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BC546 transistor failure

jonathang

Hi All,

I am new to the platform please excuse me if I am going about this in the incorrect way, I am a hobbyist and have designed a RFID module that has a 485-line driver to communicate with a master device. I have ordered the bords from JLC and assembled the unit. I have a 12dcv supply, I have two 270E 1/4W resistors in parallel to assist with limiting the strain on the 7805 linear regulator, I since have discovered that the resistors restrict too much current and the Arduino was suffering possible brownout events.

What I am struggling with is the failure of the BC546 bjt driving the active buzzer, the buzzer is rated 8-15VDC 30mA, the failure starts as a static noise coming from the buzzer when the MCU pin is set to low and eventually the bjt fails and the buzzer keeps on buzzing.

I would appreciate some advice my errors,

Regards

image

  • Which type of buzzer are you using? Magnetic or piezoelectric.

    If you have a magnetic buzzer, you have to incorporate a flyback diode across its terminals. This diode will mitigate the voltage spikes (flyback) that occur when the BJT switch is turned off. These spikes can damage the transistor if left unchecked.

    If you have a piezoelectric buzzer you can add a discharge resistor across the buzzer as piezoelectric buzzers hold a charge momentarily due to their capacitive nature. Maybe you have to redesign your driver circuit maybe adding a buffer.

    I am not an expert on the subject, these are things that I have seen in other circuits but I think it will help the discussion.

  • When you say the bjt fails, do you mean it becomes a permanent short circuit or just that the buzzer stays on until the power supply is cycled off?

    The circuit looks like it should work so the issue is likely in how things are actually connected. For example are the emitter and collector pins reversed?

    Are all 3 bjt pins connected as shown in the schematic and to nothing else? For example is there any other path from collector to ground?

    Is the buzzer connected backwards?

    When the MCU output is low, is the bjt base also low?

    Is it an electro-mechanical buzzer as  asked?

  • in reply to javagoza

    Hi Javagoza,

    It is a piezoelectric buzzer thank you for the advice I will test it and revert with feedback.

    Regards Jonathan 

  • in reply to dougw

    Hi Dougw,

    I will doubled check the basics as you suggested, everything should be connected correctly but I might be wrong.

    It is a piezoelectric buzzer, what I find strange is that I have assembled six of the boards and 4 of them have failed all of them the components are placed on the board the same. 

    What I am wondering if I might have a cheap brand of transistors, I am considering replacing the bc546 with a bc547c. 

    To answer your questions, 

    Yes all pins are connected as in the schematic, but I will confirm. 

    There is no other connection to ground from the emitter. 

    I will measure the base of the bjt but I have removed the mcu from the board and directly grounded the base even then if I apply power the buzzer turns on immediately. 

    Thank you for your insights, 

    Jonathan 

  • in reply to jonathang

    A piezo buzzer can auto-generate fairly high voltages. High enough to destroy a transistor. (Or is this a buzzer module with driver?)