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Thread: OT What voltage is used on speakers?

  1. #1

    Default OT What voltage is used on speakers?

    Specifically, what will happen if I connect the output from a plug-in AC to 4.8-volt 50ma DC converter to the terminals of the speaker from a portable radio?

    Roger

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    At first, the speaker cone will pop toward you or away from you, depending on the polarity of your connection. In time, you may release magic smoke from the power supply as the speaker has very low dc resistance (pretty close to a short circuit).

    If the power supply were capable of more current, then the magic smoke would come from the speaker voice coil instead, burning varnish and twisting out of shape.

    Den

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    You'll hear a 'click' from the speaker, and there's a good chance you'll ruin the DC supply.

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    Thats a very good description . :-)
    ...lew...

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    Roger,

    Why?
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  6. #6

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    A rapid clicking sound is what I'm after, but I don't want to see any magic smoke.

    The converter is normally used for charging a battery pack. So, it seems like it would be OK to run the output from the converter through the speaker coil in series with the battery. Would that damage anything? Maybe adding some resistors in the circuit, and removing them in stages?

    I can't think of any way I can accurately measure the voltage that's normally on the speaker coil when it's in use, or I'd just do that.

    Roger

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    Ohm's law will do. Look on the speaker. It will usually be marked as to dc resistance such a 8 ohms or 3.2 ohms. Many will also be marked for wattage such as 0.1 watts for a small cheap speaker.

    With that you can then calculate the current needed to produce no more than 1/10 watt. P (Wattage) = I*E (I is current, R is resistance, E is voltage[Electromotive force]). Assuming 8 ohms, to produce 1/10 watt we calculate I=P/R which equals .0125 amps. If you have a known voltage to start with such as 5 volts (close enough but there is a gotcha to deal with, later on that) then we use Ohm's law to solve for the total resistance required to limit the current to .0125 amps maximum. R=E/I .

    This gives 5/.0125 or 400 ohms total resistance. We don't need to be exact here so any resistor at least that large or a bit larger will do.

    The gotcha: Small wallwart power supplies are not usually regulated and will often produce considerably higher voltage than the rating when they are lightly loaded. In that case it is necessary to measure the actual output voltage and plug in the actual voltage measured to calculate the required resistance.
    Last edited by Evan; 10-14-2006 at 02:36 PM.
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    You won't get rapid clicking unless the DC converter is doing a poor job of converting.

    If you know the orignal max output to the speaker(watts) and you know the speaker resistance(ohms), you can calculate the max volts to the speaker.

    Volts = Square Root(watts - ohms)

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    Roger,

    You won't be able to run the speaker in series with the battery being charged. You can put it in parallel using an appropriate resistor in series with the speaker. It won't make any clicking unless the charger operates by turning the voltage on and off as some do. If you need clicking with a supply around 5 to 6 volts then you could put an ordinary flashing six volt mini christmas light bulb in parallel with the speaker/resistor combination. This would give a flashing light as well as produce a click each time it turned on and off.
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    Quote Originally Posted by Evan
    Ohm's law will do.

    P (Wattage) = I*R (I is current, R is resistance). .
    Evan, I think you wanted to say that I * R is voltage. Voltage * current is wattage. Take care. Vic

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