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Topic: Can someone explain how to use this filter calculator? (Read 3588 times) previous topic - next topic

Archibald

For the filter to give you the designed frequency response, both the source and load impedances must equal the value of impedance that you enter as Zo in that online calculator.  Anyway it's better to consider this as an issue of power line decoupling.

It's very surprising if the current taken by the piezo sounder is causing the problem because the current taken by a piezo element is typically less than 4 mA. What power supply are you using?  However if your sounder is powered by DC, then it must contain electronics to generate the alternating waveform required by the piezo element. I wonder if this electronics is causing the problem.

I have driven this piezo sounder directly from an Arduino Micro.  It does not contain any electronics so needs to be driven with an alternating voltage.  You can use a PWM "analogue" Arduino output. Use analogWrite(pin,127) for maximum volume.  The frequency of the PWM does not correspond with the resonant frequency of the piezo element but the sound output level is remarkably adequate for most purposes.

MarkT

This thread is a classic instance of the xyproblem: http://xyproblem.info/

You should have asked how can I stop interference from my piezo buzzer affecting my sensor readings...

Designing a linear analog filter is not the best answer to this question(!)
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Grumpy_Mike

Designing a linear analog filter is not the best answer to this question(!)
Quite right it is not any sort of answer to this question.

You need to:-
1) Read this:- De-coupling
2) Post a schematic of what you have got then we can offer sensible answers.

Jiggy-Ninja

It's very surprising if the current taken by the piezo sounder is causing the problem because the current taken by a piezo element is typically less than 4 mA.
It's not the amount of current it takes, it's the digital switching that causes noise that can be conducted to the rest of the components.
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Archibald

It's not the amount of current it takes, it's the digital switching that causes noise that can be conducted to the rest of the components.
The capacitance of a piezo element is typically about 15nF but there must also be a resistive component to its impedance.  Even if a piezo element is driven with a square wave, it shouldn't be difficult to provide adequate decoupling.

Grumpy_Mike

Quote
but there must also be a resistive component to its impedance.
Yes but it is very high.

The problem is the mechanical bounce when the power is removed, you can get nearly 100V so I would try a reverse biased diode like you do for motors or inductive loads.

Jiggy-Ninja

The capacitance of a piezo element is typically about 15nF but there must also be a resistive component to its impedance.  Even if a piezo element is driven with a square wave, it shouldn't be difficult to provide adequate decoupling.
Not disputing that, I'm trying to find out if the piezo was decoupled properly.
The problem is the mechanical bounce when the power is removed, you can get nearly 100V so I would try a reverse biased diode like you do for motors or inductive loads.
Would that work? Flyback diodes work because the inductance causes the voltage across the load to reverse when the current supply is opened, which forward biases the diode and provides a path to dissipate the energy.

If a piezo is primarily capacitive, the voltage polarity will never flip so the reverse biased diode is always reverse biased.
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Grumpy_Mike

Quote
Flyback diodes work because the inductance causes the voltage across the load to reverse when the current supply is opened, which forward biases the diode and provides a path to dissipate the energy.
Have you ever seen the output of a piezo when you give it a wack? It is both positive and negative, it is based on the crystal deformation. If you have a DC voltage the crystal deforms, then when it is removed the force is removed and the crystal is restored, thus generating the reverse voltage. Exactly like a coil.

Jiggy-Ninja

Have you ever seen the output of a piezo when you give it a wack? It is both positive and negative, it is based on the crystal deformation. If you have a DC voltage the crystal deforms, then when it is removed the force is removed and the crystal is restored, thus generating the reverse voltage. Exactly like a coil.
No I haven't. Unfortunately the only piezo buzzer I currently have has an oscillator circuit built into it, so I can't experiment with it.
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polymorph

Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Jiggy-Ninja

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