Why my transistor is heating up to failure

Solution
That 741 is to be configured as a Comparator and NOT as a Linear amplifier.
Obviously, the transistor is operating in a Linear mode and not as a Switch.
It will heat up and fail.
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That 741 is to be configured as a Comparator and NOT as a Linear amplifier.
Obviously, the transistor is operating in a Linear mode and not as a Switch.
It will heat up and fail.
 
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Solution
CHILLFACTOR said:
Then please suggest me the modification for proper working of the ckt..
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The 100k resistor should be connected between the non-inverting input and the output of the op-amp, instead of between the inverting input and the output.

Also the minimum output voltage of the 741 may be too high, which would cause the transistor to be conducting some current when it was supposed to be off. You should add a 470 Ohm resistor between the base and emitter of the transistor.

There is no decoupling capacitor, and there is the risk that the op-amp is oscillating.

The 741 was invented in 1968, so the design is 56 years old. Unsurprisingly, there are now better op-amps available.
 
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The 741 is a poor choice, this is a 741 Data Sheet. I suggest you read it. The 741 is also a dual supply operational amplifier, not a single supply. Since you want a comparator verse op amp I suggest using a comparator circuit design. However if you insist on the ancient 741 operational amplifier I suggest a Google of 741 Op Amp Comparator Circuit and again the 741 requires a dual supply not a single supply.

Ron
 
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If you move 100k from the 741 out to the inverted collector , it will have hysteresis, work OK and be cold. This will easily work on a 741 but I won't shame this choice. There are better ways.

Previously the 12 V, 0.36W coil will split this load evenly with Q1 when Vcoil was 6V making this your hot error with -ve feedback.

Using +ve feedback latches the output Off (high at 10Vout or On at 0.5V).

If one can make it work easily with a 741 and understand why it works, it is a learning experience. Swapping the resistor to Vin+ is a slightly worse design choice since Vo was not rail to rail and potential pot low values make the 100k R ratio for hysteresis almost nil. But either way, it will have some functional differences but work. The 100k may be increased to 1M for <10% hysteresis
 
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The main error is the incorrect connection of the 100kΩ feedback resistor, as noted by others.

But even with that change, the circuit will not work well, since the 741 output will only go to a minimum of about 2V, thus likely keeping the transistor partly turned on.
Better to use a single-supply comparator, such as the LM339 or LM393, or the LM324 op amp in place of the 741.
 
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You also have the common mode range limitation of the 741 :



When the LDR value drops you must insure it worst case keeps the V into 741 > 3V, or use a opamp/
comparator with input common mode range = to its negative supply pin V, in your circuit ground.


Regards, Dana.
 
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The reason, your transistor got hot is due to errors on the schematic from another website, unfortunately. If Vce is at 6V when driving 40 ohms at 20 mA it dissipates 1/8th Watt and gets hot. (pinterest and pcbhaven had this

It is true that the 741 cannot drive much current within 2V of Vee (-) but in this situation, it can get down to 0.9V which is still not low enough to go below Vbe threshold around 0.6V for this coil drive current of about 40 mA.

Using 1k to 1.2k to attenuate the output will make it work if added as an attenuator across base-emitter.

It would be better to say what LDR is used and what light thresholds you expect for on and off. and start with no feedback resistance of 100k... and look for better sources of schematics with simulators.

Here even the 741 works to adjust the LDR at low brightness levels to turn off the relay.


A better solution is the open collector comparator suggested by crutschow

Have you heard of Murphy's Law?
 
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This demonstrates if you can make it work with a 741 over a wide range of LDR & pot settings, you can always make it work with better parts. The 741 emulation here uses the discrete model of a 741.
 
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With all its limitations, the 741 can be used with a Single supply ( min 12 Volts). Just have to keep in mind that the output voltages cannot reach the Bus voltages - 0 or Vcc.
 
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Below is the LTspice sim of a 741 circuit to drive a transistor from a single supply:

It has two diodes in series with the transistor base to insure the transistor is off when the 741 output is at it's minimum low voltage.
R4 is connected from the 741 output to the (+) input (not the minus input as shown in the original schematic) to give a small amount of hysteresis so the relay doesn't chatter around the trip point.

The relay turns on (red trace) when the LDR resistance (green trace) drops below 100kΩ for the 50% setting of the pot.

 
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At that rate, I'd rather grab an old LM386 audio amplifier, wire it as a comparitor and drive the relay coil directly from the output.
 
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