Electronic – Converting 8V AC to 8V DC – bridge rectifier gets very hot while idling

ac-dcbridge-rectifierpower supplystep-down

I am tinkering with an Arduino and recently had to convert 8V AC to 5V DC in order to run it.

The project is a custom doorbell, which gets the power from the doorbell transformer (transformer is located in the basement).

The doorbell transformer is a 220V AC to 8V AC transformer.

Since it is 8V AC, I needed to make it to 8V DC.
So I soldered a bridge rectifier and put a compensating capacitor to smooth the output (see supplying image).

This 8V DC output is an input of a DC-DC step-down module, which is based on a LM2596 power regulator. It delivers 5V DC and supplies the Arduino and a module, which is responsible for the bell sound. This module can drive up to 3W speaker.

The speaker connected to it is 2W 8 Ohm.

Unfortunately the bridge rectifier and e-cap are getting very hot even in the idle state (e.g. at night, no doorbell button pressed).

What am I missing in my circuit in order to get proper 8V DC for the step-down module and keep the bridge rectifier at the normal temperature?

29.07.2019 EDIT
Transformer is located in the basement and cannot heat up the bridge. It gets hot on it's own.

current wiring

23.09.2019 EDIT
So I have found a probable root of that issue.
I have not checked, if there is any current on the doorbell button.
Both existing doorbell buttons are connected to the 8V AC line, causing lot of trouble when connected to the Ground and to a digital input on arduino (red arrow).

Dengerous connection, that caused the issue

For the workaround see my answer below.

Best Answer

It should not be getting hot. You are using a switching regulator, which should run cool and draw less current than the output current. The Arduino draws tens of mA and the sound module should draw little quiescent current. So even a small bridge such as a W04M should not get noticeably warm.

The transformer, on the other hand, might well get hot and heat the rest of the parts. It's best to keep the electrolytic cap in particular cool for long life.

If you are sure the bridge is getting hot, check the connections and consider replacing it. If it's an impedance-protected transformer you might be shorting the transformer out every half-cycle. Maybe there is a hidden path (possibly through a ground connection) somewhere.

Also the typical output voltage across the capacitor might be more like 10-15VDC depending on how lousy the transformer regulation is. It's extremely unlikely you will measure 8VDC. As you've got a switching regulator, that's all fine and good.

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