What Is a Float Switch?

Lori Kilchermann

A float switch is a switch mounted inside of a float that is activated when the float reaches a certain level. Often, a mercury switch is activated once the float switch has reached a particular angle and then switches off once the float has once again settled at a lessor angle or level. Occasionally, the float switch will consist of nothing more than a float connected to a micro-switch by a rod that turns the switch on and off as the float rises and falls. Commonly used on sump pumps and water wells, the float switch is considered a simple, yet reliable, method of activating pumps, drains, warning lights and sirens in a myriad of settings and applications.

A sump pump uses a float switch that turns on the device when water rises to a certain level.
A sump pump uses a float switch that turns on the device when water rises to a certain level.

A version of the float switch is used in the toilet tank and controls the flow of water into the holding tank. As the water level drops at flushing time, a large float attached to a metal rod drops and activates a water valve allowing the water to enter the toilet tank. As the water level is restored inside of the tank, the float rises on the water's surface. When the float reaches the top of its setting, the switch is turned off, thus stopping the flow of water into the tank. This process is repeated after every flush, thereby demonstrating the effectiveness and durability of the float switch.

When used in a typical sump pump application, the float switch is used to turn an electric pump on and off. As water enters the reservoir of the sump pump, a plastic or metal float begins to rise. The float is commonly fastened to a rigid structure at one end, thereby, causing the float to pivot or roll over as it sises on the water level. A mercury switch inside of the float is connected to an electric pump by wiring that runs through the chassis of the pump. Once the proper angle is reached by the float, the mercury runs to the low side of the switch and completes the circuit, allowing electricity to reach the pump motor and begin to pump the water out of the basement or structure.

Once the water has been pumped out of the reservoir and the float reaches its resting level once again, the mercury flows out of the switch and the contact is broken within the wiring harness, turning off the pump motor. While plastic is used for many of the float switch designs, occasionally a metallic float will be used on higher-end sump pumps and well motors. In this design, the float switch is commonly housed in a copper, aluminum or tin float canister to prevent rust and deterioration in the submerged position.

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