Improved switch with hysteresis

Abstract

The present disclosure relates to an improved switch with hysteresis, in particular to a switching circuit comprising an input terminal connected via a first resistor to the base of a first transistor, which is an NPN bipolar gate transistor, the switching circuit further comprising a second resistor connected between the base of the first transistor and ground, and comprising an output line or terminal connected to the collector of the first transistor, and wherein the emitter of the first transistor is connected to ground, the circuit further comprising a second transistor, which is a PNP bipolar gate transistor, wherein the collector of the second transistor is connected via a third resistor to the base of the first transistor, and the emitter of the second transistor is connected to the input terminal, and wherein the emitter of the second transistor is further connected via a fourth resistor to the base of the second transistor, and the base of the second transistor is further connected via a fifth resistor and a diode to the output terminal.

Description

Technical Field

[0001] This disclosure relates to solid-state switches and switching circuits, as well as simple switching / switching circuits including bipolar junction transistors (BJTs). Background Technology

[0002] A single BJT switch has no hysteresis. An unstable input voltage to the base that is at or near the threshold level (e.g., with noise) can cause the switch to repeatedly turn on and off.

[0003] This problem can be overcome by using more complex designs that can be used with multiple BJTs (three or more BJTs) or comparator solutions, which introduce hysteresis.

[0004] One object of the present invention is to overcome this problem and to create a simpler and cheaper switching circuit that includes hysteresis.

[0005] By adding another BJT (PNP) switch to enhance the on-current to the NPN base, the problem of a single BJT NPN open-collector switch without hysteresis being susceptible to unstable input voltage at the threshold level of the BJT base is solved. Summary of the Invention

[0006] In one aspect, a switching circuit is provided including an input terminal connected to the base of a first transistor (Q1), which is an NPN bipolar gate transistor (Q1), via a first resistor (R3). The switching circuit further includes a second resistor (R5) connected between the base of the first transistor (Q1) and ground, and includes an output line or terminal connected to the collector of the first transistor (Q1), wherein the emitter of the first transistor (Q1) is grounded. The circuit also includes a second transistor (Q2), which is a PNP bipolar gate transistor, wherein the collector of the second transistor (Q2) is connected to the base of the first transistor (Q1) via a third resistor (R8). The emitter of the second transistor (Q2) is connected to the input terminal, and wherein the emitter of the second transistor (Q2) is also connected to the base of the second transistor (Q2) via a fourth resistor (R11); and the base of the second transistor (Q2) is also connected to an output terminal (3) via a fifth resistor (R10) and a diode (D1).

[0007] The diode (D1) can be electrically connected between the fifth resistor (R10) and the output line / terminal (3). Attached Figure Description

[0008] The invention will now be described by way of example with reference to the accompanying drawings, in which:

[0009] Figure 1 A known simple switching device is shown;

[0010] Figure 2 A known switching device with a comparator is shown;

[0011] Figure 3 Examples of the present invention are shown; and

[0012] Figure 4 It shows Figure 3 The operation of the circuit. Detailed Implementation

[0013] Figure 1 A known simple switching device is shown with an input line 1 or terminal (e.g., from switching source 2) connected to the base of a BJT (NPN) labeled Q1 via resistor R3. Resistor R5 is included between the base and ground. Output line / terminal 3 is shown, which is effectively connected to the collector of Q1. Thus, this describes a conventional open-collector NPNBJT switch with BJT Q1. R3 is used to limit the current to the base, and R5 is used to ensure that Q1 is off when there is no voltage at the input.

[0014] As mentioned above, this circuit has no hysteresis, therefore there is no Schmitt trigger or hysteresis in the typical open-collector NPN switch below. Noise at the input can trigger the switch to turn on and off, meaning it can be unstable.

[0015] The known solution is to add hysteresis by adding a comparator. Figure 2 This device is illustrated. Furthermore, comparator 4 requires a separate power supply because it needs a regulated voltage. These additional requirements / components mean the circuitry is more complex and expensive.

[0016] invention

[0017] In one example, the circuit provides a switch with effective and reliable hysteresis. Figure 3 An example is shown. (Compared to...) Figure 1 Similarly, there is an input line or terminal 1 (e.g., electrically connected or potentially electrically connected to switching source 2), which is connected to the base of the BJT (NPN) (labeled Q1) via resistor R3. The circuit includes a resistor R5 connected between the base of Q1 and ground. An output line or terminal 3 is shown, which is effectively connected to the collector of Q1 and can be considered an open-collector output. The emitter of Q1 is directly connected to ground (earth).

[0018] In addition, the circuit includes another BJT, labeled Q2, which is a PNP transistor. The collector of Q2 is connected to the base of Q1 via resistor R8, and the emitter of Q2 is connected to input line / terminal 1. The emitter of Q2 is also connected to the base of Q2 via resistor R11; the base of Q2 is also connected to the output terminal via resistor R10 and diode D1; diode D1 is located between resistor R10 and output line / terminal 3. Optionally, the positions of resistor R10 and diode can be interchanged. Resistor R10 can be located (i.e., electrically connected) between diode and terminal 3.

[0019] Therefore, the circuit effectively provides a Schmitt trigger switch by adding a second BJT Q2 as a PNP BJT, along with resistors R8, R11, R10 and a diode.

[0020] operate

[0021] The input voltage rises to just enough to turn on Q1 through R3. When Q1 is on, it will also turn on Q2 through R10 and the diode. When Q2 is on, more current is supplied to the base of Q1 through R8. This forces Q1 into the "on" state. This increases the on-state current to the base of Q1.

[0022] operate

[0023] Figure 4 It shows Figure 3 An example of circuit operation is shown below. The setup is as follows: i) The input switches on / off in a slow rising and falling manner; ii) A DC voltage source is connected to the open-collector output 3 via a resistor. Curves A, B, and C respectively illustrate... Figure 3 The voltages at points A, B, and C in the diagram.

[0024] When Q1 NPN turns on (output voltage becomes zero), the base voltage of Q1 NPN rises from 0.5V to 0.7V. This is a result of Q2 providing more current to its base. In this example, the turn-on voltage (low to high) is 6.2V and the turn-off voltage (high to low) is 2.8V. The threshold voltage and hysteresis range can be adjusted by changing the resistance value of the resistor.

Claims

1. A switching circuit comprising an input terminal (1) connected via a first resistor (R3) to the base of a first transistor (Q1), said first transistor being an NPN bipolar gate transistor (Q1), said switching circuit further comprising a second resistor (R5) connected between the base of the first transistor (Q1) and ground, and comprising an output line or output terminal (3) connected to the collector of the first transistor (Q1), and wherein, The emitter of the first transistor (Q1) is grounded. The switching circuit further includes a second transistor (Q2), which is a PNP bipolar gate transistor. The collector of the second transistor (Q2) is connected to the base of the first transistor (Q1) via a third resistor (R8). The emitter of the second transistor (Q2) is connected to the input terminal (1). The emitter of the second transistor (Q2) is also connected to the base of the second transistor (Q2) via a fourth resistor (R11). The base of the second transistor (Q2) is also connected to the output terminal (3) via a fifth resistor (R10) and a diode (D1). This is such that when the first transistor (Q1) is turned on, the second transistor (Q2) is turned on through the fifth resistor (R10) and the diode (D1), thereby increasing the on-current to the base of the first transistor (Q1) via the collector of the second transistor (Q2) and the third resistor (R8).

2. The switching circuit of claim 1, wherein, The diode (D1) is electrically connected between the fifth resistor (R10) and the output line / output terminal (3).

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