensors are not susceptible to E.D.I., where electrostatic discharge may often severely damage the Hall effect device.

High Voltage

Reed sensors are capable of withstanding much higher voltages (miniature sizes are rated up to 1,000 Volts). Hall effect devices need external circuitry for ratings as high as 100 Volts.

High Carry Current

Reeds are capable of switching a variety of loads, whereas the Hall effect sensor delivers only smaller voltages and currents.

High Shock Resistance

The Reed sensor is typically tested to withstand a three-foot drop test, which is comparable to the Hall effect sensor.

Long Life Expectancy

Because the Reed sensor has no wearing parts, low-level loads (<5V @ 10 mA and below) will operate satisfactorily well into the billions of operations. This rivals semiconductor MTBF figures.

Wide Temperature Range

The Reed sensor is unaffected by the thermal environment and is typically operated from -50°C to +150°C with no special additions, modifications or costs. Hall
effect sensors have a limited operating range.

No External Power

Ideal for portable and battery-powered devices. There are many very good applications of reed products. Selecting the proper reed for the proper application is often critical. Some reed/relay companies are excellent at designing in reeds in critical applications where quality, reliability and safety are paramount.