More noob questions... :P

Well, I'm back with another (possibly) obvious question :D!

This, time, I'm pondering about Ohms (Impedance?) in Speakers.

I was looking through the forums, and I came across something having to do with Tone Generation, and it talked about using an 8 Ohm speaker. I have found both a 16 Ohm speaker and a 150 Ohm speaker, but I have no idea what the difference is. I know Ohms have something to do with resistance, but in the real world, what does that have to do with the amount of Power required/power maximum, etc. The thread I read also said something about the resistor you use has something to do with a speaker's "impedance".

If someone could shed some light on this subject, it would be much appreciated.

ohms is the unit of measurement for resistance

without getting "into" it you need resistance in your ciruits to add load, without it (or too little of it) and you have a lovely smoke generator with optional fire

on the arduino each pin has an absolute max of 40ma and if we go over that (actually near it is pretty bad too) we have a very good chance of burning out that pin. how do we know what is safe ... Ohms law

Ohms law is a fairly simple algebraic equation that can be mixed and matched to find out all sorts of things, in this case we have 2 known values, the voltage the arduino lets out, and the resistance of a speaker, we need to find out how much amperage this is going to draw so we dont go killing our chip (or anything else in its path)

I = V / R or 0.625 = 5 / 8

woah! 625ma is defiantly too much so we have to add more resistance, but how much? instead of 40 im going to be safe and use 20 so, we want 0.02 amps at the 5 volts the arduino pushes out

R = V / I or 250 = 5.00 / 0.02

if you now plop 250 into the first equation it should work out to 0.02 amps, which is happy for an arduino even if there was nothing else in between the pin and ground except this resistor

now take the ohms of your speaker and subtract it from 250 and you will have the correct resistor value, if you dont have one of the exact value (as you almost always wont) just choose the one you have closest to the value (preferably bigger in value)

for example 250 ohms + 8 ohm speaker 220-240 ohms + 16 ohm speaker 100 ohms + 150 ohm speaker

would all equal 20ma (+ or - a few microamps) at 5 volts

Thanks SO MUCH! I think I really get it now. You have no idea how much that'll help in the long run! :D

EDIT: But I have another question: If you put a larger than necessary resistor on there, will the speaker just be quieter than normal?

yes, and if you go gonzo with it the resistor will need to be able to dissipate more wattage, if your using standard issue 1/4 or 1/2 watt issues you really should be ok for most things, check out these pages

http://www.the12volt.com/ohm/page2.asp http://www.bcae1.com/ohmslaw.htm

which is a bunch of fun ohm's law calculators (power = wattage)

In the case of your speakers, the ohms value is not it's resistance value but rather it's impedance value. Impedance can be thought of as resistance in a AC circuit.

Because a speaker has inductance (it's voice coil) and the signals it handles covers AC frequencies (audio frequencies) it's impedance is a combination of it's DC resistance (of the wire of the coil) and it's reactance, and uses a different formula then Ohm's law.

So while both resistance and impedance use ohms as their units of measurements, they are different qualities. One is covered when one studies DC theory and the other when studying AC theory.

Lefty

In the case of your speakers, the ohms value is not it's resistance value but rather it's impedance value. Impedance can be thought of as resistance in a AC circuit.

Because a speaker has inductance (it's voice coil) and the signals it handles covers AC frequencies (audio frequencies) it's impedance is a combination of it's DC resistance (of the wire of the coil) and it's reactance, and uses a different formula then Ohm's law.

So while both resistance and impedance use ohms as their units of measurements, they are different qualities. One is covered when one studies DC theory and the other when studying AC theory.

Lefty

Ah....Well, with that said, would Osgeld's statement:

now take the ohms of your speaker and subtract it from 250 and you will have the correct resistor value

Still hold true? :-?

Still hold true?

His explanation is OK, the math is not fully correct, in that what he was trying to explain was that an Arduino output pin has maximum amount of current it can pass before it might be damaged, just like passing more current through a fuse then it's maximum rating will cause the fuse to blow open.

So while the formula he used are not correct for determining the true impedance of a resistor + speaker combination, the resistor's value dominates and does therefore does establish a safe current level for the Arduino I/O pin.

Lefty

His explanation is OK, the math is not fully correct, in that what he was trying to explain was that an Arduino output pin has maximum amount of current it can pass before it might be damaged, just like passing more current through a fuse then it's maximum rating will cause the fuse to blow open.

So while the formula he used are not correct for determining the true impedance of a resistor + speaker combination, the resistor's value dominates and does therefore does establish a safe current level for the Arduino I/O pin.

Alright, so although he's not technically right, it's close enough to be safe? Good. ;D

thats all i ever aimed for :P