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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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  • Hi Andy,

     

    You can try a 470 ohm which will limit the current to the speaker to 8 mA. The problem is that most of the power will be across the resistor and the speaker may not be loud enough to hear. You could also try a small audio output transformer with a 500 ohm input and an 8 ohm output. This at least would put more of the power through to the speaker. Probably a good idea to put a flyback diode across the transformer primary so the spikes don't kill your FPGA's output. Another option would be to use a logic level MOSFET to drive the speaker and use the FPGA to drive the MOSFET.

     

    John

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  • Hi Andy,

     

    You can try a 470 ohm which will limit the current to the speaker to 8 mA. The problem is that most of the power will be across the resistor and the speaker may not be loud enough to hear. You could also try a small audio output transformer with a 500 ohm input and an 8 ohm output. This at least would put more of the power through to the speaker. Probably a good idea to put a flyback diode across the transformer primary so the spikes don't kill your FPGA's output. Another option would be to use a logic level MOSFET to drive the speaker and use the FPGA to drive the MOSFET.

     

    John

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