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Low amps cause my amplifier to smoke?

Discussion in 'General Electronics Discussion' started by kcroy, Oct 27, 2012.

  1. kcroy

    kcroy

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    Jan 29, 2012
    I have a noob test station set up - Voltage generator that does 20v and 2 amps & a multimeter. My background in electronics is minimal, but learning as i go.

    I wanted to see if a car amplifier would work ( 12v / 30 amps ) - so hooked it up to my test generator. Lights came on. Seems good.

    Plugged in a speaker, and tried to run some music through it. A few clipped bits of music and then the amplifier started smoking.

    It was only pulling 12v, and was max at 2 amps. I don't understand why/how this would have damaged the amplifier.

    Any input appreciated!
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi kcroy

    wow up there lol .... you said the generator puts out 20V, what made you think that your poor 12V amplifier could handle 20V ?

    Rule1 ... you MUST supply a piece of gear with the voltage its designed to work at else it will let out the magic smoke as you have discovered

    Rule2 ... if you expect a piece of gear to work properly it must be able to get the current it requires. 2 Amps from supply doesnt = 30 Amp requirement

    Rule3 ... You always use a power supply that is capable of more current ( Amps) than what the gear needs.
    Else the power supply will be working to its max and is also likely to overheat and fail

    so say you have a bit of gear or a project you have built and it requires 2 Amps, you power supply should be capable of at least 3 Amps, so its not being too heavily loaded

    here's some basic power requirements ....

    P( Watts) = V (Volts) x I ( Amps)

    12V x 30 Amps = 360Watts

    20V x 2 Amps = 40Watts

    But the main problem in your case was the 20V .... the components in the amplifier are designed for 12V NOT 20V ;)

    see the problem ?

    cheers
    Dave
     
    Last edited: Oct 27, 2012
  3. kcroy

    kcroy

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    0
    Jan 29, 2012
    Hi Dave,

    Thanks for replying. I left out some important information:

    In regards to your #1: The generator does 0-20v, I had it putting out 12.00 v. I can see why mismatching the voltage would be a problem, but don't believe that to be the problem.

    #2 - I can understand that the equipment would not work properly - hence the clipped sound - I am not sure how this would cause the amp to smoke. This would put stress on the voltage generator, NOT the amplifier correct?

    #3 - yes - I didn't realize the draw was that high when I first hooked it up. Looks like I'll need a second voltage generator that handles higher amperage.

    update: popped the amp open and don't see anything obviously burned - caps look ok. What is a good next step? Hook it up to a proper power source ( car battery ), and see if it still works?
     
  4. duke37

    duke37

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    Jan 9, 2011
    You could connct it to a 12V battery in series with a headlight bulb. If the quiescent current is low, the lamp will not be bright.

    If the lamp is bright, measure the voltage across the radio. If it is very low then there is a short inside. Check for any hot components.
     
  5. kcroy

    kcroy

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    Jan 29, 2012
    awesome, thanks. Neat trick with the headlight!
     
  6. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Darned right, duke 37.
    I've often used an incandescent lamp in the place of a fuse when I wasn't sure what
    was going to happen in a circuit. A fuse just pops. A lamp in series handles the current,
    and allows you to find the problem in the circuit.
    Great suggestion here.
     
  7. penfold

    penfold

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    Oct 24, 2012
    The problem that caused the smoke was probably the current limiting action of the power supply. As the current begins to exceed what the supply can handle the voltage will be throttled back in an attempt to maintain a constant current. As this happens, the internal circuits of the amplifier will not longer be under the voltage conditions they were designed and poor old semiconductors really don't like that.

    My bet is that smoke was silicon. I've had a lot of FETs find the way out of their packaging leading hardly any trace.

    As mentioned before you should be using a power supply capable of nearly 1.5 times what you're expecting to use it for, but in particular with audio the RMS power is really drawn in low frequency chunks, think the beat of the music, these extra chunks have to come from somewhere and typically be too low a frequency for the decoupling caps to smooth. (Just though this could be a useful note (that is seriously not a pun).)
     
