JPS6133653Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to a bias circuit for phototransistors.

FIG. 1 shows a conventional bias circuit for a phototransistor that performs light detection, where 1 is a first bias power supply, 2 is a power switch, 3 is a lamp serving as a light source,
4 hash and light body 5 is a phototransistor, 6 is a current limiting resistor 7 is a transistor, 8 is a resistor that determines the threshold of light detection, 9 is a collector load resistance of the transistor 7, Vc is a control signal and a load 10 added to. In such a conventional circuit, the light body 4
When the optical switch 2 is closed, power is applied to the collector of the phototransistor 5 through the resistor 8, the base emitter of the transistor 7, and the resistor 6. At this time, current flows to the base of the phototransistor through the capacitance between the collector and base. This current is amplified and flows to the collector. That is, the capacitance between the collector and emitter of the phototransistor is equivalent to the current amplification factor times the capacitance between the collector and base. When the phototransistor is Darlington connected, the current amplification factor is about 100,000 times, and if the collector-base capacitance is 10PF, the collector-emitter equivalent capacitance is 1 μF. Therefore, if the resistor 6 is 100KΩ, the time constant is 0.1 seconds, and the collector current of the phototransistor 5 flows for about 0.1 seconds after the switch 2 is closed. This is the same as no light emitting, and the transistor 7 is turned on, the control signal Vc reaches the power supply potential, and the load 10
is driven. In other words, the conventional circuit had the disadvantage that when the power was turned on, the light would temporarily disappear and no light would be emitted.

This invention was developed in view of the above-mentioned drawbacks, and provides a phototransistor bias circuit that is equipped with a bias power supply that is independent of the switching operation of the power supply in order to prevent the phototransistor from malfunctioning when the power is turned on.

This invention will be explained below using an embodiment shown in FIG.

In FIG. 2, 11 is a second bias power supply, and 12 is a resistor for applying the second bias power supply 11 to the collector of the phototransistor 5.

According to the present invention, the collector-emitter of the phototransistor 5 is always biased by the second bias power supply 11. Therefore, if the tape 4 illuminates when the switch 2 is closed,
Since the collector voltage of the phototransistor reaches the voltage of the second bias power supply, it is possible to prevent the transistor 7 from being energized. This occurs under the following conditions.

V ₁ −V _BE ×R ₈ +R ₆ /R ₈ < V ₁₁ ×R ₈ +R ₆ /R ₁₂ +R ₆ +R ₈ …(1) The left side of this equation (1) is the phototransistor when transistor 7 is turned on. 5, and the right side shows the collector voltage of phototransistor 5 when switch 2 is open. However, lamp 3 is O.
_VBE is the base-emitter forward voltage of transistor 7.

If this condition cannot be satisfied, the condition shown in the following equation (2) may be achieved by inserting a diode 13 in series with the resistor 6 as shown in FIG.

V ₁ −V _BE ×R ₈ +R ₆ /R ₈ <V ₁₁ ...(2) As described above, according to this embodiment, even if the switch 2 is closed, the transistor 7 does not turn on. A transistor bias circuit can be constructed, and when a tape is used as a light source, it can be applied to a circuit that detects the beginning or end of the tape.

As explained above, since this invention uses the second bias power supply, the circuit can be configured normally even when the power is turned on, and has a great practical effect.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of a conventional phototransistor bias circuit, and FIG. 2 is a circuit diagram showing an embodiment of this invention. FIG. 3 is a circuit diagram showing another embodiment of this invention. In the figure, 1 is a first bias power supply, 2 is a switch, 4 is a phototransistor, and 11 is a second bias power supply. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Scope of utility model registration request] a phototransistor, a first bias power supply, a circuit that applies the first bias power supply between the emitter collector of the phototransistor via a switch, a second bias power supply, and a circuit that applies the second bias power supply to the phototransistor. and a circuit applied between the emitter and the collector of the transistor, wherein a second bias power source is applied to the photo transistor regardless of whether the switch of the first bias power source is opened or closed.