Reed Sensors Vs. Hall Effect Sensors

John Beigel, Meder Electronic

Reed Relays and Switches: What's New?

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 in solid state that it's a more reliable approach, particularly when comparing it to electromechanical devices. However, several remarkably interesting advantages are observed when comparing the reed sensor technology to the Hall effect technology.

But first, let's take a closer look at the reed sensor technology. The key component in the reed sensor is the reed switch, invented by Western Electric back in the 1940s. The other major component is the magnet or electromagnet used to open or close the reed switch. Over the last 60 years the reed switch has undergone several improvements, making it more reliable, improving its quality and reducing its cost. Because of these dramatic improvements of 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 testimony of the reed's quality and reliability is its use in automatic test equipment (ATE). Here this technology is used exclusively. The 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. An electromechanical device with this quality level used to be unheard of. 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 life. 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 utilize its high capital expense; and therefore, billions of operations may be required during the reed relay's lifetime.

Another example of its favored 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 (brake fluid level sensing, etc.), along with many medical applications, including defibrillators, cauterizing equipment, pacemakers and medical electronics, where they isolate small leakage currents. ec05ss01a.eps

12. The reed sensor is unaffected by the thermal environmental, and is typically operated from 65° to +150°C with no special additions, modifications or costs. The Hall effect sensors have a limited operational range.

There are many very good applications of reed products. Selection of the proper reed in 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. MEDER Electronic is very much involved in several of the critical requirements mentioned above and represents an excellent choice for those critical safety related applications.

Table Comparing Hall Effect Sensors With Reed Sensors

Specifications     Hall Effect     Reed Sensor

Input requirements     External magnetic field 15 Gs Min.     External magnetic field 5 Gs Min.

Sensing Distance     Up to 20 mm. effectively     Up to 40 mm effectively

Output required     Continuous Current 20 mA dependent on sensitivity     None

Power Required all the time     Yes      No

Requirements beyond sensing device     Voltage regulator, constant current source, hall voltage generator, small signal amplifier, chopper stabilization, schmitt trigger, short circuit protection, external filter, external switch     None

Hysteresis     Fixed usually around 75%     Ability to adjust to meet design requirement

Detection circuit required     Yes, and generally needs amplification     None

Ability to switch loads directly     No, requires external switching     Yes, up to 2 A and 1,000 V depending upon the reed selection

Output switching power     Low mW     Up to 100 W depending upon switch selection

Voltage switching range     Requires external switch     0 V to 200 V (1,000 V available

Current switching range     Requires external switch     0 A to 2 A

Output sensitivity to polarity     Yes, critical for proper operation     No

Output offset voltage sensitivity     Yes, exacerbated by sensitivity to overmolding, temp. dependencies, a thermal stress     None

Chopper circuit requirement     Yes, helps reduce output offset voltage. Requires additional output capacitate external     None required

Frequency Range     Requires external switch     DC to 6 GHz

Closed output on resistance     200 ohms     .065 ohms

Expected life switching 5 V 10 mA     1 billion operations     1 billion operations

Capacitance across output     100 pF typical     .2 pF typical

Input/Output isolation     10 ⁺¹² ohm min.     10 ⁺¹² ohm min.

Isolation across output     10 ⁺⁶ ohm min.     10 ⁺¹² ohm min.

Output dielectric strength     10 V typical     250 V typical (2,500 V available

EDI susceptibility     Yes, requires external protection     No, requires no external protection

Hermiticity     No     Yes

Shock     150 Gs     150 Gs

Vibration     50 Gs     10 Gs

Operating temperature     0° to +70°C typical above or below range will degrade specifications     55° to +150°C no specifications degradation

Storage temperature     55° to +125°C     -55° to +150°c

Author Information

John Beigel is President and Chief Executive Officer of Meder Electronic, 766 Falmouth Rd., PO Box 2207, Mashpee, MA 02649; (800) 870-5385; Fax: (508) 539-4088; www.meder.com.