 Newibie questions about current, voltage, resistors

I'm a newbie and I do not have any electronics background. I googled about it different times but I really don't understand if my calculations are right.

Here's my issue:

I have a buzzer. The technical specifications are:

Rated Voltage: 3.5V Operating Voltage: 2V to 5V Rated Current: 30mA at 3.5V Coil Resistance 17 Ohm

If I apply a 5V voltage from an arduino pin to the buzzer I should or not use a resistor?

My understanding is

• Rated voltage is 3.5V
• Rated current is 30mA then 30mA + 40mA (of the arduino PIN) = 70mA that is too high… Is that right?
• Then 5V (supplied from arduino) - 3.5V (rated voltage) = 1.5V
• According to the Ohm law R = 1.5 / 0.03 = 50 Omhs
• Since the buzzer has a coil resistor of 17 Ohms I should use a 33 Ohms resistor

Am I right?

And what does operating voltage means? Since my buzzer has an operating voltage of 2 to 5 V does that means that I can also use it without a resistor?

And what if I do not have any rated current info but I only have infos about the rated voltage and the coil resistance? is there any way to measure the rated current of a component having only these two values?

40mA (of the arduino PIN)

Where has this come from?

40mA is the limit of current you should draw from a pin. It is not added to anything.

Since the buzzer has a coil resistor of 17 Ohms I should use a 33 Ohms resistor

No, 5V at 40mA will be drawn from a resistor of 5/0.04 = 125R you have 17R in the load so use a 125 - 17 = 108R As you can't get those go for the next highest up, 120R

But he wants only 30ma through the buzzer, so R=(5/.03) - 17 which is about 150 ohms.

Where has this come from?

I read it in a tutorial somewhere...

But he wants only 30ma through the buzzer, so R=(5/.03) - 17 which is about 150 ohms.

That make sense. But why you are using 5V if the buzzer specs says 30mA at 3.5V?

No wonder you are confused. 17 ohms at 3.4 volts equals 200ma.

A buzzer is not a simple DC device since it is switching current on and off (that's what makes it buzz)

The 17 ohms in the DC resistance (R)- it will have inductance and therefore an inductive reactance (X)L - which is almost always greater than R

The 30ma is the MEAN (DC) current that it draws when operating.

If it is a true buzzer it will create lots of high voltage spikes which WILL damage your arduino.

I therefore recommend that you use an opto-isolator between the arduino output pin and the buzzer to isolate these spikes

But why you are using 5V if the buzzer specs says 30mA at 3.5V?

Because you are feeding it with 5V not 3.5V.

pablito0078: That make sense. But why you are using 5V if the buzzer specs says 30mA at 3.5V?

5V is what you have available. If we assume that the minimum resistance of the buzzer is 17 ohms, then the resistor given will keep the current under the maximum allowable. While operating, as jackrae suggested, the resistance will be likely be higher and the resulting current lower. If you were operating it at the right frequency and at 3.5, you might indeed get an average current draw of about 30ma.

In our case though, we have 5V available so I calculated a resistance that would keep the buzzer safe at DC. You can use this as a starting point if you need to get maximum volume out of the buzzer. Getting optimum performance would require more than you a likely able to follow for now. Seeing as you are an admitted newbie though, safety is the best plan.

Drop-out voltages are typically specified at given current levels - usually fairly high.

Your mcus and lcd display will consume far less than that. In those cases, a non-ldo regulator will drop close to 0.7v, leaving room to spare in your case.

Also, your nokia lcd runs just fine at 5v.

Looks like I responded to the wrong thread, :)

Looks like I responded to the wrong thread

You can use the delete button to remove a post you know.

Thanks for your replies. Some concepts are clearer now but I'm still a bit confused.

I can use a max of 40mA for each Arduino pin in/out then the components I use should draw less current than 40mA at 5V.

Then: (case 1) My buzzer draws 30mA at 5V ==> I can use it without resistor

(case 2) My buzzer draws 30mA at 3.5V. I feed it with 5V ==> I must use a resistor R=5/0.03 = 166R - 17R (my buzzer 's coil resistance) = 150R.

(case 3) My buzzer draws 30mA at 3.5V. I feed it with 5V and no resistor ==> Will Arduino have issues? Or the buzzer?

(case 4) My buzzer draws 50mA at 5V ==> What will happen in this case?

I'm looking for some online resources to understand how all this works. I found tons of them but if someone can give some advice about where I can start from I would really appreciate it.

1 Like

I can use a max of 40mA for each Arduino pin in/out then the components I use should draw less current than 40mA at 5V.

YES

(case 1) - Yes
(case 2) - sort of yes but the buzzer is an inductor and feeding pulses into an inductor is a situation ohms law does not cope with.
(case 3) - yes both the arduino and the buzzer will be working outside the limits.
(case 4) - you will damage the arduino pin, it might stop working now or at some time in the future. Like putting a nail in your car’s tire, the wheel might go down immediately or you may be able to continue to drive it for many miles but it is damaged.

1 Like

Thanks! Now everything is clearer...