This invention relates to a voltage detecting circuit for generating an output voltage in response to an input voltage.
Conventionally, in industrial equipment which uses microcomputers such as numerical controllers, industrial robots, etc., a voltage detecting circuit is used which includes an input circuit section to detect a voltage applied to a predetermined load and an output circuit section, which is coupled to this input circuit section by means of photocoupling, to generate a control signal in accordance with the result of voltage detection of this input circuit section. FIG. 1 shows an example of such a type of conventional voltage detecting circuit. This voltage detecting circuit includes an input circuit section 2 to which a voltage to be detected will be applied and an output circuit section 4 coupled to this input circuit section 2 by means of photocoupling. This input circuit section 2 has a full-wave rectifying circuit 6 to rectify an AC input voltage applied across the input terminals, a resistive voltage dividing circuit 8 to divide an output voltage of this full-wave rectifying circuit 6, and a series circuit of Zener diode ZD and light emitting diode D1 which is connected between the output terminals of this voltage dividing circuit 8.
The output circuit section 4 includes a pnp transistor TR1 having an emitter connected to a power source terminal VC and a collector which is grounded or connected to a power source terminal VS through a resistor R1, and an npn phototransistor TR2 having a collector which is connected to the power source terminal VC through a resistor R2 and which is connected to a base of the transistor TR1 through a resistor R3 and an emitter connected to the power source terminal VS. This phototransistor TR2 together with a light emitting diode D1 constitutes a photocoupler.
When an AC voltage with a smaller amplitude than a predetermined value is applied to the full-wave rectifying circuit 6, the Zener diode ZD is turned off and no current will flow through the light emitting diode D1, so that the light emitting diode D1 will not light up. Thus, the transistors TR1 and TR2 remain non-conductive, and an output voltage between the output terminal VO and the power source terminal VS is kept at a low level. When an AC voltage with a larger amplitude than the predetermined value is applied to the full-wave rectifying circuit 6, the Zener diode ZD is turned on, so that a large enough current will flow through the light emitting diode D1 for successful light emission. Hence, the phototransistor TR2 is made conductive in response to the light emitted from this light emitting diode D1, and the transistor TR1 is also made conductive. Therefore, an output voltage between the output terminal VO and the power source terminal VS will be at a high level.
However, since a Zener voltage of the Zener diode ZD is constant, a fluctuation of the output voltage of the voltage dividing circuit 8 due to noise or the like from the outside may cause a malfunction in the Zener diode ZD. For example, when an output voltage from the voltage dividing circuit 8 is lower than a predetermined level and the Zener diode ZD is turned off, if a noise voltage is added to the output voltage of this voltage dividing circuit 8, the Zener diode ZD will be turned on, resulting in the light emission of the light emitting diode D1.