Switches

The operate time for a hall sensor is typically 5 µs, a reed sensor is 100µs and the emr is up to 10 ms.

Hall sensors cannot switch any voltage directly.  Reed and emr sensors can switch up to 1000 volts directly.

Hall sensors supply a microwatt level signal, reed and emr sensors can switch up to 100 watts directly.

Only a reed and emr sensor can be switched directly.

Reed sensors can adjust the hysteresis from 35% to 95%.  Hall and EMR sensors have fixed hysteresis.

Yes – chopper circuits and drivers are required for the hall sensors only.

Only hall sensors are sensitive to input polarity.

A current is required for proper operation only on the Hall sensor.

Yes, they only supply a small milli-volt signal in the presence of a magnetic field. The signal needs to be amplified and then fed into a switching circuit.

A voltage is produced on a semiconductor material when in the presence of a magnetic field.  The voltage is proportional to the strength of the magnetic field.

Best to use a small copper plated reed switch in an application where the carry current is about 3 amps RF.  Greater than 3 amps you should use a large copper plated reed switch.  The RF will be riding on the outside ‘skin’ of the switch.

A Hall sensors dielectric strength is less than 10 volts, for emrs its typically 250 VRMS, and reed sensors the dielectric strength can be up to 5000 volts.

The output capacitance for a Hall sensor is typically 100pf, a reed sensor is only 0.2 pico-farads, and emrs are typically 20 pico-farads.

The release time for a Hall sensor is typiclly 5µs, reed sensor 20µs and the emr 5 ms.

Hall sensors can not switch any output current, the reed sensor and emr can typically switch up to 2 amps directly.

Hall sensors are typically 200+ ohms, the reed and emr sensors are typically 50 milliohms.

Yes output polarity is critical for proper switching operation with hall sensors only.

Use the Standex Electronics KSK-1A85 reed switch series.

Use ORD228, the ORD211 iridium, or the ORD311.

For a sensor use the ORD228 with iridium or the ORD2210 for a relay.

Small electromechanical relays are not good for switching low levels of voltages and currents.  Electromechanical relays need a hefty voltage and/or current to break any film buildup.  It is this film buildup that won’t allow very low voltages and currents to pass through the contacts.   Reed switches are clearly the best.  Using sputtered ruthenium contacts or iridium contacts are the best materials for these low level loads.

Most reed switch blades are made of nickel/iron that has a relatively high resistance to current flow when compared to copper and silver.  Most of the time it is not a problem. However, when the reed switch is asked to pass high current, whether DC or AC the contacts will heat up.  The heat can become so high that the curie point is reached > 700°C.  At this point, the nickel/iron loses it ferromagnetic properties.  Therefore, the relay coil or magnet holding the contacts together will no longer hold the contacts open due to the excess heat.  To solve this problem plating the entire reed switch with 50 to 100 µm of copper will improve the conductivity so much that the problem will disappear.

Switching and breaking voltages of 250 volts and above is best done with a vacuum reed switch.  Up to 4000 volts can be effectively done with the ORD2210V as long as the current levels are not too high.  Above 4000 volts use the Hermetic reed switches.

Miniature reed switches less than 20 mm (0.80 inches) glass length can effectively break up to 250 Volts.  This depends on the pull-in AT (mT) used.  The higher the better.  Reed switches less than 10 mm will shrink this value to around 150 volts.  Minimizing the current flow at the time of opening will improve this value.

Reed switches whether they are used in sensors or relays all will be asked to switch some load.  Generally there are two aspects to this load.

1. Its steady state load
2. Is the actual switching taking place during the first 50 nanoseconds.  This is also called the signature of the load.

This signature takes into consideration not only the steady state load but also any transient voltages or current that may be present during the first 50 nanoseconds.  These transients may be from stray capacitance, inductance in the line and/or common mode voltages.  From a reed switch designer standpoint, the signature is all there is.  The most important time during the switching of a load is that first 50 nanoseconds.  That is when all the damage to the contacts with occur if you are switching the contacts ‘hot’.  If a customer is having a problem with early failures, this is the first place to look.  Equally important and not to be overlooked is what voltage and current is actually being broken when the contacts open.  Any healthy voltage and/or current present will chew up the contacts rapidly leading to sticking reed contacts.

