JPS586195B2 - Fire detector constant voltage circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a constant voltage circuit for a fire detector using a depletion type field effect transistor.

In order to suppress the current consumption of the constant voltage circuit of a fire detector to the necessary minimum even if the input voltage fluctuates, a constant current circuit is added in series with the Zener diode of the circuit, so that the constant current always flows through the Zener diode. Some devices were known that consisted of bipolar transistors that allowed current to flow, and Zener diodes.

However, the constant voltage circuit described above has the disadvantage of being expensive because it requires a constant current circuit, and since bipolar transistors operate under current control, a current above a certain level must always flow through the base circuit. There was also a limit to how much current consumption could be reduced.

In view of the above points, the present invention uses a depletion type field effect transistor whose operation is controlled by voltage and operates even when the voltage is zero in a constant voltage circuit, thereby eliminating the need for a constant current circuit, resulting in low cost and low current consumption. This provides a constant voltage circuit for fire detectors.

An embodiment of the present invention will be described below with reference to FIG. 1. A fire detection circuit D consisting of an ion chamber or a semiconductor heat-sensitive element that requires a constant voltage, and an amplification element including a switching element thereof, has no operation. A depletion type field effect transistor FET that is controlled by a voltage and operates even when the voltage is zero is connected in series, and the drain d of the transistor FET and one end of the detection circuit D are connected to the positive and negative power terminals a and a of the fire detector. ' is connected to '.

Further, a voltage divider constituted by a series circuit of a Zener diode ZD and a resistor R is connected in parallel to the fire detection circuit D, and the transistor FET is connected to both ends of the resistor H.
The gate g and source S of are connected.

When a current is applied to the above circuit, a drain current flows to the fire detection circuit D through the field effect transistor FET,
When the voltage of the detection circuit D exceeds the Zener voltage of the Zener diode ZD of the voltage divider, a small current flows through the Zener diode ZD to the resistor R, and the voltage across the resistor R, that is, the gate-source voltage of the transistor FET. ,
A constant drain current determined by the impedance of the detection circuit D and the voltage divider flows through the detection circuit D, and the detection circuit D is held at a constant voltage.

In such a state, even if the input voltage fluctuates, the transistor FET has a constant current function, so the drain current does not change and the voltage of the fire detection circuit D is held constant.

In addition, the fire detection circuit D in a fire detector is generally designed to have little load fluctuation during monitoring, so even if the current flowing through the detection circuit D changes somewhat due to load fluctuation of the detection circuit D, this will not cause any change in the current flowing through the detection circuit D. Since the voltage change across the resistor H is small, and therefore the output voltage of the constant voltage circuit is also small, the voltage of the detection circuit D is maintained almost constant.

FIG. 2 shows another embodiment of the present invention, in which the field effect transistor is composed of a field effect transistor FET connected complementary to a bipolar type transistor T, and the transistor FET, T is connected to one end of the fire detection circuit D and the power supply. Except for the connection between terminals a, the structure and operation are the same as those of the embodiment shown in FIG.

In the above circuit, fire detection circuit D includes both transistors FET,
A constant voltage circuit that can flow a large current determined by the product of T's ID and hFE and is resistant to load fluctuations can be obtained.

Furthermore, in the embodiment shown in FIG. 1, if the switching element of the fire detection circuit D is not provided between the power supply terminals a and a', sufficient signal current may not be obtained when the sensor is operating, but in the embodiment shown in FIG. In the example, there is no need to provide a switching element between the power supply terminals a and a'.

Further, in the above embodiment, the voltage divider is composed of the Zener diode ZD and the resistor R, but if the fire detection circuit D has almost no load fluctuation, a resistor can be used instead of the Zener diode ZD.

In this case, there is no need to flow a current through the Zener diode for regulating its operation, so that the current consumption of the constant voltage circuit can be further reduced.

Since the present invention is constructed and operated as described above, it is possible to obtain an inexpensive constant voltage circuit for a fire detector that does not require a constant current circuit and consumes less current.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing different embodiments of the constant voltage circuit of the fire detector of the present invention. D: Fire detection circuit, FET: Depension type field effect transistor, T:
・Bipolar transistor, R... Resistor forming part of the voltage divider, ZD... Zener diode forming part of the voltage divider.

Claims (1)

[Claims] 1. A fire detection circuit is characterized in that a depletion type field effect transistor is connected in series and a voltage divider is connected in parallel, and the gate and source of the transistor are connected to the output terminal of the voltage divider. constant voltage circuit for fire detectors. 2. A constant voltage circuit for a fire detector according to claim 1, wherein the voltage divider is constituted by a series circuit of a resistor and a zener diode. 3. A constant voltage circuit for a fire detector according to claim 1, wherein the voltage divider is comprised of a resistor. 4. The constant voltage circuit for a fire detector according to claim 1, 2, or 3, wherein the field effect transistor is constructed of a field effect transistor connected in combination with a bipolar type transistor.