Reed Relays and
Electronics India Limited

# Frequently Asked Questions on Reed Switches

### Theory

What is a Reed Switch ?

The basic reed switch consists of two identical flattened ferromagnetic reeds, hermetically sealed in a dry inert-gas atmosphere within a glass capsule, thereby protecting the contact from contamination. The reeds are sealed in the capsule in cantilever form so that their free ends overlap and are separated by a small air gap.

What is a Reed Sensor ?

A reed sensor is a device built using a reed switch with additional functionality like ability to withstand higher shock, easier mounting, additional intelligent circuitry, etc.

How does a Reed Switch work ?

When a magnetic force is generated parallel to the reed switch, the reeds become flux carriers in the magnetic circuit. The overlapping ends of the reeds become opposite magnetic poles, which attract each other. If the magnetic force between the poles is strong enough to overcome the restoring force of the reeds, the reeds will be drawn together.

What magnetic force is required to close a Reed Switch ?

Minimum force – expressed in Ampere Turns, will cause the reeds to close, and is called the just-operate force. Since the force between the poles increases as the gap decreases, a force of approximately half the just-operate force will maintain the operated state. Speed of operation of the reed switch is determined by the excess of operating force over the just-operate force.

What are the advantages in using Reed Switches ?

They are hermetically sealed in glass environment, free from contamination, and are safe to use in harsh industrial and explosive environments. Reed Switches are immune to electrostatic discharge (ESD) and do not require any external ESD protection circuits. The isolation resistance between the contacts is as high as 1015 ohms, and Contact Resistance is as low as 50 milli-ohms. Reed Switches can directly switch loads as low as a few micro-watts without needing external amplification circuits, to as high as 120W. When used in combination with magnets and coils, they can be used to form many different types of relays.

What are the advantages of Reed Switches from RRE?

RRE’s reed switches are manufactured with flattened leads. This makes it easier for orienting and mounting for maximum in-group sensitivity, and no extra force (which could damage the seals) is required while SMD forming because the leads are already flattened. Ruthenium Sputtering provides low, stable Contact Resistance, long life, and prevents cold welding.

What are the different types of Reed Switches available ?

The basic reed switch is a normally open, Form A contact. A normally closed, form B contact is provided by biasing the Form A with a permanent magnet, and another magnet of opposite polarity needs to be used to actuate it. A form C or a form D contact can be made by combining a Form A contact and a form B contact in the same operating coil. The form B, C and D contacts made in this manner have the same characteristics as the basic Form A.

Are single capsule change-over Reed Switches available ?

Single capsule Form C reed switches are available in many configurations depending on the manufacturer. All configurations fall into two main categories, those which use a magnet bias to hold the armature against one pole, and those which have the armature mechanically preloaded against one pole. The former is polarity sensitive, while the latter has a higher Contact Resistance on the closed contact.

What is the effect of temperature on a Reed Switch?

The normal operating temperature range for a reed contact is -40°C to +200°C, but a steep rise or drop in temperature may crack the glass to metal seals. As the temperature rises, the Reed Blades lose some of their ability to carry magnetic flux and Reed Switches become less sensitive.

What is the relationship between Ampere-turn and Gauss?

In the CGS system, 1 Gauss (B) = 1 Oersted (H) in air. Assuming that the coil which is used to measure the Ampere-turn of the reed switch is longer than 5 times its mean diameter and the coil dimensions are taken in centi-metres,

B = H = $\frac{0.4\cdot\pi\cdot AT}{Coil Length}$

What are the differences between Reed Switches and Hall Effect Devices ?
 Reed Switches Hall Effect Sensors Are hermetically sealed Are susceptible to changes in the environment Can be operated from -50°C to 150°C Can be operated from 0°C to 70°C only Immune to electrostatic discharge (ESD) Requires ESD protection Insulation resistance >1012 minimum Insulation resistance >106 minimum Typical breakdown voltage >250V Typical breakdown voltage <10V Contact Resistance ~50mΩ Contact Resistance >200Ω Does not require power for operation Requires external power source for operation No components needed to generate output Requires many other components to generate output Configurable hysteresis Fixed hysteresis Signal does not require any amplification Requires amplification circuits Switches a range of loads directly Require external devices for switching loads Does not drain battery in mobile devices Constant drain of battery in mobile devices
How does a Thermal Reed Switch sensor work ?

