EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: nicknml on September 13, 2014, 01:50:53 am
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A while back I bought a bag of cheap buzzers (internal oscillators) off of Ebay. Most of the listings don't provide an actual part number/datasheet and I was wondering if mine had a diode to protect against back EMF. I heard that most of them do, but I wanted to make sure.
Since it was a rainy day, I decided to tear one of them down. After some work and removing the epoxy with some paint thinner I finally found a PCB. It's just a single sided board (I'm assuming that the internal oscillator is somehow a part of the magnet/coil assembly.) I whipped out my multimeter and probed around a bit.
Below are the photo of the pcb and a crude sketch (I did scrape that diode a bit from taking a layer of epoxy off with a Dremel in attempt to embed a diode that went terribly wrong lol):
(http://i1241.photobucket.com/albums/gg517/nickl6/pcb3.jpg)
(http://i1241.photobucket.com/albums/gg517/nickl6/pcb.png)
Note that there were no labels on any of the parts or pcb itself.
From my probing, D1 does in fact appear to be a diode. Pins 3 and 1 appear to be anodes and 2 and 4 appear to be cathodes. I used the diode check function to check the diode. I got Vf readings of around .68V R1 appears to be a resistor and I measures about 18.6K Ohms. It was a fun mini-teardown. I've come to the conclusion that it does have a diode to protect against back EMF. So did I come to the correct conclusion? I'm a bit curious as to the purpose of the resistor.
EDIT: it's a actually a magnetic buzzer not a piezo buzzer.
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Since piezos are mostly capacitve, there is no back EMF.
I do not think it is a diode, it makes not sense. Piezo buzzers with internal oszillator normally use a piezo disc with 3 electrodes to get a feedback signal to the driving transistor. The layout looks like it had more than 2 connections to the piezo. Therefore I think D1 is a CMOS inverter used as part of the oscillator. The resistor is probably for protection or biasing of the feedback signal.
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Thanks bktemp. Unfortunately I had a stupid moment when writing the OP. I realized that it's actually a magnetic buzzer not a piezo buzzer.
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Another note about magnetic buzzers and back EMF. Old electromagnetic buzzers, the type which use a mechanical interrupter, which is basically a relay with a normally closed contact, wired in series with the coil and connected to a diaphragm need a back EMF diode and possibly a capacitor and ferrite bead to filter out any RF emissions. Newer electromagnetic buzzers, which have an electronic oscillator connected to the coil, don't need a blocking diode.