JPH03218226A - Sensor power supply reverse connection destruction prevention circuit - Google Patents

Abstract

PURPOSE:To suppress a forward voltage drop to a small value by composing a reverse connection preventing circuit, at the input section of a power supply built in a sensor such as a photoelectric sensor, etc., of an FET and a Zener diode. CONSTITUTION:In a photoelectric sensor, etc., for detecting a person or an article, a sensor 21 is formed of a power supply 28 and a sensor circuit 29. A surge absorber 23 and a reverse connection preventing circuit 24 are provided at the power supply connecting section of the sensor 21. If a power supply is externally connected to a DC power supply input terminal 22 erroneously in polarity, the potential at the gate electrode of a P-channel FET 25 becomes higher than that of a source electrode. Accordingly, the FET 25 is not conducted, and the circuit 21 is protected against damage due to the flow of a reverse polarity current. When the polarity of the terminal 22 is normal, the voltage drop of the circuit 24 becomes about 0.1V, and hence the lower limit of the voltage range of the external supply power source can be lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a reverse power connection damage prevention circuit provided on the input side of a built-in power supply circuit of a sensor such as a photoelectric sensor, a proximity sensor, or an ultrasonic sensor.

[Conventional technology]

Photoelectric sensors, proximity sensors, and ultrasonic sensors that detect people and objects are usually installed at a location remote from the device to be controlled, such as an automatic door drive unit, and therefore are supplied with a separate power source. In addition, it is possible to use various power sources depending on the installation environment, and in order to tolerate voltage drops in long power supply lines, power can be supplied in a wide voltage range of 3.8 to 30V, for example.

The circuit configuration of a conventional sensor, mainly showing the power supply section, is as follows:
For example, as shown in FIG.

In FIG. 3, (1) is a direct current power supply terminal, to which a direct current voltage of, for example, 3.8V to 30V is supplied. (2) is a surge absorption circuit connected to the next stage of the DC power input terminal (1), and is composed of a resistor R and a capacitor C. (3)
is a switching diode that is forward-connected to the next stage of the surge absorption circuit (2) and protects the internal circuit from reverse connection of the power supply. (4) is a power supply circuit connected to the next stage of the switching diode (3) and generates a constant voltage. (5) is a sensor circuit, which, in the case of a photoelectric sensor, consists of a light emitting element, a light receiving element, a driving circuit thereof, and the like. (6) is an operation indicator light made of an LED, which receives the voltage supplied to the sensor circuit (5) through a resistor (7).

The outline of the power supply to the sensor (8) is that the surge component of the external power supply supplied to the DC power input terminal (1) is removed by the surge absorption circuit (2), and the voltage is made constant by the power supply circuit (4). The voltage supplied to the DC power input terminal (1) is set to be within a wide range of 3.8V to 30V, for example, by the constant voltage operation of the power supply circuit (5). Furthermore, the switching diode (3) protects the sensor circuit (5) by blocking current flow when an external power source with incorrect polarity is connected to the DC power input terminal (1).

[Problem to be solved by the invention]

Although the sensor (8) accepts a wide range of externally supplied voltages, consider the voltage levels at the lower end of that range.

In the conventional sensor (8) shown in FIG. 3, let us consider a case where 3cm of DC is supplied to the DC power input terminal (1). Assuming that the sensor circuit (5) consumes approximately 20mA, the resistance R (4.7Ω) of the surge absorption circuit (2) is approximately 0.IV.
, a forward voltage drop of about 0.9 cm in the switching diode (3), and a voltage drop of about 0.2 cm in the power supply circuit (4). Here the switching diode (3)
The voltage drop is a typical value, and the voltage drop of the power supply circuit (4) varies greatly depending on the voltage of the external power supply connected, but 0.2 V is the minimum level.

In this way, in the conventional power input circuit, when a voltage as low as 3■ is supplied, the distance between the DC power input terminal (1) and the sensor circuit (5) is 0.1+0.9+0.2=1.
There is a voltage drop of 2V, and the sensor circuit (5) has a voltage drop of about 1V.
．． Only 8V voltage is supplied. As a result, the sensor circuit (5) does not operate, and its operation indicator light (6) cannot be turned on.