A special Ferrite compound which loses its magnetic permeability at its Curie Temperature, is sandwiched between two permanent magnets. At temperatures lower than the Curie Point, the magnetic flux lines between the two exterior magnets are connected and enlarged as a whole. This keeps the reed switch contact closed. When the ambient temperature reaches the Ferrite’s Curie Temperature, the flux no longer passes between the exterior magnets, and the reed switch contact opens. Normally open type of thermal reed sensors are made by positioning the magnet assembly, slightly away from the reed switch contact overlap.

### Usage

How do two Reed Switches having different Release Ampere Turn differ ?

When two identical magnets move away from two closed reed switches with identical Operate AT but different release AT, the one with the higher release AT, being less sensitive to magnetic fields, will open first

How do two Reed Switches having different Operate Ampere Turn differ ?

When two identical magnets approach two reed switches of different Operate AT, the one with the lower Operate AT, being more sensitive to magnetic fields, will close first. When an application requires a reed switch to only operate within a specific distance, and is flexible in terms of release distance, reed switches with an “L” in the ordering code denoting low release, suit best.

What are the pros and cons in using Reed Switches having Operate Ampere Turn >20 ?
 Pros Cons Bigger magnets can be used Arriving at the right magnet strength takes time Higher resistance to shock and vibration Stronger magnets need to be used Less receptive to stray magnetic fields Magnet needs to be closer to actuate Can switch higher voltage and current Higher operate time
What are the pros and cons in using Reed Switches having Operate Ampere Turn <20?
Pros Cons
Very sensitive to magnetic fields Lower resistance to shock and vibration
Can be used with inexpensive magnets Very low contact gap acts like a capacitor
Magnet can actuate from farther away Lower breakdown voltage
Lower operate time Higher Contact Resistance
What is the recommended test sequence for measuring the AT of a reed switch?

All measurements should be taken using Kelvin sockets. The first step is to apply a saturation pulse to the test coil, then increase the coil drive gradually up to the point the reed contact closes. This is the operate AT of the reed switch. Contact resistance across the reed switch leads should be measured after giving a 25% over-drive to the test coil. The coil drive should now be decreased gradually to the point the reed switch opens. This is the release AT of the reed switch. More information on testing can be found here.

How is contact rating linked to switching voltage and switching current ?

Switching voltage and switching current should not exceed their individual maximum ratings and the product of voltage and current should not exceed the maximum contact rating in watts.

How does cropping or bending the leads affect a Reed Switch?

When the leads of a Reed Switch are cropped or bent, the Operate AT and to a lesser extent, the Release AT, increase. However, since the contact gap is not altered, the Breakdown Voltage is not changed. Severe shock will permanently alter the contact gap and all other parameters.

What kind of Coils can be used with Reed Switches?

In general, there will be little benefit derived from using a coil whose dimensions are significantly greater than those of the appropriate test coil for a reed contact. If the object is to operate the reed switch with the minimum coil dissipation, the inner diameter of the winding should be made as small possible and the winding length should be approximately two-thirds the overall length of the switch. Sensitivity can be further increased and, therefore, coil dissipation further reduced, by wrapping a low-coercivity Ferro-magnetic foil around the outer diameter of the coil. The foil also constitutes a shield which reduces inter-action between the reed relay and its environment.

How can the operate time of a Reed Switch be reduced?

When using reed switches with a coil and when the actuating coil current is interrupted, the release time is invariably less than 0.5 ms. If the coil transient is suppressed with a resistor capacitor network, the release time will be shortened. But if the coil current is sustained by a suppression diode in parallel with the coil, the reed switch release time will be increased by two to three times.

How can bounce time be reduced?

Bounce time can be reduced by using stronger magnets or by giving a higher Drive to the Coil. The leads of a Reed Switch should not be cropped too short as reed switches with shorter leads tend to bounce more.

Summary
Article Name
FAQ on Reed Switches and Reed Sensors
Author
Description
Frequently asked questions: Whats is a Reed Switch ? What is a Reed Sensor ? How does a Reed Switch work ? How do Reed Switches of different AT band differ ? etc.