There are several key factors:

1. You need to have an idea of the required load.  What voltage and current is being switched at the time of closure for the first 50 nanoseconds?
2. How many operations will be required during the life of the product?
3. What are the size requirements?  How much room is needed?
4. How will the product be mounted? Surface mount, thru hole, etc.
5. For long life and low levels, use a ruthenium or an iridium sputtered/plated switch.
6. For switching applications from 50 Volts to 200 volts use the Philips/Coto/Comus sputtered ruthenium switch.
7. For switching currents 25 ma to 1 amp, the KOFU thickly plated rhodium is good along with our KSK-1A35.
8. For higher voltages above 200 volts  up to 4000 volts at relatively low current use the  OKI ORD2210V.
9. For voltages above 1000 volts up to 10,000 volts with higher currents use the Hermetic vacuum switches.  This represents a start.  One could write a book on this subject.  Best to find out the exact customer load and run a life test with a few or several reed switches to make the final determination.

A Form C reed switch is essentially a single pole double throw reed switch.  It is hermetically sealed with 3 leads:

1. A common lead
2. A normally open lead
3. A normally closed lead

When operated the common contact will swing from the normally closed contact to the normally open contact. This is caused by the magnetic field produced by a coil or the magnetic field from a magnet.  When the magnetic field is removed the common contact will revert back and come to rest on the normally closed contact.

The reed switch once it is sealed is subjected to a partial annealing process.  This partial annealing process leaves a stress on the glass to metal seal (hermetic seal) and is actually done to make the seal stronger.

When metal is subjected to a very high temperature bath, that process is called annealing.  The temperature is slowly increased to a max temperature where it is stabilized for a period of time, and then the temperature is slowly reduced back to room temperature.  This process will leave the metal in its softest state.  For a reed switch this is very important because this point is also where the nickel/iron leads have near zero magnetic retentivity.  This means when the reed switch contacts are subjected to a magnetic field and then the magnetic field is removed, there will be no residual magnetism on the leads.

Most metals do not like to be connected to different metals.  A few metals like other metals.  The most popular ones are gold and copper.  These two metals when brought together with other metals will diffuse into that other metal.  These metals are like the glue that holds two different metals together. This process gives rise to multiple levels of plated or sputtered metals.

A hermetic seal is considered to have 3 types:

1. Glass to glass seal
2. Glass to metal
3. Metal to metal seal

These seals by definition completely isolate the outside environment from what is hermetically sealed inside.  These seals are meant to have zero porosity, so there is no leakage even at a molecular level.

Sputtering is a new process where the material is embedded in the soft nickel/iron layer, where plating is simply electroplated on the soft metal.  The problem is that if the plating is not perfect, flaking can occur between the very hard outer plated level and the soft inner metal.

No.  There is no net effect on the reed switch, once the magnetic field saturates the reed switch contacts it no longer has any effect.

A magnet and reed switch can be turned into a temperature sensor by using a magnet that has a certain curie temperature for the temperature you want to sense.  When that curie temperature is reached the magnet loses its magnetic properties whereby the reed switch contacts open.  When the temperature drops below the curie temperature, the reed contacts will close.

Currie Temperature.  The reed contacts become so hot that they reach the currie temperature of the nickel/iron material.  At the currie temperature the materials lose their ferromeagnetic properties.

The nickel and iron are relatively soft.  When you switch voltage and current  across the contacts some of the metal will melt and transfer to the other reed contact.  When you switch often enough, a sufficient amount of  metal will transfer and sticking will occur. Plating and/or sputtering a harder metal like rhodium or ruthenium will dramatically reduce the amount metal that is transferred and therefore directly increasing the life time or number of cycles before sticking will occur.

A magnetic field will only influence a metal that is ferromagnetic.  Both nickel and iron are ferromagnetic.  52% nickel is critical because its thermal coefficient of expansion exactly matches the glass’s expansion rate.