In the conventional sensor (8), the main factor determining the lowest level of external supply voltage that can be used is the voltage drop across the switching diode (3), as exemplified above. As described above, since the switching diode (3) was conventionally used, there was a problem in that the minimum voltage of the external power supply that can be used was greatly restricted, and the conditions of use were narrowed.

Therefore, an object of the present invention is to provide a circuit for preventing reverse connection breakdown, which has a forward voltage drop that is considerably smaller than that of conventional switching diodes.

[Means to solve the problem]

The present invention is a power supply reverse connection destruction prevention circuit that is inserted and connected between a built-in power supply circuit of a sensor that is placed apart from a controlled device and outputs a detection signal to the controlled device, and its DC power input terminal, The source electrode is connected to the built-in power supply circuit side, the gate electrode is grounded via a resistor, and the drain electrode is connected to the active terminal side of the DC power input terminal (5). ) channel type FET and this FE
Disclosed is a reverse power supply breakdown prevention circuit for a sensor, characterized in that it is comprised of a Zener diode connected in reverse bias between the source and gate of a sensor.

[Effect]

As shown in FIG. 1, the P (N) channel FET (25) of the reverse connection breakdown prevention circuit (24) has its gate electrode grounded via the resistor (27), so the DC power input terminal When power is supplied from (22) with the correct connection, a bias voltage is applied between the source and gate electrodes,
Electrical conduction occurs between the drain and source electrodes.

The voltage drop between the drain and source electrodes during this conduction is approximately 0.
．． It is as small as about 1V, and the minimum usable voltage of an externally connected power source can be lowered by about 0.9-0.1=0.8V compared to when a switching diode is used. ．． m(7) FET (25) The zener diode (26) connected between the source and the gate electrode prevents a high-ass voltage exceeding the rating from being applied between the source and the gate electrode, thereby protecting the FET. On the other hand, if power is supplied to the DC power input terminal (22) with opposite polarity, the reverse bias voltage between the source and the gate will be F.
The ET (25) is brought into a non-conductive state, and the flow of current into the power supply circuit (28) is interrupted.

Next, in order to further clarify the operation of the circuit of the present invention, although it looks similar to the reverse connection breakdown prevention circuit (21) of the present invention,
For reference, a constant voltage circuit whose operation is completely different is shown in FIG. 4 and will be explained. This constant voltage circuit (9) differs from the reverse connection breakdown prevention circuit (21) of the present invention in that it uses an N-channel FET (10) for the positive power supply, and a Zener diode is connected between its gate electrode and the ground line. (11) is connected, and a resistor (12) is connected between the drain and gate electrodes.

This circuit (9) drops the voltage between the drain and source electrodes so that the output voltage of the source electrode is kept at the same level as the voltage of the gate electrode determined by the Zener voltage of the Zener diode (11). The invention is FET
The polarity of the resistor (12) and the Zener diode (11) are completely different.

〔Example〕

An embodiment of the present invention is shown in FIG. 1 and will be described below.

Figure 1 shows the circuit configuration of a sensor (21) incorporating the reverse connection breakdown prevention circuit (24) of the present invention, (22) is a DC power input terminal, and (23) is a surge sensor consisting of a resistor R and a capacitor C. The absorption circuit (24) is a reverse connection breakdown prevention circuit, and is composed of a P-channel FET (25), a Zener diode (26), and a resistor (27).

(28) is a power supply circuit that outputs a voltage stabilized to a constant voltage, and (29) is a sensor circuit, which in the case of a photoelectric sensor includes a light emitting element, a light receiving element, and their control circuits. (30) is a sensor operation indicator light, which uses an LED and receives the voltage supplied to the sensor circuit (29) via a resistor (31).

