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Electrical Considerations

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. Accessories: Relays and Controllers

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.

LoadLoad ExampleProtectionDiagramCommon Failure ModesFailure 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
contacts closed
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

Level Switch Electrical Considerations and Reed Switch ProtectionFor 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.

Click here to read more about Voltage Ratings.

Click here to read more about Tank Level control and to view our Wire Color Guide.