Load Switching and Contact Protection
The Reed Switch contact rating is dependent on the switch size, gap size or ampere turn rating, contact material and atmosphere within the glass capsule. To receive the maximum life for a given load some precautions may be necessary.
Because a Reed Switch is a mechanical device and has moving parts, there are circumstances where life will be shortened due primarily to contact wear. Switching no load or loads where the voltage is less that 5 Volts @ 10 mA or less, the contacts undergo little or no wear. Here life times in excess of billions of operations are expected and realized. In the 10 Volt range,higher contact wear will take place. The amount of wear is dependent upon the current switched. Generally speaking, switching 10 Volts @ 10 mA, life times of 50 million to 200 million operations can be expected. If one is looking for more life under these circumstances and you can not eliminate the actual switching of the load, mercury wetted contacts may be the correct solution. Here the contacts actually have a small amount of mercury on them so that no net metal is ever transferred from contact to contact. Life for most âßßhotâßß switching loads using mercury wetted contacts will also be in the billions of operations even when switching 100âßßs of Volts at 10âßßs of mA.
Switching pure DC loads is always advised. All the data shown in our life test section, has been taken under this condition. Avoid loads with a leading or trailing power factors.
Loads having a net overall inductance will play havoc when opening the contacts. Because the opening transition is so fast the di/dt term in Equation #1 becomes very large and the contacts will undergo sustained arcing.
When the contacts see a net overall capacitive load, an inrush of current will occur when closing the contacts. Contact damage and even sticking will occur depending up the total capacitance, voltage present and series resistance.
Tungsten filament lamps, a very popular switching load for Reed Switches particularly in automotive, have inrush currents due to their cold filaments. Once the light is on the resistance in the filament rises rapidly reducing the current flow. Typically current surges in the order of 10 to 20 times the stead state current can be expected. Knowing the cold filament resistance is important to determine the size of the inrush current. Adding some series resistance to the same circuit can have a dramatic improvement on the life of the switch.
Capacitive and Inductive Loads
Stray capacitance may be present, to some degree, when switching any voltage and current. When closing and switching a given voltage and current, the first 50 nanoSeconds are the most important (see Figure #46). This is where the exact amount of arcing will occur. If there is a significant amount (depends on the amount of voltage switched), of stray capacitance in the switching circuit, a much greater arc may occur and thereby reduce life. When switching any sizable voltage, it is always a smart idea to place a fast current probe in the circuit to see exactly what one is switching in the first 50 nanoSeconds. Generally speaking, when switching voltages over 50 Volts, 50 picoFarads or more can be very significant to the expected life times. If the Reed Switch is operated remotely with a long cable connection, that cable can act like a long distributed capacitance. Shields and other potentially capacitive components can also lend their capacitance to high inrush currents.
Figure #46. Surprisingly large inrush currents can be generated across the contact when stray capacitance is charged to compliance voltages. Contact life may be dramatically shortened.
Common mode voltages are also another area to be watchful of. They can have a significant effect on the life of a Reed Switch as well. Depending upon the circuit and the environment, common mode voltages can in effect, charge stray capacitances in the switching circuit and dramatically reduce Reed Switch life in an unexpected manner. (See Figure #47) Again, a fast current probe can reveal a startlingly higher than expected voltage and current switched in that first 50 nanoSeconds, having no bearing on oneâßßs actual load. When line voltages are present in or near sensitive circuits, be cautious. Those voltages can be coupled into the circuit creating havoc with your life requirements. Typically, a faulty Reed Switch is blamed for this reduced life, when in actuality, it is a product of unforeseen conditions in the circuit.
Figure #47. Common mode voltages charging stray capacitance and then be switched across the contacts will reduce life times rapidly. Avoiding this condition is important.
Ideal for position and limit switches, security, and level sensor applications
Just Released! A Brand New Reed Sensors vs. Hall & Electromechanical Product Training Video
Instead of an actuating magnet, a simple piece of iron will immediately drive the reed sensor to...