Electronics > Repair

Odd 220V AC 7805 Circuit

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daneduplooy:
Hi! Anybody have any idea how the following 220V AC to 5V DC (7805) circuit works?

[attachimg=1]

Thanks! :-)

mariush:
You have the switching power supply controller right there, labeled U1, by the D3 diode. 

In very simple explanation, the chip sends pulses of high DC voltage (ac rectified by that bridge rectifier and smoothed by a capacitor) through the transformer and the output gets converted back to DC.
The lower voltage then goes to the linear regulator to get 5v.

daneduplooy:
Thanks!!! Makes sense...

I was surprised to see the AC Neutral connected straight to the 7805 positive input - so the switching circuit must be generating a "negative" voltage relative to the AC Neutral.

So if this is not working (measuring 0V at the regulator input), then likely there is a problem with either U1 or the transformer? Or perhaps the 7805 itself is blown?
DC voltage after the bridge rectifier is measuring fine.

Ian.M:
That looks like a non-isolated power supply.  Its pretty dangerous for novices to work on as when its connected, just about everything on that board is mains live and could electrocute you.   I  certainly wouldn't want to work on it powered on without an isolating transformer and all CAT II rated test equipment suitable for making floating measurements, and I've been in the trade for several decades.

TLDR: If you need to ask how it works, you don't know enough to work on it safely!

m k:
Pictures are named Airwave 12.

That seems to be misleading.

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