An under voltage protection device includes a voltage detector and a starting unit. The voltage detector includes a diode, a variable resistor, a transistor and a fixed-resistance resistor. The starting unit includes a second transistor. When a voltage of the external voltage source is smaller than a voltage of the diode, a voltage between an emitter node and an collector node of the first transistor is smaller than a threshold voltage present between a gate node of and a source node of the second transistor such that the starting unit is turned off. When the voltage of the external voltage source is larger than the voltage of the diode, the voltage between the emitter node and the collector node is larger than the threshold voltage present between the gate node and the source node such that the starting unit is turned on.