Reed Switch Info
An information guide to reed switch technology
This site was designed to provide a comprehensive technical reed switch information overview aimed to answer some of the most common questions pertaining to basic reed switch operation, characteristics, selection criteria and functionality. The information herein was developed by a expert team of engineers who collectively have in excess of 50 years experience in designing and manufacturing reed switch products.
What is a Reed Switch?
- A Reed Switch is a small electromechanical device having two ferromagnetic reeds that are hermetically sealed in a glass envelope. They range in length from 2.0 inches long to as small as 0.025 inches long.
How Does a Reed Switch Work?
- When brought into a magnetic field the reeds, which are ferromagnetic will close, creating a switching function.
- The orientation and direction of the permanent magnet determine when and how many times the switch will open and close.
How a Reed Switch is Manufactured
Manufacturing a Reed Switch requires the use of micro technology in order to mass produce switches at a very high quality. A clean environment is essential to ensure no contaminants are introduced into the glass capsule during the hermetic sealing process.
How Are Reed Switches Constructed?
- The two reed leads are made up of Nickel/Iron (NiFe) alloy (52% nickel).
- To be affected by a magnetic field the reed leads must be ferromagnetic.
- The three most popular materials in nature and easy to anneal are ferromagnetic: iron, cobalt, and nickel.
- The tips of the two reed contacts are either plated or sputtered with rhodium, ruthenium or iridium, with an under layer of either gold, copper or Tungsten.
Glass Hermetic Seal
- A glass tube is used for the outer packaging whose Temperature Coefficient of Expansion (TCE) exactly matches the NiFe alloy.
- Both ends of the glass tube are heated and the glass melts and forms the hermetic seal encompassing both ends.
- During the glass sealing process the glass cavity is usually filled with an inert gas (typically nitrogen) or the cavity may be evacuated creating a vacuum. This vacuum usually supports high voltage switching (in excess of 1000 Volts).
What are its Operating Characteristics?
- Pull In (PI) – is the point where the reed switch contacts close
- Drop out (DO) is the point where the reed switch contacts open
- Most companies measure the reed switch in Ampere Turns (AT). Universally milliTesla (mT) is a more generally accepted magnetic measurement unit. MEDER electronic is the first and only manufacturer to convert over to mT, but continues to reference AT.
- Pull-in and Drop-out are the points referred to when the contacts close and open. AT or mT indicate the relative magnetic strength at these opening and closing points. Also, its very convenient to specify the closure and opening points in distance for specific applications.
- Hysteresis is also another parameter that is useful to measure particularly in liquid level measurement. Its simply the ratio of the drop out and the pull in, and is measured as a percent (%) or decimal.
- If the liquid being measured is in any type of moving vehicle or a vibrating environment, the hysteresis can play an important role in a successful application
- Once the sensing takes place the hysteresis will keep it in that state even after a considerable movement of the liquid level.
Example: Pull-In / Drop-Out x 100% = % Hysteresis
Example: 12 mm / 20 mm = 0.6 x 100 % = 60 % Hysteresis
meaning the switch will activate at 60 % of it’s release point
What are its Electrical Parameters?
| Rated Power (Watts)|| up to 100|
| Switching Voltage (Volts DC/AC)|| 0 to 10,000|
| Breakdown Voltage (Volts DC)|| 200 to 15,000|
| Switching Current (Amps)|| 0 to 3.0|
| Carry Current (Amps)|| 0 to 15.0|
| Contact Resistance (milliOhms)|| < 100|
| Isolation Resistance (Ohms)|| up to 10E15|
| Operating Time (milliseconds)||<1.0|
| Release Time (microseconds)|| < 50|
| Capacitance (picoFarad)|| 0.2 typical|
Reed Switch Life Test
What is DCR testing?
- Dynamic Contact Resistance (DCR) testing will eliminate early failures and improve long term reliability in the customer’s equipment and/or technical systems.
- DCR testing is a great way to qualify a new sensor or relay to make sure that all tools involved are not adversely affecting the fragile reed switch.
- This is particularly true in any operation involving bending or forming the reed, along with any over-molding of the reed.
What causes DCR disruptions?
- Over-stressed reed switch
- A small crack on the reed switch hermetic seal
- A broken reed switch
- Plating or sputtering peeling or flaking off the contact area
- Improper air mixture (moisture) inside the glass capsule
- Particles on the reed contacts
What are the DCR parameters?
- Reed switch size and the subsequent inductance of its coil can have a major influence in the dynamic switching characteristics.
- When the reed contacts come together, they do so with a certain momentum. That momentum makes the reeds vibrate in a simple critically damped harmonic motion.
- Critically damped harmonic motion is an important concept in our DCR testing.
- Larger reed switches have more inertia and the reed blades are stiffer. This in effect will create three things:
- The need for a magnetically stronger more inductive coil is required
- It will increase the initial reed closure time
- It will increase the effects of the critically damped harmonic motion.
- Conversely, smaller reed switches have less inertia and are not as stiff. Therefore, they will behave in an opposite manner compared to larger switches.
- Taking the size of the switch into consideration, therefore, is an important step in determining the parameters of the DCR testing.