The power supply to the sensor (21) is carried out by using a surge absorbing circuit (21) to supply surge components of an external power source connected to the DC power input terminal (22), similar to the conventional sensor (8) shown in FIG.
), the voltage is stabilized at a constant voltage by a power supply circuit (28), and the voltage is supplied to a sensor circuit (29). However, the reverse connection breakdown prevention circuit (24) is configured using a P-channel type FET (25), unlike the conventional switching diode (3).

The operation of this reverse connection breakdown prevention circuit (24) is as follows.

If an external power source is connected to the DC power input terminal (22) with the wrong polarity (+ to the ground terminal, - to the active terminal), the P-channel FET (25) will be damaged by the action of the resistor (27).
Since the potential of the gate electrode of F becomes higher than that of the source electrode, F
ET (25) is not conductive, and the internal circuits of the sensor, such as the power supply circuit, are protected from destruction due to the inflow of reverse polarity current.

On the other hand, when an external power supply is connected to the DC power input terminal (22) with the correct polarity, the potential of the source electrode of the P-channel type FE (9) T (25) becomes higher than the potential of the gate electrode, and the voltage between the drain and source electrodes becomes Conduct. At this time, the Zener diode (26) prevents the voltage between the source and gate electrodes from exceeding the maximum rating. for example,
If the maximum rating is 6■ and the Zener voltage is selected to be 5.6■, the P-channel FET (25) becomes almost completely conductive, and the voltage drop between the drain and source electrodes is approximately 0.
．． It's as small as 1v.

As a result, the voltage drop at the power input stage of the sensor (21) of the present invention is reduced to approximately 0.1 V = 0.8 V, compared to when a conventional switching diode is used. Therefore, surge voltage absorption circuit (23) and power supply circuit (28)
However, if it is the same as the conventional sensor (8) shown in Fig. 1, the voltage of the external power supply that can operate the sensor (21) can be lowered by about 0.8 V than the conventional one. can. For example, even if the external power supply voltage is 3■, the sensor circuit (29) will have a voltage of 3-0.1-0.2-2.
．． Since 6V can be supplied, the sensor (21) can be operated (10) and the operation indicator light (30) can also be lit.

Note that the embodiment of the sensor (21) shown in FIG. 1 above shows the case where a positive power source is applied to the DC power input terminal (22), but when applying a negative power source, as shown in FIG. A channel type FET (25a) may be used and the polarity of the Zener diode (26) may be reversed and incorporated.

〔Effect of the invention〕

According to the present invention, by using a P (N) channel type FET, a voltage drop can be reduced by about 1% compared to the conventional one as a reverse connection breakdown prevention circuit for an external power supply that is incorporated in the front stage of a sensor power supply circuit.
/9 can be provided. As a result, the lower limit of the voltage range of the external power supply that can operate the sensor is
For example, it can be expanded from the conventional 3.8V to 3V, making the operating conditions of this type of sensor gentler and increasing the reliability of operation against voltage drops in the external power supply.
To facilitate the installation work.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a sensor incorporating the reverse connection breakdown prevention circuit of the present invention, and FIG. 2 is a circuit diagram of the reverse connection breakdown prevention circuit of the present invention used for a power source with a polarity different from that in FIG. 1. . FIG. 3 is a circuit diagram of a conventional sensor, and FIG. 4 is a circuit diagram of a constant voltage circuit similar to the reverse connection breakdown prevention circuit of the present invention. (21) - Sensor, (22) - - DC power supply input terminal, (24) - Reverse connection breakdown prevention circuit, (25) - P channel type FET, (26) - Zener diode, (27L - - resistance , (2B) One power supply circuit.

Claims (1)

[Scope of Claims] A power supply reverse connection breakdown prevention circuit that is inserted and connected between a built-in power supply circuit of a sensor that is placed apart from a controlled device and outputs a detection signal to the controlled device, and its DC power input terminal. Then, the source electrode is connected to the built-in power supply circuit side, the gate electrode is grounded via a resistor, and the drain electrode is connected to the active terminal side of the DC power supply input terminal.
) A reverse power supply breakdown prevention circuit for a sensor, comprising a channel type FET and a Zener diode connected in reverse bias between the source and gate electrodes of the FET.