Buzzer resistor value

[barakg]

Hello

I would like to know how did the circuit designer of the circuit in the image calculated resistor R100 value.
I have tried to figure it out by myself but I'm not sure about it. I would like to know if my calculations are right.
My guess is that the 100ohm resistor is to limit the current to ~20mA.

buzzer's datasheet http://www.puiaudio.com/pdf/SMT-0540-S-R.pdf​

The buzzer's datasheet says that Vp-p = 3V, Imax = 100mA.

To use Ohm's law to calculate R100 we have to 'translate' Vp-p to Vrms (kind of avg voltage) so,
Vrms = 0.35355 * Vp-p = 1.06V.

Then by Ohm's law:

R100 = (3.3 - 1.06) / 0.020 = ~100Ohm.

My calculations are right?

Thank you!

[robertferanec]

I am not sure if someone really calculated it precisely. In this cases, very simple calculation like 3.3V / 100 OHM = 33mA is just enough .. as you said, just to be sure, that the current will not be bigger than 100mA.

PS: For safety, you may want to use the 3.3V as the maximum voltage value (even there will be some voltage drop on the transistor and resistor, it is just more simple for calculation), but you definitely need to count with situation when the transistor is open for a long time (e.g. mistake in programming the controller). Also, I would not use Vrms for calculation - when transistor is open, the full voltage will be applied, and the current at that specific moment needs to be limited.

[Abhi P Choksi]

It's calculated like this.
One thing we pretty sure that buzzer would handle only 3.00V DC Supply maximum.
Another thing we sure is current limit of buzzer is 100mA.
So when it comes to design if we put 100ohm resistor and if transistor is switched on then 0.3V will be dropped across resistor and buzzer will get 3.00V and current through buzzer would be 33mA and that's under limitation.

[robertferanec]

The datasheet is actually a bit confusing as it says RATED VOLTAGE (SQUARE WAVE) 3V, but OPERATING VOLTAGE RANGE 2-4V. Normally they should say like Absolute maximum voltage - that information is missing.