May I know the value of the burnt resistor?Thanks!!

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It could be anywhere from 82R to 680R, with no other data it's hard to tell. If the resistor is burned, the transistor base is likely open. Check the 240R resistor, it dissipates more than a watt and may be damaged as well.
 
It could be much higher than that, it's really EXTREMELY uncritical, and in many cases could be left out entirely.
 
Boncuk said:
... because it's absolutely senseless and worthless in this position.
Click to expand...

Not really, it ensures the output transistor stays turned OFF if there is slight leakage in the driver transistor - it's good practice to include it, but it's value can be almost anything higher than the lower resistor - preferably a LOT higher though.

Quick thought - if that resistor has burnt up, it can only happen if the output transistor has failed (O/C BE), otherwise there's only 0.7V across it.
 
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hi,
I agree, a 'high' base/emitter resistor is recommended.

The fact that resistor has burnt out, suggests the pnp is a short circuit [base/collector]
 
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ericgibbs said:
hi,
I agree, a 'high' base/emitter resistor is recommended.

The fact that resistor has burnt out, suggests the pnp is a short circuit [base/collector]
Click to expand...

And has to be O/C base/emitter as well, or just O/C base/emitter could make it burn up as it would have far too much voltage across it.
 
The value of the base-emitter resistance affects the turn-off time of the transistor. See below.
 

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Roff said:
The value of the base-emitter resistance affects the turn-off time of the transistor. See below.
Click to expand...

It's of VERY little concern though, if it was of concern you would make better arrangements than that so it turned off much faster still.
 
Nigel Goodwin said:
It's of VERY little concern though, if it was of concern you would make better arrangements than that so it turned off much faster still.
Click to expand...
I was taking issue with those who said that Rbe is unnecessary, or at least irrelevant. It may or may not be important in this particular circuit.
 
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