Reed Switch Pull-in vs. Drop-out: Key Differences Explained

This article explains the terms “pull-in” and “drop-out” as they relate to reed switches and highlights the key differences between them.

What is a Reed Switch?

• A reed switch is a magnetically activated mechanical switch.
• It comprises two metallic contacts sealed inside a small enclosure, typically a glass capsule.
• These contacts are magnetic and move in response to an applied magnetic field.
• Activation requires a permanent magnet or electromagnet.
• Two main types exist: SPST (Single Pole Single Throw) and SPDT (Single Pole Double Throw). The SPST type, either normally open (NO) or normally closed (NC), is the most common. Bistable versions are also available.

Figure 1: Reed switch construction and circuit symbol.

Reed Switch Pull-in

• Pull-in refers to the minimum magnetic field strength needed to activate the reed switch.
• It’s measured in ampere-turns (AT).
• Essentially, it’s the point at which the magnetic field is strong enough to cause the reed switch contacts to close (in a normally open configuration) or open (in a normally closed configuration).

Reed Switch Drop-out

• Drop-out signifies the maximum magnetic field strength at which the reed switch contacts relax and return to their original state.
• Like pull-in, it’s measured in ampere-turns (AT).
• In simpler terms, it’s the point where the magnetic field weakens enough for the switch to de-activate.
• The pull-in value is always higher than the drop-out value. This difference provides a degree of hysteresis, preventing rapid on/off switching when the magnetic field fluctuates around the pull-in threshold.