Preparation for Troubleshooting : • A labeled schematic with proper dc and signal voltages. • You should know the expected voltages in the circuit should be before you can identify an incorrect voltage The Complete Troubleshooting Process : • Identify the symptom(s): Check the input and output voltages. • Perform a power check: Confirm dc supply is correct. • Perform a sensory check: Check for wrong connections or extremely hot components. • Component Replacement: Determine a certain symptom to indicate a component may be defective. • Apply a signal-tracing to isolate the fault to single circuit: Determine the faulty component which caused the incorrect voltages. • Use replacement or repair to fix the problem Troubleshooting Transistor Many digital multimeters(DMMs) have a diode test position that provides a convenient way to test a transistor. If the junction is good, you will get a reading of 0.7V being typical for forward bias. If it is reverse bias, you will see the reading of OL, indicates that the junction has an extremely high reverse resistance Summary of Transistor Bias Circuits You can view the transistor as two diodes connected as shown in the figures below The base-to-emitter junction voltage must be in the range from 0.5 to 0.8 volts if working properly. The test includes the measurement of the emitter-to-ground voltage and the base-to-ground voltage and subtract the emitter voltage from the base voltage. If there is problem, the junction voltage will not even be close to the correct (0.6-0.7V) value. Quick Test on Collector-to-Emitter Voltage The voltage (V CE ), ideally is equal to roughly one-half the supply voltage. V CE voltage of 0.2 to 0.8 volt usually indicates saturation and an excessive base bias condition. Any V voltage of less than about 3 volts merits further The trick in this test is to force the transistor’s collector voltage to change, to see if the base actually has control over the collector current. Connecting a temporary jumper from base to emitter to turn the base current off. If the collector current turns off, as it should, the collector voltage will rise to approximately the power-supply voltage. If this doesn’t happen, the transistor is probably defective. Troubleshooting Techniques Quick-Test on Base-Emitter Junction Voltage Quick-Test on Pull-Down Test for Transistor Operation BASE BIAS • Q-point values • Collector Current: • Collector-to- emitter voltage: B BE CC DC c R V V I C C CC CE R I V V E I C I VOLTAGE-DIVIDER BIAS • Q-point values • Collector Current : • Collector-to- emitter voltage: E BE CC C R V V R R R I 2 1 2 E C C CC CE R R I V V E C I I COLLECTOR-FEEDBACK BIAS • Q-point values • Collector Current : • Collector-to- emitter voltage : DC B C BE CC C R R V V I C C CC CE R I V V E C I I E C I I EMITTER BIAS • Q-point values • Collector Current : • Collector-to- emitter voltage: DC B E BE EE C R R V V I E C C EE CC CE R R I V V V E I C I

Troubleshooting Techniques

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Preparation for Troubleshooting : A labeled schematic with proper dc and signal voltages. You should know the expected voltages in the circuit should be before you can identify an incorrect voltage The Complete Troubleshooting Process : - PowerPoint PPT Presentation

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Preparation for Troubleshooting :

• A labeled schematic with proper dc and signal voltages.• You should know the expected voltages in the circuit should be before you can identify an incorrect voltage

The Complete Troubleshooting Process :

• Identify the symptom(s): Check the input and output voltages.• Perform a power check: Confirm dc supply is correct.• Perform a sensory check: Check for wrong connections or extremely hot components.• Component Replacement: Determine a certain symptom to indicate a component may be defective.• Apply a signal-tracing to isolate the fault to single circuit: Determine the faulty component which caused the

incorrect voltages.• Use replacement or repair to fix the problem

Troubleshooting Transistor

Many digital multimeters(DMMs) have a diode test position that provides a convenient way to test a transistor. If the junction is good, you will get a reading of 0.7V being typical for forward bias. If it is reverse bias, you will see the reading of OL, indicates that the junction has an extremely high reverse resistance

Summary of Transistor Bias Circuits

You can view the transistor as two diodes connected as shown in the figures below

The base-to-emitter junction voltage must be in the range from 0.5 to 0.8 volts if working properly.

The test includes the measurement of the emitter-to-ground voltage and the base-to-ground voltage and subtract the emitter voltage from the base voltage.

If there is problem, the junction voltage will not even be close to the correct (0.6-0.7V) value.

Quick Test on Collector-to-Emitter Voltage

The voltage (VCE), ideally is equal to roughly one-half the supply voltage.

VCE voltage of 0.2 to 0.8 volt usually indicates saturation and an excessive base bias condition.

Any VCE voltage of less than about 3 volts merits further investigation.

The trick in this test is to force the transistor’s collector voltage to change, to see if the base actually has control over the collector current. Connecting a temporary jumper from base to emitter to turn the base current off. If the collector current turns off, as it should, the collector voltage will rise to approximately the power-supply voltage. If this doesn’t happen, the transistor is probably defective.

Troubleshooting Techniques

Quick-Test on Base-Emitter Junction Voltage Quick-Test on Pull-Down Test for Transistor Operation

BASE BIAS• Q-point values

• Collector Current:

• Collector-to-emitter voltage:

BECCDCc R

VVI

CCCCCE RIVV

VOLTAGE-DIVIDER BIAS• Q-point values• Collector Current :

• Collector-to-emitter voltage:E

BECC

VVRR