Sensors

A hall sensor is affected by EDI and RFI and must be protected against them.  Reed sensors and emrs are not affected by EDI or RFI.

The reed sensor is the only sensor that is hermetically sealed.

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.

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 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.

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 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.