  8. kcroy

    kcroy

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    Jan 29, 2012
    Thanks - hopefully if I damaged anything, it is something I can fix.

    I'm still trying to wrap my head around electricity having frequencies like music...
     
  9. davenn

    davenn Moderator

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    Sep 5, 2009
    ?? what do you mean by that ?


    Dave
     
  10. kcroy

    kcroy

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    Jan 29, 2012
    "RMS power is really drawn in low frequency chunks"
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Perhaps better to think of it as drawing power in bursts (in this case).

    I have read through Penfold's answer a few times (disagreeing with it) before I have come to the realisation that he's almost certainly right.

    Incidentally, this would also mean that connecting a bulb in series would be a bad thing for this amplifier too...
     
  12. kcroy

    kcroy

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    Jan 29, 2012
    because that would drain current, and we don't want to under deliver?

    Light would be a pretty trivial drain, wouldn't it?
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, now it's my turn to be confused.
     
  14. kcroy

    kcroy

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    Jan 29, 2012
    why would connecting the bulb in series be a bad idea?
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Penfold's assertion was that the amplifier was essentially damaged by undervoltage, even at a current much lower than it would normally operate at.

    The entire point of a series bulb is to react to sustained overcurrent demands by reducing the voltage to the device. Essentially it will cause exactly the same situation that appeared to have damaged the amplifier in the first place.

    Of course, this may only happen over some restricted range of voltages and currents, but it's enough to recommend caution because there is no way of knowing if you're going to be in that danger zone or not.
     
  16. kcroy

    kcroy

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    Jan 29, 2012
    If the headlight requires 10 amps, does the amplifier have to work "harder" to get the 20 amps it needs?

    Or does the battery just need to "push" harder.

    Some of these basic electrical concepts frustrate me.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If the headlight is rated at 10A (120W) then from a 12V supply the amplifier, if in series with the bulb, will NEVER get 20A.

    That would translate to a higher voltage. Yes, that would work, but the entire reason for having the bulb there is to reduce the maximum current available.

    Yes, and you have an odd situation here.

    Running an amplifier from a lower voltage/current would normally be like driving a car slower than it is normally driven. If you hit something you will do less damage.

    Your amplifier seems to be behaving like a plane. If you fly it slower it falls out of the air and crashes.
     
  18. kcroy

    kcroy

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    Jan 29, 2012
    oh - I thought the purpose of the bulb was to show if the amp was pulling enough current.

    I'll have to think about this more tomorrow. Thanks for responding to my posts!

    For now though - I hooked up the amplifier properly to a car battery, and it seems to be working. Not sure if any damage will show up once I hook it up to some proper speakers.
     
  19. penfold

    penfold

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    Oct 24, 2012
    Now, I would advise initially connecting the bulb in series, that way under no real load (just quiescent conditions) the amp *shouldn't* be drawing too much current and the bulb will present very little barrier (the resistance of a bulb is very low at low current remember). If however there is now a fault that will cause excessive current the bulb will light.

    Assuming that the bulb does not glow, before actually amplifying anything, short out the bulb, this will prevent any undesired under voltages.

    I am in no way certain that an under voltage would have damaged your amp, but I have had a few rare cases where similar has happened. Current limits from power supplies also have a tenancy to causes funny oscillations when you start drawing current close to the limit, that could also be a possibility.
     
  20. duke37

    duke37

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    Jan 9, 2011
    You do not give details of the amp however, at low output powers the current drawn should be very low and the bulb would not light or just glow.

    Consider a drum, to get sound out you have to give it a thump with a stick to put energy in. An amplfier has to provide an electrical thump to the speaker to reproduce the sound. Power amplifiers will not take much power when they are not driving the speakers hard so start with the volume at zero and go gently.

    If the power supply cannot provide the peak current the amplifier requires then the signal will be clipped and you will get distortion.

    If smoke has come from the amplifier it seems to me that it is likely to be damaged.

    The bulb trick is very useful to find shorts in vehicle wiring.
     
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