Schmitt trigger circuit and power supply monitoring apparatus

1. A Schmitt trigger circuit comprising:

a voltage dividing circuit configured to divide an input voltage and output a divided voltage; and

a basic Schmitt trigger circuit configured to include a plurality of transistors as current controlling elements and control current flowing through a light emitting diode (LED) included in an external photocoupler on the basis of an output voltage of the voltage dividing circuit proportional to the input voltage,

wherein the voltage dividing circuit has a positive temperature coefficient,

wherein the plurality of transistors include a first transistor and a second transistor, and

wherein a cathode terminal of the LED is connected to a junction point between a collector terminal of the first transistor and a base terminal of the second transistor.

2. The Schmitt trigger circuit according to claim 1 , wherein the voltage dividing circuit includes:

a Zener diode connected in a reverse direction, and

a voltage-dividing resistor connected to a low-potential side of the Zener diode.

3. The Schmitt trigger circuit according to claim 1 , wherein the voltage dividing circuit includes a Zener diode having a positive temperature coefficient.

4. The Schmitt trigger circuit according to claim 2 , wherein the Zener diode has a positive temperature coefficient.

5. The Schmitt trigger circuit according to claim 1 , wherein the voltage dividing circuit has the positive temperature coefficient which cancels out a negative temperature property of a base-emitter voltage of the transistors.

6. A Schmitt trigger circuit comprising:

a voltage dividing circuit configured to divide an input voltage and output a divided voltage; and

a basic Schmitt trigger circuit configured to include a transistor as a current controlling element and control current flowing through a light emitting diode (LED) included in an external photocoupler on the basis of an output voltage of the voltage dividing circuit proportional to the input voltage,

wherein the voltage dividing circuit has a positive temperature coefficient,

wherein the transistor includes:

a first NPN transistor including a base terminal connected to an output terminal of the voltage dividing circuit, and

a second NPN transistor including a base terminal connected to a collector terminal of the first NPN transistor, and

wherein the LED has a cathode terminal connected to a junction point between the collector terminal of the first NPN transistor and the base terminal of the second NPN transistor.

7. A power supply monitoring apparatus comprising:

a Schmitt trigger circuit including:

a first voltage dividing circuit configured to divide an input voltage from an external power supply and output a divided voltage, and

a basic Schmitt trigger circuit configured to include a second voltage dividing circuit having a positive temperature coefficient and dividing an output voltage of the first voltage dividing circuit, and a plurality of transistors as current controlling elements, and control current flowing through a light emitting diode (LED) included in a photocoupler on the basis of an output voltage of the second voltage dividing circuit; and

an output circuit configured to include a phototransistor included in the photocoupler and output power source monitoring information to outside,

wherein the plurality of transistors include a first transistor and a second transistor, and

wherein a cathode terminal of the LED is connected to a junction point between a collector terminal of the first transistor and a base terminal of the second transistor.