Describe pull-in and dropout current.

The minimum current required to actuate a device is called the pull-in current, and is determined by the applied voltage.The pull-in current must be enough to create the magnetic forces necessary for the device to mechanically overcome the tension of the spring holding the armature in the de-energized position, and any friction losses there may be in the device.Sometimes pull-in current is also called "take-up" current.The current level where the armature returns to the de-energized position is called the dropout current.Dropout current is sometimes called the "let-go" current.Relays and contactors normally operate at a control voltage level considerably higher than the voltage necessary to cause pull-in current, to guarantee positive actuation of the device.