Help me understand Buzzer behavior

[amazed]

Hello,

I have a custom board with a TMB12A03 buzzer on it, I use the recommended driving circuit from the document (the connectors are jumpers).

I measure the uC drive signal and the voltage on R8 / 150R. Blue is the drive signal, red is the voltage measured between BJT and 150R. The Collector-Emitter voltage looks good (basically 0V), when the drive signal is at high level.

In this case the red voltage is between Buzzer and 150R. Approx. 20mA (2,87V / 150R) flows when the drive signal is at high level.

The interesting part is when the drive signal is at low level (lower picture green bracket). First I thought that the BJT cannot turn off and current still flows, but as I realized that the voltage is the same at both ends of the resistor, current cannot flow.

Could it be the induced voltage of the Buzzer after BJT switch OFF? The relatively big spike seen immediately after switch off suggests that some kind of inductance is there.

Thanks in advance.

[TomGeorge]

Hi, @amazed
You have got the gnd of the Vbatt connected to the gnd of the uC?

Tip, When describing test points that the scope is connected to, put the test points a A, B, C on your schematic, is easier than trying to understand descriptions;

@amazed

In this case the red voltage is between Buzzer and 150R.

Make all your scope pattern measurement with respect to gnd, that makes comparisons a lot easier to diagnose.

Tom....

[amazed]

@TomGeorge

You have got the gnd of the Vbatt connected to the gnd of the uC?

Yes.

@TomGeorge

Tip, When describing test points that the scope is connected to, put the test points a A, B, C on your schematic, is easier than trying to understand descriptions;

I thought of that, but I was lazy. Thanks for the ABC designations , they are correct.

A: blue signal on both pictures
B: red signal on upper picture
C: red signal on lower picture

[2112]

The TMB12A03 is an active buzzer. You shouldn't pulse it. Apply a constant DC voltage to activate the buzzer.

[amazed]

@2112

You shouldn't pulse it. Apply a constant DC voltage to activate the buzzer.

Actually I thought of that, but haven't tried it yet.

The datasheet is confusing regarding this.

It says resonant frequency square wave.

Anyway, I'll give it a try soon. Thanks!

[amazed]

@2112

Apply a constant DC voltage to activate the buzzer.

Yes, the built-in driver circuit works.

[2112]

@amazed

Yes .....

Happy to help