My BC547 is getting very hot

Discussion in 'General Electronics Chat' started by pprmz89, Jun 21, 2018.

  1. pprmz89

    Thread Starter New Member

    May 28, 2018
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    Hi. I made very simple audio amplifier with circuit from youtube video. The amplifier work very well for me, but after some time the transistor BC547 is getting so hot. And even it failed to work. I saw this treat here:
    https://forum.allaboutcircuits.com/...t-where-should-i-be-putting-a-resistor.24714/
    and I think I must use resistor to cool my transistor, but my circuit is different then the @samoz's circuit.

    PPRMZ_amplifier.jpg

    Please tell me what to do.
    Thanks in advance.

    Moderators note : copied image to forum
     
    Last edited by a moderator: Jun 22, 2018
  2. ArakelTheDragon

    Well-Known Member

    Nov 18, 2016
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    The circuit is missing. You need a heat sink. Even if it is a 5W amplifier you still need a heatsink. The resistor is in order to stabilize the current.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    5,393
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    That circuit is trying to put hundreds of milliamps through the collector when the maximum rated collector current is only 100 milliamps.

    Put about 15 Ohms in series with the speaker, change the 1K to 10k, put a 1.8k resistor from the base to ground, and put a 1k resistor in series with the input capacitor. That should lower your collector current to something like 30 milliamps average and power dissipation to a little over 100 milliwatts. It won't be as loud as it was before but the transistor is much less likely to burn.

    upload_2018-6-22_15-47-28.png

    It is highly recommended that after you see this work you get back on the internet and look for a better amplifier or even build one around the LM380 audio amplifier IC. It is very likely that you will receive more suggestions in this thread.

    Apologies to @Audioguru for wrecking the speaker damping if there was much to start with.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    9,493
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    A BC547 is totally unsuitable for that post #1 circuit. It is rated at ~0.5W, but in your circuit it will be dissipating about 1.2W. No wonder it gets hot :eek:.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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  6. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    If the intention is to experiment with making an amplifier then that's OK.
    If the intention is to make an amplifier then how about the LM386?
     
  7. pprmz89

    Thread Starter New Member

    May 28, 2018
    3
    0
    Nice suggestions. Thank you very much.
    @MrChips, your circuit sims very interesting, but how to add second speaker?
    Also I'm looking for circuit that works with 2 8ohms 10 watts speakers. Can you suggest something? Thanks in advance.
     
  8. ArakelTheDragon

    Well-Known Member

    Nov 18, 2016
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    I am glad to hear, you need 2x10w and not 2x1000, when someone tells me he has a 1000W speaker and amplifier in his car and it works 5hours without draining the battery at all, while the car is not running, I tell him the nuclear powerplant is working for him.

    You already got a suggestion using LM380 or LM386. You can also try tda2050 but I have not tested it and I do not know how good it is.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Where, why and how do you want to add a second speaker?
    Do you want to add the second speaker in series or in parallel with the first on the same amplifier output?
    Or do you want to create a second audio channel for stereo?

    10W is substantially more than what has been presented so far. You need to tell us your complete objectives of this project.
     
  10. pprmz89

    Thread Starter New Member

    May 28, 2018
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    First I thought to add second speaker to this amplifier, but now I'm looking for circuit to make amplifier with TDA2050. My idea (for now) is to make 2 amplifiers(one for left and ane for right) with TDA2050 and to connect to them one speaker, and add double potentiometer for volume control.
     
  11. ebp

    Well-Known Member

    Feb 8, 2018
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    You cannot drive a single speaker with two separate amplifiers unless the amplifiers have a common input signal, and even then it is generally a bad thing to do. A slight imbalance in the phase shift caused by the input coupling capacitor and terminating resistor could cause one amp's output to be at a different voltage than the other, resulting in high current between the amp's outputs. If you have a stereo source and want just one speaker, then you can mix the two channels before they are applied to the input of a single amplifier.
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Do make up your mind. Do you want mono channel or dual channel (for stereo)?

    As ebp says, if you add two channels into one speaker (for more output power) you could end up blowing your amps.
    [​IMG]
    Doing the above is not recommended.


    If you want more power into a single speaker, you can bridge the outputs of the amp into one speaker if you invert the signal going into one of the amps.

    [​IMG]

    The two amps are driving the speaker with 180° phase difference.
     
  13. Audioguru

    Expert

    Dec 20, 2007
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    You do not increase the power to one speaker with amplifiers in parallel.

    Pick a modern amplifier IC. The TDA2050 is obsolete and is not made anymore.

    If you want to convert stereo to mono then you need a mixer circuit, not just shorting the source channels together.
     
  14. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    But the mixer could be as simple as two resistors, one from each input signal to the amplifier input.
     
  15. Audioguru

    Expert

    Dec 20, 2007
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    Correct. Two resistors mix audio signals together perfectly without destroying the signal sources like shorting them together might do.
     
  16. ArakelTheDragon

    Well-Known Member

    Nov 18, 2016
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    A lot of things are not made anymore, but we still use them.

    And yes putting the 2 cables on 1 input with 2 resistors is enough.
     
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