1. Field of the Invention
The present invention relates to a digital signal input device that converts an input direct-current voltage into a digital signal, and more particularly, to a digital signal input device suitable for using at an electrical station such as a substation.
2. Description of the Related Art
A digital signal input device includes a charging circuit arranged between a first input terminal and a second input terminal capable of inputting a direct-current (DC) voltage, and a digital signal detection unit that outputs a logical value “1” to an internal circuit when a charging voltage of the charging circuit exceeds a predetermined level, and outputs a digital signal of a logical value “0” to the internal circuit when the charging voltage of the charging circuit is below predetermined level. The whole of the charging circuit and the digital signal detection unit can be referred to as a so-called input circuit for the internal circuit. For ease of description, the word “input circuit” is used in the following to refer to the whole of the charging circuit and the digital signal detection unit.
A conventional digital signal input device is shown, for example, in FIG. 3 of Japanese Patent Application Laid-open No. 2002-84169. In the conventional digital signal input device, the input circuit is shown as a digital signal input unit.
The charging circuit is constituted by a current limiting resistor and a noise-filtering CR filter connected in series across an input terminal (in the following, the input terminal in this connection mode is, for example, referred to as a “first terminal”) connected to a positive terminal of a power supply via a switch and an input terminal (in the following, the input terminal in this connection mode is referred to as, for example, a “second terminal”) connected to a negative terminal of the power supply. The CR filter is a circuit of a capacitor and resistor connected in parallel.
When a DC voltage is applied across the first input terminal and the second input terminal at the circuit where the current limiting resistor and the CR filter are connected in series, a capacitor constituting the CR filter is charged according to a time constant at the series circuit. On the other hand, when a DC voltage is not applied across the first input terminal and the second input terminal, the electric discharge is carried out according to a time constant for a closed circuit of the capacitor and the resistor constituting the CR filter.
The digital signal detection unit is constructed so that at the digital signal input unit, a Zener diode, and a built-in light-emitting diode (LED) of an insulating photocoupler are arranged in series across the ends of the CR filter. A built-in phototransistor of the insulating photocoupler then outputs a digital signal (“1”, “0”) to the internal circuit. With this configuration, the sum of an on-operating voltage of the Zener diode and an on-operating voltage of the insulating photocoupler is provided as a detection level for the charging voltage at the charging circuit.
Taking the case of an electrical station such as a substation, this type of digital signal input device converts the states of a plurality of items of equipment connected to the same DC control power supply installed at the electrical station into a digital signal of logical values of “1” and “0” using the DC control power supply. The digital signal is then incorporated into a control panel and the like.
The input circuit at the digital signal input device used at the electrical station includes a plurality of first input terminals provided using a one-to-one relationship with a plurality of switches connected in parallel at the positive electrode of the DC control power supply and a second input terminal connected to the negative electrode of the DC control power supply. The input circuits of the same circuit are provided so that one input terminal is connected to terminals corresponding to a plurality of first input terminals and the other input terminal is connected to a common second input terminal.
With the input circuit of the conventional digital signal input device, a current limiting resistor and a Zener diode generate heat by a drive current for emitting light at a built-in LED of the insulating photocoupler. When the drive current flows in the same period at the plurality of input circuits arranged in parallel, a considerable amount of heat is generated.
In order to solve the problem of heat generation, in Japanese Patent Application Laid-open No. 2002-84169, as shown in FIG. 1, a time-division phototransistor is connected in parallel with just the number of the first input terminals to a single insulating phototransistor via a Zener diode and a CR filter. Each time-division phototransistor is then selectively turned on by a time-division control signal. The DC voltage applied to each first input terminal is then converted into a digital signal using time-division. If the digital signal input device used at the electrical station adopts this configuration, a pulse current flows for a short time at the current limiting resistor and the Zener diode. The heat generation can then be considerably reduced.
However, with the digital signal input devices used at electrical stations, there are also problems that cannot be dealt with using the configuration shown in Japanese Patent Application Laid-open No. 2002-84169 (FIG. 1) in addition to the heat generation.
DC control power supplies of a wide variety of voltages such as DC 48 volts, DC 110 volts, and DC 220 volts are used at electrical stations. The digital signal input device used at the electrical station needs to satisfy various requirements stipulated in the electrical standard JEC-2500 and the like for protective relays for electrical power use. Such requirements include, for example, detecting “voltage input present” when the input DC voltage is within a rated voltage fluctuation range. The requirements also include detecting “voltage input present” for one rated voltage and detecting “no voltage input” for another rated voltage when a DC voltage of the same value is a determination target for two different rated voltages of ranges of voltage fluctuation that do not overlap.
In addition to countermeasures for suppressing the heat generation, it is necessary for digital signal input devices used at electrical stations to make conversions into digital signals correctly according to DC voltages of different values and according to the requirements.
Regarding this problem, it is necessary to appropriately set the value of a current limiting resistor and the time constant of a CR filter according to different rated voltages and according to the requirements for a detection level decided by the sum of a Zener voltage of a Zener diode and an on-operation voltage of an insulating photocoupler. It is therefore difficult to achieve this with input circuits of the same configuration.
For this reason, input circuits of conventional digital signal input devices used at electrical stations have different configurations for every voltage of the DC control power supply installed at the electrical station. The input circuit selects elements that are resistant to the power consumed according to the corresponding DC voltage for the current limiting resistors and Zener diodes for every voltage of the DC control power supplies installed at the electrical station. The insulating photocouplers are also selected as necessary and the values of the current limiting resistors and the time constants of the CR filters are decided according to the corresponding DC voltages.