A friend's remote doorbell has failed. This is the part that plugs into the wall and makes noise when the remote button is pressed. It's stone-cold dead. No indicator LED, nothing.

I popped it open and as you can see, it uses a capacitor power supply. I find full AC mains voltage across the big ceramic capacitor, but no voltage on the larger electrolytic. There's also no voltage on the zener, although there seems to be a coating that might be interfering and making it hard to connect my multimeter probes. I have not checked the voltages on the bridge rectifier M10S.

My question is, does this kind of supply fail in a predictable way? Is it common for the ceramic cap to fail, or the zener? I'm just trying to get a feel for whether it's worth trying to repair this thing. I guess I could put 12V onto the electrolytic and see if the rest fires up.

 

That large brown capacitor is the likely culprit. If you can check its value you will probably find it is very low. You need a class X2 capacitor to replace it.

 

I give up. I tried pulling that 1000µF electrolytic. It holds charge for at least minutes. Placing 5VDC across the empty pads caused slight arcing. It's obviously pulling too much current there. So there's more wrong than just the power supply.

 

It's gonna that brown film capacitor, believe me.

 

It's gonna that brown film capacitor, believe me.

+1, As I understand it, mains capacitor is a voltage dropper, go for it!

 

What voltage do you have on the bridge rectifier DQ1.??

The 220nF ceramic cap and R9 220 ohms is the mains dropper..

 

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