Level Switch Electrical Considerations and Reed Switch Protection
When using Madison level switches, it is important to consider the applications electrical parameters. Our level switches utilize reed switch technology, which are glass encapsulated, magnetically actuated switches. Madison generally provides electrical ratings for resistive loads; however, where the maximum current of the load permits, the switches are capable of controlling devices such as motors, solenoids or coils that produce capacitive or inductive electrical loads. Where possible, Madison recommends the use of general-purpose/isolation relays or controllers to protect the switch.
Protect your level switch: Protection Techniques and Common Failure Modes
Reed Switch protection is the most successful method of increasing the performance and life of your level sensor. Since every application varies, it is important to understand your protection options. The life of the reed switch is typically 106 - 109 operations, dependent upon load conditions. The special reed blade material is selected to ensure a mechanical life of at least 3 x 109 operations. The table below is a guide to suggested protection techniques and common failure modes associated with each load type.
|Load||Load Example||Protection||Diagram||Common Failure Modes||Failure Mode Description|
|Resistive (DC)||Indicator Lamp, Heaters||Current Limiting Resistor||A||In-rush Current (Switching)||In-rush current exceeds rating and welds switch closed|
|Over-Current (Carry)||Carry-current exceeds rating and switch welds or burns open like a fuse|
|Inductive & Capacitative (DC)||Relay Coil, Solenoids, Motor||Reversing Diode||B||Over-Voltage (Arcing)||Voltage arcing during switching welds
|Inductive & Capacitive (AC or DC)||Resistor & Capacitor Network||C|
|Resistive, Inductive & Capacitive (AC or DC)||Indicator Lamp, Heaters, Relay Coil, Solenoids, Motor||Varistor or MOV||D||Over-Voltage (Arcing)||Transients voltage spikes exceed breadown voltage and weld switch closed|
For DC circuits: Insert a 1N4004 diode across the load (i.e.: relay coil) with the cathode end (marked with circular line) connected toward the positive side. This way the diode conducts only when the field collapses. General rule is to use a diode with a voltage rating at least three times the circuit voltage. A 1N4004 has a rating of 1 amp continuous, 30 amp surge, 400V max.
For typical 120v AC circuits: Insert a 50 to 100 ohm, 1/2 watt Resistor in series with a .1 micro farad 400 to 600 volt Capacitor across the switch. The capacitor is a high impedance to 60 hertz, but is essentially a short circuit to high frequencies of generated voltages. Alternately, a Varistor V130LA10A by itself across the switch will also work for 120v AC.