 # Choosing right resistor for speaker

Hi there, I'm new here.
I read many discussions posted here but I never really understood the solution.
I have an arduino uno and I just bought a 8 ohm 3 watt speaker.
The producer suggest a 1kOhm resistor and I really don't get why.
I know ohm's law but I just don't get why I would use a 1kohm resistor.

What value would you suggest?
(And why?)

I don't know, this is why I'm asking, I am probably doing wrong calculations using ohm's law.
Since 5v are the volts of the arduino and 20 mA of the pin,I would've done 5-3.4/0.020
But It's probably wrong

@loddem, your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with your project (or questions about it). See About the Installation & Troubleshooting category.

Where does the 3.4 in your calculation come from? You're on the right track, 5V / 20mA should be around 250 Ohm.

I thought 3.4 were the volts of the speaker and therefore 5-3.4V.
Since 5 are the volts of the arduino and 20 mA the amperes of the pin, wouldn't I always obtain 250 ohms and therefore I would use the same resistor for everything... I'm a bit confused

What would be the difference between the resistor for a 3 watt 4 ohm speaker and a 3 watt and 8 ohm speaker?
I've been trying to understand this for quite a bit

There are two formulas
V = I * R (or I = V / R)
P = V * I

Just in case, P stands for power / watt.

You can substitute V or I in the second formula so you get
P = V2 / R
P = I2 * R

3W / 8 Ohm and reworking the formula
P * R = V2.so V = 4.9V
P / R = I2, so I = 0.6A

If you want to protect the Arduino, the current will be 0.02A (whicg requires the 250 Ohm resistor) and the power in the speaker is 3.2mW.

1 Like

I got a bit lost... may I please ask you to explain it again?
I know v=IR and P=VI... I got lost after that..
What would i do if I had a 4 ohms and 3 W speaker?. ?

There is no difference, you use the same resistor. If you want to do it correctly, there is a minor difference. To limit the current to 20 mA, the total resistance needs to be 250 Ohm. This is the combination of resistor and speaker. So the resistor is 250 - 8 = 242 Ohm or 250 - 4 = 246 Ohm.

But seeing as you would never be able to tell the difference anyway, a common 270 Ohm resistor will be just fine. I think my main problem is that I just dont really get how to do the calculation..

So basically you do the voltage of the arduino, divided by the current or the pins and then the result is gonna be the ohms.
Finally you subtract the obtained ohms by the ohms of the speaker?

Yes

Replace that with
Divide by the current you want to flow through the speaker.

The speaker’s power rating has nothing to do with this calculation.

In theory, yes, but the point is that an 8 Ohm speaker resistance is in practice insignificant compared to 250 or so Ohms and in fact, you simply cannot audibly detect the difference between a 220 and a 270 Ohm resistor.

So in the future I'll just apple ohms law to find the resistance and subtract the ohms of the speaker to the obtained ohms.
I was thinking, i saw that in some threads people subtracted a value off the amperes, especially when they were doing circuits with LED and resistances..why is that?

No, they (are supposed to) subtract the forward voltage of the LED; LEDs do not have a resistance as such. Let's say that a LED has a forward voltage of 2V and you want to limit the current through the circuit to 10mA; we assume a 5V output pin.

The voltage over the resistor = 5V - 2V = 3V.
You want to limit the current to 10 mA so the resistor value is 3V / 10mA = 300 Ohm.

1 Like

So we do this because they don't have a resistor,but they have a volt value

If we posit that a pin should draw no more than 20mA
then no pin should have a resistance of less than 250Ω
connected to it.
Given that it is an 8Ω speaker → 250Ω ─ 8Ω = 242Ω.

You really need a separate amplifier to drive your speaker if you want to do it the right way.