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Testing & Operate Characteristics

How soon after the contacts open can a voltage be applied to the contacts?

Typically we indicate the reed contacts open within 50 microseconds.  However, this does not mean that you can immediately apply a voltage across, particularly if you are working with high voltages.  Breakdown voltage or dielectric strength is stipulated for a given AT range (mT range). For high voltages we use the highest range that will work for a given set of pull-in voltage requirements.  With that said, when the contacts first open they will waiver for at least a few milliseconds.  During this time the contacts will come close to closing in a periodic manner.  During these close encounters the dielectric strength will be much reduced.  If this is a consideration in a customer circuit, care must be taken to wait a sufficient amount of time before re-applying the high voltage.

What is breaking voltage?

Breaking voltage is that voltage that can safely be opened by the contacts.  This voltage is quite different than switching voltage.  Although most specifications don’t really call out the difference.  This becomes increasingly important as the switching/breaking voltage is increased above 200 volts.  Typically when you close, say 400 volts, the contacts will switch this voltage along with any current the circuit is capable of offering.  The event is all over after the first 50 nanoseconds.  Typically, any metal transfer will occur only during this brief period.  Now when you break this voltage an arc will occur and depending upon the amount of current available will sustain that arc.  If you are trying to break a high current, say above 50 ma.  The arc may be sustained for a sufficient amount of time to cause severe damage to the contacts.  Extreme care must be exercised when breaking higher voltages with any current associated with the break.

What is dielectric strength?

Dielectric strength is just another way of saying standoff voltage or breakdown voltage.  No arc-over will occur at this voltage that is either across the contacts, or between any two points.  This has nothing to do with the voltage that will be switched.

What is breakdown voltage?

Breakdown voltage is just another way of saying dielectric strength or standoff voltage.  No arc over will occur at this voltage that is either across the contacts, or between any two points.  This has nothing to do with the voltage that will be switched.

What is standoff voltage?

Standoff voltage is just another way of saying dielectric strength or breakdown voltage.  No arc-over will occur at this voltage that is either across the contacts, or between any two points.  This has nothing to do with the voltage that will be switched.

What does a 10 watt load actually mean?

Many customers misunderstand what a 10 watt load is.  We typically rate the same contacts with 200 volts and 0.5 amps.  Since the wattage requirement can be derived from ohms law, namely Wattage = Voltage x current, one can easily misconstrue this to mean we can switch up to 100 watts with the above voltage and current.  What we really mean is that we can switch up to 200 volts but with a maximum wattage of 10 .  This means if we are switching 200 volts, the maximum current we can switch is 0.050 amps.  Conversely is we are switching 0.5 amps, the maximum voltage we can switch will be 20 volts .  Perhaps specifying these parameters differently may be a better approach that will cause less confusion.

What is drop-out distance minimum?

The magnet distance as it is withdrawn from the sensor with the magnet parallel to the sensor when the contacts are guaranteed to open.  This distance is referenced from the center axis of the reed sensor.

What is pull-in distance maximum?

The contacts will close at a greater distance or equal to the max pull-in distance as a reference magnet is brought in parallel to the sensor.  This distance is referenced from the center axis of the reed sensor.

What is drop-out voltage minimum?

The minimum drop-out voltage is the point where any voltage across the coil less than this minimum, opens the contacts.

What is nominal voltage?

The nominal voltage usually refers to the voltage supplied by the customer’s power supply or his voltage supply circuitry.

What is maximum pull-in voltage?

Maximum pull-in voltage refers to that point where Standex Electronics guarantees the contacts will have closed.  This is measured at 20°C.  This value is usually chosen to be 75% to 80% of the nominal voltage.  This is to take into consideration the variables potentially effecting closure: ambient temperature, power supply variation, and voltage drops across semiconductor switching elements.

What is overdrive?

To close a set of reed contacts takes a certain amount of force.  At this point the contact resistance may be relatively high because there is no force pressing the contacts together. So typically we use a 40% overdrive.  This means if the contact close with 2 volts applied we will apply 40% above that to measure the contact resistance.  So we would measure the contact resistance at 2.8 volts.

What does it mean to overdrive the coil?

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 reed contacts 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.

Why would our customers want us to test their reed relay and/or sensor for dynamic contact resistance (DCR)?

Very simply it will improve the quality and reliability of the product by weeding out these various problem types:

  • Overstressed reed switch usually from assembly
  • Small crack on the reed seal
  • Broken reed switch
  • Plating or sputtering pealing off
  • Air contamination in the glass capsule
  • Particles on the contacts
What is release time?

It is the time initiated when when the coil turns off until the reed contact blades first open.  This is typically in the 20µs to 50µs range.  If a diode is used to suppress the negative coil spike, the opening time will typically increase to 200µs to 350µs.

What is dynamic noise?

Dynamic noise begins after the last bounce and settles usually within 1/2 ms to 1 ms.  The dynamic noise is generated by the wavering reed switch contacts in a magnetic field (the coil) which generates a current. This current is the dynamic noise.

What is bounce time?

The bounce time is measured after the first closure to the last closure.  This does not include dynamic noise.

What is operate time?

This is the time measured when the coil is first energized to when the contacts close and settle.  This includes the bounce time.

Why test for DCR?

DCR (dynamic contact resistance) testing is a great way to qualify a new reed 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 contacts, along with any over-molding of the reed.  DCR testing will eliminate early failures and improve long term reliability in the customer’s equipment and/or technical systems

What can cause a DCR (dynamic contact resistance) failure?

DCR (dynamic contact resistance) failures are caused usually by some malfunction of the reed switch and they include:

  • an over stressed reed switch usually produced during assembly
  • a small crack on the reed switch seal
  • a broken reed switch
  • plating or sputtered contact materials cracking or peeling
  • improper air mixture internal to the contacts
  • particles on the reed contacts
Is it normal for one side of the reed relay to measure 5KV as expected and the other several hundred volts?

Whenever you make a voltage measurement, one must consider the effect of creating a voltage divider.  What is the resistance on the other side of the switch?  For instance, if the switch measures 1E10 ohms and it is connected to a 100 mega ohm (1E8) resistor and 10,000 volts is applied to the other end of the switch away from the resistor, and series circuit is set up such that some of the voltage will be dropped across the switch and some will be dropped across the 100 mega ohm resistor.  One has a series circuit set up with basically two resistors in series. One resistor is the switch at 1E10 ohms and the other the load resistor 1E8 ohms.   When applying 10,000 volts to this circuit approximately 1 µA of current will flow to the open switch and through the load resistor.  Simply using ohms law the 1 µA will generate 100 volts across across the load resistor.  Now if the insulation resistance across the switch is 1E11 ohms the voltage across the resistor would only be 10 volts.  However, if the insulation resistance across the reed switch is 1E9 ohms, then the voltage across the load would be up to 1000 volts.I hope this all makes sense to you.  Obviously, the insulation resistance across the reed switch is very important as is load resistance.  Hope this explains better what you and the customer are seeing.

What does dynamic contact resistance mean?

This is a method used by Standex Electronics to test its contact resistance on reed relays, reed switches and reed sensors.  Essentially we operate the reed contacts at about 100 times per second and look for the a measurement of the contact resistance about 1 millisecond after the contacts close.  If the contacts are clean (no contaminants on the contacts) and the reed switch is tact, we usually get a good reading.  But if there is the slightest problem, the contact resistance will not have settled down within one millisecond, and we reject it.


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