Driving a piezo hard - Page 1
Forum section: electronics / projects

author: trevwhite
author_rank: Frequent Contributor
timestamp: August 13, 2013, 07:15:57 pm
content: Hi all.I am looking to drive a standard piezo disc but I need to generate some real loudness from it. I have seen a few designs that use a transformer to generate 80V from a small 5 volt supply and this seems to work. I am trying to source such a transformer. It is only a 3 pin device as the isolation isnt required. It just pulses the one side and the other side outputs a higher voltage at the right frequency and the piezo is connected across the winding.Is there a specific name for such a transformer? I looked in Farnell and couldn't find any.ThanksTrev

author: fcb
author_rank: Super Contributor
timestamp: August 13, 2013, 07:21:33 pm
content: You could use a small PCB mount mains transformer (in-reverse) - it's bigger than you need but quite cheap ($2 in qty). Another possibility is a 100v line matching transformer, these are used in PA systems. Or wind your own.

author: nctnico
author_rank: Super Contributor
timestamp: August 13, 2013, 09:14:53 pm
content: Quote from: trevwhite on August 13, 2013, 07:15:57 pmThere is not much sound pressure you can get from a disc. You need to add a resonance chamber to it.

author: trevwhite
author_rank: Frequent Contributor
timestamp: August 14, 2013, 10:57:58 am
content: Thanks for the comments. Yes, a resonance chamber makes sense and I have seen these in designs. But I still need to drive the piezo at higher voltage I thought to get some noise out of it?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 14, 2013, 11:21:23 am
content: sound chamber design makes a huge difference. And of course you must drive at the resonant frequency2 common ways to drive at high voltages without special parts :Open-collector transistor with inductor (10-100 millihenries) to positive with piezo across the inductor. Ideally use a high voltage transistor like MPSA42 to protect against a disconnected piezo, as voltage can go very high without the load.This is very simple and cheap, and can do typically 20-40V under load depending on inductor and supplyboost converter plus H-bridge. This can be done very simply with a few transistors, driving from a few pins of a microcontroller, and a PWM peripheral to generate the boost waveform. No regulation needed - you're effectively just charging and discharging the piezo capacitance on alternate cyclesThere are some chips designed for HV drive for haptic feedback devices - I think TI does one, which integrates the latter approach

author: nctnico
author_rank: Super Contributor
timestamp: August 14, 2013, 11:22:15 am
content: If you drive it from a bridge (normal signal and inverted signal) you can get a lot of noise from it even from 5V. You can use a function generator to find out at which frequency it sounds the loudest (somewhere between 800Hz and 2000Hz). Also drive it with a square wave. A square wave has harmonics so people who a deaf for certain frequencies can still hear it.

author: trevwhite
author_rank: Frequent Contributor
timestamp: August 14, 2013, 11:49:18 am
content: The inductor idea is avery interesting one, thanks. As you say it may well generate quite a bit of noise, etc.I have attached the 3 pin transformer I found in a window alarm. You can see they created a sound chamber as well to help as discussed. I was hoping to find one of these as it sounded a nice solution. Pulse the primary and the secondary drives the piezo.

author: RDXx
author_rank: Newbie
timestamp: December 15, 2015, 07:55:50 pm
content: Did you find a solution?I am looking for the same.Piezo element + resonant chamber <3mmAnd transformer

author: SeanB
author_rank: Super Contributor
timestamp: December 16, 2015, 11:26:14 am
content: You can drive the piezo to quite a high level using a simple inductor across the piezo, inductance calculated to resonate with the transducer capacitance at somewhere between 1-4kHz. Then drive with a simple bipolar high voltage transistor, preferably a unit with a Vceo over 300V and with the supply rail decoupled locally by a low ESR capacitor so the loop around the inductor and transistor is low area. Drive with a short pulse ( under 20% duty cycle as a beginning) at the desired frequency, and the high voltage induced in the parallel LC circuit of the inductor and transducer will generate a very loud output. Inductor need low DC resistance and should not saturate at the supply voltage.

author: Pjotr
author_rank: Frequent Contributor
timestamp: December 16, 2015, 12:22:40 pm
content: Indeed you need to combine all resonance together, the mechanical resonance of the disk, the resonance of the cavity and the LC resonance of disk capacity and the inductance of the inductor. If they match well you can get over 110 dB SPL from a 30 mm disk at say 3 kHz from a single lithium coin cell.What is called a transformer in this case is not exactly a transformer. It is merely coupled inductor that is in resonance wit the disk's capacitance.

