Reduce voltage in power supply of audio amp?

Discussion in 'General Electronics Chat' started by Mossen, May 26, 2011.

  1. Mossen

    Thread Starter New Member

    Dec 21, 2010
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    I have a transformer which steps 120V down to 25VAC. I was going to use some capacitors in my circuit that were rated for 25V but when I tested the output of my transformer I found it was actually closer to 30V.

    I've since bought new capacitors rated to handle that voltage (which cost me $8 at my LES :eek:, hence this question), but in the future is there anything I could have done, like with resistors or voltage dividers, to reduce the voltage so my 25V caps could have handled it?

    This is for an audio amp, my first one. Apparently its supposed to spit out 11W of power or so. If I used a voltage divider, does this mean I'd need to use resistors rated for 11W? Would it even make sense to do this or would you just buy the proper capacitors to handle the voltage?

    Thanks
    Mossen
     
  2. DigitalReaper

    Member

    Aug 7, 2010
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    25V AC is the RMS voltage, the peak voltage will be 1.414 times that. Unloaded voltages will be higher because manufacturers need to add a few extra turns to compensate for resistive losses at full load.

    For a 25v AC transformer (35.4v peak) you need 40v capacitors at minimum, preferably a little higher.
     
  3. Mossen

    Thread Starter New Member

    Dec 21, 2010
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  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Actually it is the center tap that is key here. The AC voltage is actually two signals of 12.6VAC tied together; each voltage makes half the DC output.

    Look at how ground connects the caps back to the transformer.

    Thus the caps will see less then 18V and should be just fine.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Electrolytic caps tend to act as a voltage clamp (sort of like a Zener diode) when their rated voltage is reached. If the current is not limited, the current through the capacitor may become excessive, causing the electrolyte to heat and boil, which will rupture the package.

    Are you measuring from -vcc to +vcc, or from +vcc to ground?

    You should actually be reading somewhere around 17.8v under load from ground to +vcc, and -vcc to ground. You will most likely read higher voltages under no load conditions.
     
    Last edited: May 26, 2011
  6. Mossen

    Thread Starter New Member

    Dec 21, 2010
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    Thank you Sgt

    As you probably guessed I am rather new at this. I was measuring the voltage with no load, so that is probably why I read 30V.

    Just for discussion's sake, and my understanding, what would happen in this circuit if those caps were 3300uf, or 1000uf instead of 4700uf? How would that affect the way it works? I am still trying to understand what components do and why the designer chooses certain types...

    Thanks
    Mossen
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    The lower the uF, the greater the ripple voltage will be under load.

    With 4700uF caps and a 2A load, you'll have about 3.5v ripple on your supply.
    With 3300uF caps, ripple goes over 5v.
    With 1000uF caps, ripple is around 17v. You'd hear nothing but 60Hz hum at full volume.
     
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