Quick question about inductor in this circuit.

[FenderBender]

This I am hoping will be an easily answered question. I just seem to be too dumb to figure out the answer.

I'm building up an analog multimeter kit I got as a present from a relative. There's an inductor in the parts list and it came broken (leads snapped of). It was a 57.4mH inductor...not sure where they got that value from, rather strange.

I need to replace it. I am going to use a 56mH inductor as I'm pretty sure that it's in a non critical part of the circuit, and inductors usually have a pretty big tolerance as it is. 5-10% for prewound inductors.

Anyway, I was looking at the schematic and I'm a bit confused on what it's actually doing in the circuit. Is it part of a filter of some sort? I think I spot an oscillator by the BJT for the buzzer, but I don't think it's related.

It is connected to ground.

What's it doing?

[IanB]

I don't recognize that circuit configuration, but my guess is that the buzzer behaves like a capacitor and the inductor forms part of an RC oscillator circuit with the buzzer and the transistor to make the buzzer buzz. By sitting on the emitter of the transistor the inductor is effectively providing feedback to make the oscillator oscillate. It might be amusing to make up this circuit in Spice and see what happens (replacing the buzzer with ordinary capacitors).

[FenderBender]

I don't recognize that circuit configuration, but my guess is that the buzzer behaves like a capacitor and the inductor forms part of an RC oscillator circuit with the buzzer and the transistor to make the buzzer buzz. By sitting on the emitter of the transistor the inductor is effectively providing feedback to make the oscillator oscillate. It might be amusing to make up this circuit in Spice and see what happens (replacing the buzzer with ordinary capacitors).

Well now that you say it. If you look at how to buzzer is, drawn it seems they are trying to show it is capacitive. Interesting.

[firewalker]

They could be dumping the "coil reaction" to the buzer. I have seen this to some ~100 db mini alarm systems.



Alexander.

[westfw]

It's pretty common to use an inductor to boost the voltage (and thus the volume) in piezo driver circuits, bit I don't see that exact circuit described anywhere (with a quick scan.)

[SeanB]

Never seen that particular variant yet, but it is very common to use a LC circuit with a piezo unit to make a louder tone. The inductor is resonated with the quite large capacitance of the piezo unit to generate large voltage across the piezo unit. Normally you will see the LC as a parallel circuit with the transistor  driving the bottom and the feedback point coupling into the base like here with just enough bias to start the transistor conducting.  Here it looks like it is used in a series resonant circuit ( actually a parallel resonant if you consider the supply decoupling capacitor should be a very low impedance at the oscillator frequency) so the inductor peak voltage ( and thus the volume) is damped at a certain level dictated by reverse breakdown of the transistor BE junction.

You should add a  capacitor across the 3V battery of around 100uF, otherwise you will find this buzzer will not start reliably as the battery ages, as it does need a turn on transient to get it oscillating. Place it where the leads connect to the board