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Ratings (Power / Loads)

Power

Mid-West Instrument offers the reeds in various power options.  Reed switch resistive load ratings are defined by  Maximum  Power (Watts), Maximum switching Voltage and Maximum switching current.  The product of the switching voltage and switching current cannot exceed the power rating.

Ie;  The 3W SPDT switch has a 0.25 A max current rating and  a 120 VAC/VDC maximum voltage rating.  If the application calls for 120 VAC switching voltage, the maximum current (resistive load) you can switch is  = 3/120 or  .025 Amps.

A great advantage of reed switches is that they are not limited when switching low current or used in low voltage applications.  Unlike higher current capable  micro-switches they do not suffer from oxide buildup on their contacts.  Reed switches are the product of choice when interfacing to modern DCS (Digital Control Systems) using opto-isolator  interface circuitry.

Inductive Loads

Inductive Loads such as a relay or a solenoid will generate an inverse voltage proportional to the current flowing and the speed at which the contact opens.  This voltage will often exceed the dielectric breakdown voltage of the reed switch. Therefore it is recommended that switch protection should be provided to reduce the potential of premature contact deterioration. Reducing inductive spikes is also recommended from an EMI/EMC perspective and is good design practice.

For AC applications a MOV (Metal Oxide Varistor), Bidirectional TVS (Transient Voltage Suppressor) or a RC (Resistor-Capacitor) network are used.

For DC applications a diode or a back-to back Zener and diode combination are used across the coil of the Inductive element.

There are many online articles and application notes regarding Inductive kickback suppression.

Capacitive Loads

Capacitive loads can also cause contact deterioration.  When applying power to a discharged capacitor, the capacitor appears as a short circuit.  Over time and repeated on-off cycles, the switch contacts appear to “stick”.  Often a slight “rap” will cause the contact to release. However, continued operation when this happens will lead to eventual failure.

Long cable runs result in distributed static capacitance.  For the lower power 3W switches switching 120 VAC/Vdc cable runs should be less than 50 ft.  If cable runs are  greater than 50 ft, it may be necessary to install an inductor or current limiting resistor in series with the load.  The resistor implementation works very well for DCS opto-isolator interfaces.

We have not seen any issues with the higher powered switches (60W) due to long cable runs  nor with the 3W switch switching lower voltages (ie; 24 VAC/VDC).

The above recommendations are not absolutely necessary to guarantee reliable performance for many years, but will allow operation of the reed switch for millions of cycles.   For example if you were to use a 3W reed switch switching 120 V  to control the coil of relay that has a 2W rating, we would recommend that some suppression circuit be implemented.  However if you were to use the 60W, 3A, 240 V rated reed to control that same coil,  you could possibly achieve reliable switching for many years without implementing any suppression.

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