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Electrical Considerations for Switches


It is important to consider the application’s electrical parameters when using Madison level switches. Our level switches utilize reed switch technology - glass encapsulated, magnetically actuated - switches.

 

Madison generally provides electrical ratings for resistive loads; however, where the maximum current of the load permits, the switches can control devices such as motors, solenoids, or coils that produce capacitive or inductive electrical loads. Where possible, we recommend using general-purpose/isolation relays or controllers to protect the 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 1 million cycles within rated load conditions. The table below suggests 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 In-rush current exceeds rating and welds switch closed
Inductive & Capacitive (DC) Relay coil, solenoids, motor Reversing diode B Over-voltage (arcing) Voltage arcing during switching welds contacts closed
Inductive & Capacitive (AC or DC) Relay coil, solenoids, motor Resistor & capacitor network C Over-voltage (arcing) Voltage arcing during switching welds contacts closed
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 a circular line) connected toward the positive side. This way the diode conducts only when the field collapses. The general rule is to use a diode with a voltage rating of at least three times the circuit voltage. A 1N4004 has a rating of 1 amp continuous, 30 amp surge, 400V max. Refer to Diagram B.


For typical 120V AC circuits: Insert a 50 to 100 ohm, 1/2 watt Resistor in series with a .1 microfarad 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. Refer to Diagram D.

Current and Voltage Ratings of Liquid Level Switches

We recommend that switches stay below the noted ratings for non-resistive loads.

Current and Voltage Ratings are for resistive loads only. For inductive loads, maximum life will be achieved if appropriate arc suppression is used.


Each switch has a power rating in watts. These power ratings are for resistive loads that are at a steady state and are calculated as Power (Watts) = Voltage (AC/DC) * Current (Amps-resistive load). Most devices

have other things to consider, such as current or voltage spikes caused when powering up or shutting down. Common devices such as pumps, coils, and light bulbs can create these types of spikes. For these more complex cases, one must remember to stay within the maximum current and voltage ratings of the switch regardless of the power rating.

For example, our 30-watt standard switches have a maximum switching current of 1 amp and a carrying current of 2.5 amps. If the switch is connected to a 12V circuit that spikes to 2 amps and runs at a steady

state of 1 amp, the power spike exceeds the switch rating, although the steady-state running power is 12 watts. Due to the heat and type of power dissipated during the spike, the contact may be damaged or even welded together, causing switch failure.


Custom designs have other elements and even alternative ground paths that may affect the power delivered to a switch. It is important to consult your engineering department before assuming the system's power requirements. Contact us for additional assistance.

Ratings noted in the below chart are for resistive loads only.

Maximum voltage ratings
SPST Switches - 240V AC  / SPDT Switches - 120V AC

UL Rating UL Rating UL Rating UL Rating Madison Co Calculation
Nominal VA* Amperes (resistive) at 240V AC Amperes (resistive) at 120V AC Amperes (resistive) at 120V DC Amperes (resistive) at 24V DC Amperes (resistive) at 12V DC
360 1.50 3.00 0.75 3.00 3.00
100 0.40 1.00 0.40 1.00 2.00
60 0.40 0.50 0.20 0.50 0.70
30 0.14 0.28 0.07 0.28 0.56
25 – 0.28 – 0.28 0.28
15 – 0.12 100V DC, 0.10 0.30 0.30

*Note: Nominal VA is volts x amps and is the term used for Apparent Power in AC circuits. It is simply the product of voltage and current without considering the type of load (resistive or inductive). In the above table, we use the Nominal VA with resistive load in conjunction with the applied AC voltage to determine the current, we are stating currents with a resistive load. Watts and VA can be used interchangeably in the above chart.

Wire Color Guides


Multi-Point Switches with SPDT Switches or SPST Switches.  Color Code, top to bottom. Maximum of 4 levels with SPDT Switches

Wire Colors for Standard Switches


  • White wire: Polypropylene
  • Black wire: Stainless Steel, Brass/Buna-N, PBT
  • Yellow wire: Kynar
  • Brown wire: Model # M3326
  • Red wire: Model # M3326-NO 
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