Electronic – Bridge Rectifier Fails When AC Inputs are Shorted

bridge-rectifierpowerrectifier

I am working on a design that involves powering a 3.3V microcontroller off a doorbell transformer but I am running into an issue with the bridge rectifier failing when the AC inputs to the rectifier are shorted.

The bridge rectifier fails when the switch is connected.

There are two basic parts of the design. First, a full-wave bridge rectifier is used to convert the 21V AC supply from the doorbell transformer to roughly 26V DC. A switching regulator is used to deliver 3.3V for powering the microcontroller. A smoothing capacitor reduces the ripple created by the bridge rectifier. That part of the circuit is working as expected and the microntroller can be powered from the doorbell transformer.

The second part of the circuit is simple. To allow the doorbell to be used normally while powering the microcontroller, a switch shorts the AC inputs. This closes the circuit and powers the solenoid in the doorbell that sounds the chime. The issue I am running into is the bridge rectifier failing when the switch is pressed.

I would really appriciate any insight into why the bridge rectifier fails and how the circuit might be modified to prevent the failure.

Bridge rectifier datasheet: DF206ST-G

Best Answer

You should not be trying to run your power converter by wiring into the circuit across the doorbell switch. Instead you should be wiring directly across the output of the doorbell transformer.

As you have it now you are powering your converter through the impedance of the existing doorbell. Even you describe that as a solenoid which is an inductive load. When the door bell button is released there is a good chance of an inductive spike of high voltage that is taking out the diodes in your bridge.

This is a diagram of how the wiring for the circuit should be wired instead of tapping across the existing door bell button. Even with this corrected design it would be advisable to check if there are voltage spikes when the switch is released and design in appropriate high voltage spike clamping into your adapter circuit.

enter image description here