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**Unijunction Transistor Circuit Templates**

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**1 pn junction 2 pn junctions n n + + pn junction p anode cathode**

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

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**1 pn junction 2 pn junctions n n + + + pn junction p anode cathode**

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

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emitter Base 1 Base 2 e b2 b1 UJT emitter p n current meter + base 1 base 2 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. p n Low voltage signal applied to emitter OFF current meter

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

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**Always a small flow of current in this branch of circuit.**

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

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**Always a small flow of current in this branch of circuit.**

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

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**Always a small flow of current in this branch of circuit.**

+ n v threshold + 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 ON while capacitor discharge current is flowing. Light is OFF when capacitor voltage is capacitor below UJT’s threshold voltage value. Unijunction transistor in a light flasher circuit

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**Voltage across capacitor drops as capacitor current discharges n +**

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

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**n p n + Unijunction transistor in a light flasher circuit capacitor B2**

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McGraw-Hill 5-1 © 2013 The McGraw-Hill Companies, Inc. All rights reserved. Electronics Principles & Applications Eighth Edition Chapter 5 Transistors.

McGraw-Hill 5-1 © 2013 The McGraw-Hill Companies, Inc. All rights reserved. Electronics Principles & Applications Eighth Edition Chapter 5 Transistors.

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