Test circuit for bipolar junction transistor

1. Technical Field

The disclosure generally relates to test circuits, particular to a test circuit for a bipolar junction transistor (BJT).

2. Description of Related Art

Parameters of a BJT can be measured using an ohmmeter. In particular, a measuring method for the BJT is to respectively connect two pins of the BJT to a red probe and a back probe of the ohmmeter in turn to obtain impedances in each of two pins. In this way, a type of the BJT can be determined according to a relationship among the impedances. However, a plurality of tests may be time consuming and inefficient.

Therefore, there is room for improvement within the art.

Many aspects of the present embodiment can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiment.

FIG. 1 is a block diagram of a test circuit, according to an exemplary embodiment.

FIG. 2 is a circuit diagram of one embodiment of the test circuit shown in FIG. 1 .

FIG. 1 shows a test circuit 100 , according to an exemplary embodiment.

The test circuit 100 can measure a type and a common emitter current gain β of a bipolar junction transistor (BJT) 200 . The common emitter current gain β is a ratio of a collector current of the BJT 200 to a base current of the BJT 200 . The test circuit 100 includes a power supply 11 , a constant current circuit 12 , a first test circuit 13 , and a second test circuit 14 .

Referring to FIG. 2 , the power supply 11 may be a battery, for example. The constant current circuit 12 may be a current source, for example. The constant current circuit 12 is connected between the power supply 11 and the base of the BJT 200 , and outputs a steady base current Ib to the BJT 200 . An emitter of the BJT 200 is grounded. The collector of the BJT 200 is connected to the first test circuit 13 and the second test circuit 14 .

The first test circuit 13 includes a first light-emitting diode (LED) D 1 and a first resistor R 1 . An anode of the first LED D 1 is connected to the power supply 11 . A cathode of the first LED D 1 is connected to the collector of the BJT 200 through the first resistor R 1 .

The second test circuit 14 includes a second resistor R 2 , a third resistor R 3 , and an ammeter M 1 . One end of the second resistor R 2 is connected to the power supply 11 via the ammeter M 1 . The other end of the second resistor R 2 is connected to the collector of the testing BJT 200 via the third resistor R 3 .

In use of the test circuit 100 , the first test circuit 13 measures the type of the BJT 200 , and the second test circuit 14 measures the common emitter current gain β of the BJT 200 . In particular, if the BJT 200 is a npn BJT, the BJT 200 is turned on. In this way, the cathode of the first LED D 1 is connected to ground through the collector and the emitter of the BJT 200 (i.e., substantially grounded), and the anode of the first LED D 1 is connected to the power supply 11 . Thus, a potential difference between the anode and the cathode of the first LED D 1 becomes large enough to drive the first LED D 1 to emit light, thereby reminding users that the type of the BJT 200 is a predetermined type, npn, for example. Reversely, if the BJT 200 is a pnp BJT, the BJT 200 is turned off. In this way, the first diode D 1 is turned off and does not emit light.

When testing the common emitter current gain β of the BJT 200 , a value Ic of a collector current of the BJT 200 is detected via the ammeter M 1 . Since the constant current circuit 12 outputs a steady current Ib to the base of the BJT 200 , a value β of the common emitter current gain of the BJT 200 can be calculated according to a formula:

β= Ic/Ib.

In the present embodiment, the test circuit 100 further includes a protection circuit 15 . The protection circuit 15 is connected between the power supply 11 and the BJT 200 . In particular, the protection circuit 15 includes an optical coupler (OC) 151 , a first transistor Q 1 , a second transistor Q 2 , a fourth resistor R 4 , a fifth resistor R 5 , a sixth resistor R 6 , and a seventh resistor R 7 . In the present embodiment, both the first transistor Q 1 and the second transistor Q 2 are npn transistors. The OC 151 includes a second LED D 2 , and a phototransistor Q 3 . An anode of the second LED D 2 is connected to the power supply 11 . A cathode of the second LED D 2 is connected to the collector of the BJT 200 via the fourth resistor R 4 . A collector of the phototransistor Q 3 is connected to the power supply 11 by the fifth resistor R 5 . An emitter of the phototransistor Q 3 is connected to a base of the first transistor Q 1 . An emitter of the first transistor Q 1 is grounded. A collector of the first transistor Q 1 is connected between an end of the sixth resistor R 6 and a base of the second transistor Q 2 . The other end of the sixth resistor R 6 is connected to the power supply 11 . An emitter of the second transistor Q 2 is grounded. A collector of the second transistor Q 2 is connected to the power supply 11 through the seventh resistor R 7 .

In use of the protection circuit 15 , if the type of the BJT 200 meets a predetermined type (e.g., npn), the BJT 200 is turned on and an original voltage of the power supply 11 inputs to ground via the second LED D 2 and the BJT 200 . In addition, current is generated through the second LED D 2 , the collector, and the emitter of the BJT 200 . When the current passes through the second LED D 2 , the second LED D 2 emits light accordingly. The phototransistor Q 3 sensors the light emitted by the second LED D 2 , and accordingly outputs a current to the base of the first transistor Q 1 . Upon receiving the current from the phototransistor Q 3 , the first transistor Q 1 is turned on and the second transistor Q 2 is turned off. In this way, the collector of the first transistor Q 1 is connected to the power supply 11 by the seventh resistor R 7 , and generates a predetermined logic 1 (electric levels higher than a rated value).

Alternatively, if the BJT 200 is a pnp BJT, the BJT 200 is turned off, and the second LED D 2 , the phototransistor Q 3 are both turned off accordingly. Since the base of the first transistor Q 1 is connected to the power supply 11 by the OC 151 , the first transistor Q 1 is turned off and the second transistor Q 2 is turned on. In this way, the collector of the BJT 200 is connected to ground via the collector and the emitter of the second transistor Q 2 , and generates a predetermined logic 0 (e.g., electric levels lower than a rated voltage). Thus, the BJT 200 is protected from being further damaged due to an over-voltage generated by the collector of the BJT 200 .

In the present embodiment, the test circuit 100 further includes a first switch S 1 , and a second switch S 2 . The first switch S 1 is connected between the power supply 11 and the first test circuit 13 . The second switch S 2 is connected between the power supply 11 and the second test circuit 14 . In this embodiment, fixed contacts of the first switch S 1 and the second switch S 2 are both connected to the power supply 11 . Movable contacts of the first switch S 1 and the second switch S 2 are respectively connected to the first test circuit 13 and the second test circuit 14 . The type of the BJT 200 or the common emitter current gain β of the BJT 200 can be measured by operating the first switch S 1 or the second switch S 2 . For example, when the first switch S 1 is turned on and the second switch S 2 is turned off, the type of the BJT 200 is measured. When the first switch S 1 is turned off and the second switch S 2 is turned on, the β of the BJT 200 is measured.

In other embodiments, the ammeter M 1 can be replaced by a voltmeter M 2 . The voltmeter M 2 is connected in parallel to the second resistor R 2 or the third resistor R 3 . The collector current Ic of the BJT 200 is obtained according to a formula: Ic=Vc/R (Vc represents a voltage of the second resistor R 2 or the third resistor R 3 , the R represents an impedance of the second resistor R 2 or the third resistor R 3 ).

It is believed that the exemplary embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the disclosure.