What resistor value do I need?

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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jw0752 over 7 years ago +6

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…
dougw over 7 years ago +5

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…
dougw over 7 years ago in reply to andywest +4

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

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jw0752 over 7 years ago

Hi andy,
How about using a piezo speaker, then you could forget about the resistor.
John
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andywest over 7 years ago 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?
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andywest over 7 years ago 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?
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jw0752 over 7 years ago 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
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andywest over 7 years ago in reply to jw0752

That's true, jw0752. 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.
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jw0752 over 7 years ago 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
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andywest over 7 years ago in reply to jw0752

No problem at all! Thanks for all your suggestions... it looks like this is the one I'm going with.
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gecoz over 7 years ago in reply to andywest

Hi Andy,

If you interested, Murata has published a nice document on its piezoelectric sound components, which also includes some notes on the way they are connected. Although your piezo is from a different maker, you might still find some useful information that apply to your case.

Fabio
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dougw over 7 years ago in reply to andywest

I like jw0752 piezo speaker idea. You can still get double the voltage and correspondingly more volume by using it in a bridge-tied-load configuration.
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andywest over 7 years ago in reply to gecoz

Thanks! I welcome the opportunity to learn new things.
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