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What resistor value do I need?

andywest

I have an 8 ohm, 0.5 watt speaker that I want to drive directly from an IO pin of an FPGA. I just want to generate simple square wave beeps, so I think I can connect like this:

 

FPGA IO pin -> resistor -> speaker -> ground

 

The IO pin is set to 3.3V LVTTL, and I can choose between 4mA and 8mA.

 

My question is what resistor values would I need for 4mA and 8mA to get the loudest volume without drawing too much current?

 

Thanks!

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  • 3.3v / 8mA = 412.5 ohms total

    412.5 - 8 = 404.5 ohms for the extra resistor

    You might get double the effective voltage across your speaker by using a bridge tied load configuration.

    (use 2 output pins - the second one goes to the other side of the speaker)

    The two outputs are the inverse of each other.

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  • 3.3v / 8mA = 412.5 ohms total

    412.5 - 8 = 404.5 ohms for the extra resistor

    You might get double the effective voltage across your speaker by using a bridge tied load configuration.

    (use 2 output pins - the second one goes to the other side of the speaker)

    The two outputs are the inverse of each other.

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