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2015, 01:15:55 pm
content: What type of piezo disc do you want to drive?Most small piezo transducers are only rated to5V25V so 80V might damage it.https://www.sparkfun.com/datasheets/Sensors/Flex/p37e.pdfhttp://www.farnell.com/datasheets/1662482.pdfThat window alarm uses a transformer because the battery voltage is very low, probably around 3V. An inductor will work but it won't boost the voltage so much and will have a narrower frequency response.A small audio transformer will work.

author: Seekonk
author_rank: Super Contributor
timestamp: December 16, 2015, 01:53:36 pm
content: A word of general caution. We had a design that used a bridged design for higher voltage. There was a pot we adjusted to tune each oscillator to resonance. It was loud. About a year later we had dozens of units coming back with destroyed transducers. Wire bonds had fatigue breaks. These were small transducers. A close look at the data sheet would have foretold the problem.

author: RDXx
author_rank: Newbie
timestamp: December 16, 2015, 02:01:47 pm
content: I am using a CR2030 battery that I what to drive a piezo-element very loud >103dB. I am using inductors to try to give it some snap. But it is not enough.I can not match the sound from a cheep china personal alarm.figure: metal cap over a standard piezo element, I have removed the cap, so it looks like ¤%&.The china piezoalarm is a standard piezo-disk with a metal cap giving it a resonant chamber.I have the following constraints:3V batteryMax diameter: 23 mmMax height: 3 mmSLP >103 dBThe whole board should fit in a volyme of 23x8x50, including battery, driver, and alarm-speaker.The problem is that all small piezo-buzzers is either to big or has a SLP <90 dB.I can not find a piezo-disk with a metal chamber, like the Chinese one.I can not find a suitable transformer.The current design is with a inductor:http://se.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15001&langId=46&urlRequestType=Base&partNumber=2145720&storeId=10177With this inductor I get approx. 15Vpeak, but the Chinese circuit with a transformer get an output of around 40V.Currently I am driving the piezo-element with a transistor connected has an open-collector ans a inductor connected in parallel to the piezo element. Perhaps I can improve the circuit by driving the piezo-element in a push-pull, configuration with two open-collector outputs.So,Question 1: Were can I find a piezo-element with an appropriate resonant chamber?Question 2: Were can I find a suitable "transformer" like the one in the Chinese personal alarm?

author: mikerj
author_rank: Super Contributor
timestamp: December 16, 2015, 02:02:59 pm
content: Quote from: Hero999 on December 16, 2015, 01:15:55 pmThe devices in the above two links are rated to 25v or 30v pk-pk, which is very typical for these devices. The 5v devices are usually the ones that have a built in driver circuit.

author: h3po
author_rank: Newbie
timestamp: December 16, 2015, 02:04:40 pm
content: In case you weren't aware of that trick: When you want to drive a piezo with an µC, you can easily double the voltage across it by using 2 IO and alternating their polarity. I've tried it with a few piezos that I had lying around (all with small plastic resonance chamber) and found that this gets about the same volume as e.g. a smoke detector alarm.

author: RDXx
author_rank: Newbie
timestamp: December 16, 2015, 02:10:45 pm
content: Seekonk> Did your devices have alot of run time with the transducers? I am planing for a very limited activation of the transducers. Max 1h in a products life time.

author: RDXx
author_rank: Newbie
timestamp: December 16, 2015, 02:13:21 pm
content: Quote from: h3po on December 16, 2015, 02:04:40 pmI am aware of the technique but I have not tried it yet. That was what I meant with the puch-pull.

author: Pjotr
author_rank: Frequent Contributor
timestamp: December 16, 2015, 02:15:23 pm
content: Quote from: RDXx on December 16, 2015, 02:01:47 pmYou will not find such a transformer at the regular suppliers. Those Chinese ones are custom made tapped inductors that act as a transformer and resonance inductor at the same time. Probably your best bet is AliExpress etc.

author: RDXx
author_rank: Newbie
timestamp: December 16, 2015, 05:03:26 pm
content: Pjotr>Thanks, Aliexpress had the piezo-element with metal chamber.

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2015, 06:35:34 pm
content: Quote from: mikerj on December 16, 2015, 02:02:59 pmYes, it was a typographical error. I meant 25V to 30V.