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H-bridge

Alexis Polti gave us some tweeters he ordered. My goal was to determine the electrical characteristic of the tweeters in order to design the H-bridge.

As the tweeter acts as a capacitor, we’ll need to add a resistor in series with the tweeter in order to protect against intensity peaks the capacitor will generate when the voltage changes (i=C * du/dt).

So, I need to figure out what is the capacitance of the tweeter at the given frequency, so I can calculate the resistance value (it will act as a low-pass filter for the capacitor). With the tweeter was given a little circuit which generates the ultrasound frequency.

 

Oscillator circuit

 

After soldering it (and figuring out that the short circuit was because I haven’t applied the erratum. Yes, the PCB was faulty), I have been able to measure the frequency output, the voltage, and the intensity of the tweeter. The strange thing is that even if the datasheet says it will generate a 23kHz frequency, I only see a 19kHz frequency with a very crappy signal (you can see the frequency by clicking on the image and zooming — I had only two hands to take the photo and connect the circuit 🙂 ).

 

Oscilloscope

 

I measured 6.7Vpp, 19.01kHz and 202uA. That means the capacity of the tweeter at 19.01kHz is 1.59 nF. So by using a resistor of 10kohm, the cutoff frequency will be over 40kHz which will be really fine.

3 comments to H-bridge

  • 202µA : where and how did you measure it?

  • drix

    Would you explain the purpose of this H bridge?

  • ale6

    @Alexis Polti: I just measured the tweeters. You can see on the picture there is some tape on the wires, I disconnected the black one to put the ampere-meter. And yes, it’s a bad measurement, I must do this again by using the oscilloscope and a resistor to measure the peak consumption.

    @drix: The purpose would be 1. to power the tweeter with a far less voltage 2. not to stress too much the MOSFET transistors (it will multiply by 4 the overall maximal power which could be given to the tweeter).