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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!

  • 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

  • 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.

  • in reply to dougw

    Interesting. So two output pins, one set low while the other is high and vice versa, and that's it? Nothing connected to ground, and no extra components?

  • in reply to andywest

    You still need the resistor on one pin but no ground.

  • Hi Andy,

     

    If you are going to drive the speaker with a current between 4mA and 8mA, I suspect you will struggle to ear anything at all. For a 0.5W 8 Ohm speaker, nominally you get the loudest sound when you drive the speaker at maximum current. Doing a little calculation, the max current would be 250mA. Assuming you will be able to drive the speaker with 8mA current, this would only give 0.5mW.

     

    To make things worse, maximum power transfer between the source and the load requires the two to have matching impedance (8 Ohm in this case). I don't know what is the resistance of the FPGA output pin, but a typical termination resistance value is 50 Ohm, in which case, even connecting the output pin directly to the speaker (not a good idea!), there is already quite a mismatch between the source and the load. Adding 400 Ohm resistor to limit the output current will only make things worse, and most of the power will end up being dissipated there rather than reach the speaker.

     

    I think the easiest way to interface the speaker with the FPGA output is to do what John suggested: use a MOSFET to drive the speaker, and drive the MOSFET with the FPGA output pin.

     

    One note on the use of 2 output pins to get a differential output with doubled range (i.e. -3.3V to +3.3V): I would check your FPGA datasheet if negative output voltages are supported first, as most FPGAs only support positive output voltages (i.e. 0V to +3.3V). If you really want/need to increase the output range, you would need to use a level shifter (you can build one using an OPAMP, and use it as as comparator), but even doubling the range, you will still have the limitation on the output current imposed by the FPGA output pins.

     

    Fabio

  • Hi andy,

    How about using a piezo speaker, then you could forget about the resistor.

    John

  • in reply to jw0752

    That's an interesting thought. I have one of these:

     

    http://www.newark.com/multicomp/mckp1227sp1-4721/buzzer-piezo-1-3khz-86db/dp/25R0900

     

    but since it's also 8 ohms, I figured that at 3.3v I would again need a ~400 ohm resistor.

     

    Here's one with a rated max current of 3.5mA:

     

    https://www.adafruit.com/product/1739

     

    Am I correct in thinking that should work without a resistor as you suggest?

  • in reply to andywest

    Hi andy,

    It sounds like you have a buzzer there and not a speaker. The buzzers have an internal oscillator and will make a fixed tone when attached to power . The piezo speakers will have a much higher impedance and will not make a separate noise from what you put into them. they will draw considerably less current than the buzzer.

    John

  • in reply to jw0752

    That's true, . Please look at the second link (the adafruit one). I just ordered that today because it looks like it can take a square wave and has all the specs I'm looking for (1-30Vp-p operating voltage, 3.5mA max current, and a min. sound output at ten cm of 85db).

     

    What is your opinion on whether this will fry my FPGA if connected directly to IO and ground?

     

    By the way, thanks everyone here for your help and patience. I know these are basic questions.

  • in reply to andywest

    Hi Andy,

    Sorry I didn't look closer to your previous post. Yes this will work directly on the IO pins of the FPGA and will not harm the gates.

    John