Troubleshooting of a Transistor circuit

Troubleshooting  a circuit:

• 1.       Measure the Vce.
• 2.       When troubles come, they are usually big troubles like shorts or opens.
• 3.       Short: when  devices damaged, solder splashes across resistors.
• 4.       Open: when components burn out.
• 5.       These troubles produce large changes in voltages and currents

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Cause of these troubles:

1.       Trouble in the power supply

2.       An open lead between the power supply and the collector resistor

3.       An open collector resistor and so on.

4.       Open base resistor

Trouble	VB, V	VC,V	Remarks
None	0.7	12	No trouble
RBS	15	15	Transistor Blown
RBO	0	15	No Base or Collector Current
RCS	0.7	15	Check RC and Supply  connection
RCO	0.7	0	“
No VBB	0	15	Check supply and lead
No VCC	0.7	0	“

For Example:

If V_BB=15V, R_B=470kohm, R_C=1k ohm, V_CC=15 V, current gain= 100

We assume ideal condition. Suppose R_B= 1 M ohm, then base current would be 15 uA. Here R_B is 470 k ohm nearly half of the 1M ohm. Hence base current would be double, i.e 30 uA. And current gain is 100. So collector current would be 3 mA. When it flows through 1 kohm resistor, it produces a voltage drop of 3 V. So, V_CE would be 12 V, or else there is something wrong in this circuit.

Posted in: Electronics