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I am driving a piezo buzzer with a PWM signal (typical freq. 100Hz-1kHz) in order to generate 'beeps'. However due to software constrains (*), when the PWM signal is stopped I can't control if it stays at a high or a low level.

If it stays at a high level when stopped, then the power consumption of the buzzer increases, and it also generates more heat. I believe this could be solved if I could somehow block the DC component of the PWM signal that goes to the buzzer. Can anyone point at a sample circuit or application note on how to do this?

(*) Update: There is a (software) device driver which is not my own that turns the PWM signal on and off; when turning off, the PWM output stays at whatever level it was at that point.

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  • \$\begingroup\$ Not sure why you can't control the PWM, but wouldn't a capacitor block DC? What frequency are you driving it at? \$\endgroup\$ – Roger Rowland Apr 7 '16 at 7:39
  • \$\begingroup\$ How are you generating the PWM? What are the software constraints exactly? \$\endgroup\$ – Doodle Apr 7 '16 at 7:43
  • \$\begingroup\$ @RogerRowland Why: There is a (software) device driver which is not my own that turns the PWM signal on and off; when turning off, the PWM output stays at whatever level it was at that point. Freq: Typically 100Hz-1kHz. Capacitor: Yes I guess that could do the trick. Any sample circuit or app note? \$\endgroup\$ – Grodriguez Apr 7 '16 at 7:43
  • \$\begingroup\$ Is there no way for you to edit the software at all? \$\endgroup\$ – Doodle Apr 7 '16 at 7:44
  • \$\begingroup\$ Do you have a part number of the piezo buzzer? \$\endgroup\$ – Doodle Apr 7 '16 at 7:56
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A capacitor blocks DC and passes AC broadly speaking. maybe try a 10 uF capacitor. At 100 Hz it will have an AC impedance of 159 ohms. At 1 kHz it will be 15.9 ohms. Here's how: -

enter image description here

Your piezo load is the resistor in the above.

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  • \$\begingroup\$ Can we be sure that it's an AC signal controlling the buzzer? If it's still audible at a constant voltage level and is used to generate 'beeps' isn't it more likely to be one of the little DC buzzers you can get from RS or Farnell? \$\endgroup\$ – Doodle Apr 7 '16 at 8:03
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    \$\begingroup\$ @Hayman the OP says he's driving it at various frequencies so it's fair to assume it can work on AC but, if it doesn't he'll see our little conversation and have his answer! \$\endgroup\$ – Andy aka Apr 7 '16 at 8:08
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This is a recommended circuit for eliminating the DC component and avoiding the piezo as a voltage transducer which will play havoc with sensitive circuits. Use a MOSFET not a transistor. From a TDK datasheet for Piezo buzzers

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  • \$\begingroup\$ There is no DC blocking action or component in this circuit. Can you explain your thinking? In addition, you have posted a circuit showing a transistor and advised the OP to use a MOSFET. \$\endgroup\$ – Transistor Apr 7 '16 at 19:52

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