Unijunction Transistor Circuit Templates

1. Unijunction Transistor Circuit Templates This presentation is partially animated. Only use the control panel at the bottom of screen to review what you have seen. When using your mouse, make sure you click only when it is within the light blue frame that surrounds each slide.

2. pn junction p current meter n anode volt meter cathode This diode is forward biased collector current meter base + emitter 0 0 0 0 0 0 n + + p pn junctions current + meter n + + volt volt meter meter This transistor is in conduction 1 pn junction 2 pn junctions

3. pn junction p current meter n anode volt meter cathode This diode is forward biased current collector meter 0 0 + base 0 + emitter n + + + + + + 0 p pn junctions current + meter n 0 + 0 + volt volt meter meter This transistor is in conduction 1 pn junction 2 pn junctions Base 2 b2 emitter e b2 n b1 e UJT Base 1 p b1

4. Base 2 b2 emitter e b1 Base 1 UJT emitter p n n current base 2 base 1 meter 0 0 + + p n n current meter Low voltage signal applied to emitter OFF Low current flow from base 1 through base 2 A UJT (unijunction transistor) is a voltage-controlled switch that does not amplify the current in the load circuit. Low voltage signal applied to emitter OFF

5. Base 2 b2 emitter e b1 Base 1 UJT p n n 0 0 0 + + p n n current meter Low voltage signal applied to emitter OFF Low current flow from base 1 through base 2 emitter current A UJT (unijunction transistor) is a voltage-controlled switch that does not amplify the current in the load circuit. base 2 base 1 meter High voltage signal ON applied to emitter emitter base 1 High current flow from base 1 through emitter

6. + + + + + Always a small flow of current in this branch of circuit. B2 Current flows into this circuit branch until voltage at node becomes high enough to stop the flow. n p n B1 capacitor But not enough to turn the light on. Unijunction transistor in a light flasher circuit

7. v threshold + + + + + Always a small flow of current in this branch of circuit. B2 Current flows into this circuit branch until voltage at node becomes high enough to stop the flow. n p When node voltage reaches threshold value, the capacitor discharges current through the emitter to B1 circuit n (This charging current continues its flow into the capacitor until the capacitor becomes fully charged and node reaches the threshold voltage value.) B1 Light is ON while capacitor discharge current is flowing. capacitor Unijunction transistor in a light flasher circuit

8. v threshold + + + + + Always a small flow of current in this branch of circuit. B2 n p When node voltage reaches threshold value, the capacitor discharges current through the emitter-B1 circuit Voltage across capacitor drops as capacitor current discharges n B1 Light is OFF when capacitor voltage is Light is ON while capacitor discharge current is flowing. capacitor below UJT’s threshold voltage value. Unijunction transistor in a light flasher circuit

9. + + + + + Always a small flow of current in this branch of circuit. B2 When voltage across capacitor drops low enough current starts following in this branch again and cycle repeats itself. n p Voltage across capacitor drops as capacitor current discharges n B1 capacitor Unijunction transistor in a light flasher circuit

11. + + + + + B2 n p n B1 Unijunction transistor in a light flasher circuit capacitor