Application Note Magnetic Reed Switch Technology and Protection

Reed Switch Cross Section DiagramMagnetic reed switch technology has been a proven technology for more than 50 years to dependably and safely sense a wide range of liquids and positioning. The internal switches are hermetically sealed and capable of switching 1/8" accuracy and 1/32" repeatability for standard vertical switches. Magnetic reed switches, when used to electrical specification, can switch reliably for millions of cycles unaffected by everyday shock and vibration for years of dependable operation.

Magnetic reed switch operation is simple but reliable. The float moves a sealed permanent magnet along the stem as the fluid rises and falls. As the magnets pass over the switch, the reeds are pulled together to create a highly reliable contact. The sealed stem encapsulates the hermetically sealed reed switch and its wires from the liquid environment.

Most SMD Float Switches can be configured for either Normally Open (NO) or Normally Closed (NC) switching by changing the float orientation. NO and NC refer to the switches position at rest in a "DRY" tank! Single Pole Double Through (SPDT) contacts are also available. Each SMD Float switch is tested 100% before shipment for guaranteed performance!

One of the greatest failures of reed switches is caused by over current conditions. Most electrically listed products are rated for the "Steady State" operating power. This is the time when normal operation occurs and does not take into account the "turn-on" or "shut-down" loads associated with capacitive or inductive devices. The power created by these devices at "turn-on" or "shut-down" can be 5 to 10 times the "Steady State" current given as part of the power rating. The power spikes created at these moments can cause arcing which can weld or completely burn open the reed contacts. SMD reed switches list the UL resistive power ratings for the reed switches, and maximum currents and voltages to be switched. In most high power applications an inexpensive relay can be used to isolate your switch from heavier loads. Improper wiring is often the cause of switch failure. When wiring your switch and relay combination (as shown in figure 6) remember to tie the load and the switch as close as possible to the ground source to reduce any load spikes from traveling through the switch.