- When the reeds undergo the critically damped harmonic motion they are moving microscopically inside the glass capsule
- This movement is occurring in the magnetic field generated by the coil
- When a metal is in motion in a magnetic field a current will be induced in the metal
- This current is a critical part of the measurement of our Dynamic Contact Resistance
- The overdrive of the coil is also a critical parameter in making the DCR measurement. Simply defined: it is the voltage ( or current) above the actual pull-in (or closure) point where the DCR measurement is made.
- If the reeds close with 3.0 volts applied, adding an increased voltage above 3.0 Volts and testing at that point would represent the overdrive level.
- A reasonable overdrive number is 40%. Here for 3.0 Volts this represents a voltage increase of 1.2 volts or a test level of 4.2 volts applied to the coil.
DCR measured at max Pull-In
- Max pull-in as the overdrive level is another approach.
- Here if the max Pull-in spec is 3.75 volts that is where the DCR measurement is taken.
- In this case, however, if some of the population are pulling-in at or near 3.2 volt this will represent only an overdrive of less than 15%
- Again, this may represent an unfair testing approach and one may reject perfectly good reed relays (unfair in this case means throwing away perfectly good products)
DCR measured at Nominal Voltage
- Using the DCR measured at 5 volts also easy to set up by the test engineer may disguise potential problems with too much overdrive.
- It is not uncommon to have a lot of relays made that typically have a distribution of Pull-in over an expansive range.
- For a 5 volt relay it would not be uncommon to have a Pull-in of 2.0 volts. In this case, testing the DCR at 5 volts would represent 150% overdrive.
- 150% overdrive may cover up potential reed switch problems.
- Testing the DCR at 2.8 volts (40% overdrive) would be a better test
The DCR Voltage Level
- Test engineers like to select a given test voltage to do the DCR for a given specification, because it is easier. However, doing so could mask a potentially serious reed switch problem
- A 40% overdrive is generally considered a fair overdrive, but in this case the test engineer must add additional software routines to first measure the Pull-in and then calculate the overdrive. This can be done with a few line of code, thereby not taking up much test time
- An overdrive greater than 40% may be a better test for larger reed switches
- In this case, testing a population of the parts before finalizing the exact DCR voltage may be the best approach
When to measure DCR?
- Larger reed switches take longer to close as already described.
- Because of this, starting the DCR too early will mean throwing away perfectly good product
- Starting the DCR later with a smaller reed may create the opposite situation allowing too much time for settling
- 1.5 ms usually is an appropriate amount of time after the coil has been energized to perform the DCR measurement
What does a normal Reed Switch DCR test look like?
- When the Reed Switch is closed, the contacts produce a wavering momentum, also referred to as Reed Switch vibration. The natural vibration that occurs during the contact closure produces a consistent wave pattern as it gradually stabilizes. The normal vibration pattern looks similar to the below wave depending on the Reed Switch size. Smaller Reed Switches have smaller and less rigid contacts producing less inertia and shorter closure times. The opposite is true for larger Reed Switches.
- An abnormal switch contact vibration pattern would show excessive dynamic noise and typically takes much longer to completely close. The hermetic seal of the Reed Switch is the critical design element that not only protects the contacts from dirty environments but also holds the contacts in place ensuring consistent switching in the billions operations.
- The DCR (Dynamic Contact Resistance) Test is the best method for detecting a defect in the hermetic seal or contact damage from shock induced stress.
Illustration showing a single Reed Switch closure
Changing DCR on successive operations
How are Reed Switches used?
- A Reed Switch can be used as a Reed Relay and as a Reed Sensor.
How is a Reed Switch used as a Reed Relay?
- Placing a coil around the Reed Switch and passing a current through the coil produces a magnetic field equivalent to a permanent magnet.
- Placing a coaxial shield around the switch allows high frequency signals to be switched up to 20GHz.
- Because the Reed Switch has no wearing parts, the contacts can switch low level signals well in the billions of operations.
- The Reed Relay is used extensively throughout the test and measurement field.
- Reed Relays are used in test systems, matrices, RF, modems, alarms, ideal for:
- High cycle count
- High voltage applications
- Low current and low voltage switching
How is a Reed Switch used as a Reed Sensor?
- A Reed Switch used as a Reed Sensor can sense all kinds of movement using a permanent magnet.
- Reed Switches in the open state draw zero current which make them ideal in energy saving device applications.
- The magnet's field can be effective even when separated by air, plastics, and metals.
- This feature opens up a plethora of applications, where the sensing environment does not allow the movement of the magnet and switch to physically come together.
- Usually the magnet and Reed Switch are divided or separated by a physical housing or other obstacles.
- Reed Sensors are used for sensing movement, counting, detecting fluid levels, fluid level measurement, switching harsh environments, implant devices, etc.
How does a Reed Switch sensing application work?
- The magnetic sensitivity of a Reed Switch is measured in AT (ampere turns) and is used to define it's Pull-In (open) and Drop-Out (closure) points.
- The Reed Switch sensitivity produces a distinct pattern known as it's field of magnetic sensitivity.
- The field of magnetic sensitivity will vary depending on the orientation and direction of the permanent magnet which is critical to the sensing application.