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Using the experience of the E14 Community to troubleshoot a power supply

colporteur

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The attached schematic if for a wireless doorbell model WL-100A-A. It doesn't have any markings to indicate the manufacture.

 

It has been installed for a number of years and was supposedly working? I recently noticed people have been knocking more than ringing the door bell. I discovered pushing the remote button at the door does nothing. A quick battery change and still nothing. I removed the cover of the unit plugged directly into the wall and discovered that sweat smell of burnt Bakelite.

 

My initial question was "where is the AC step down transformer?" An internet search of transformerless power supply (PS) reveal a number of hits. One immediately caught my attention because of the warnings about mains voltage. This is my first experience with a PS that doesn't use a transformer. After doing some reading it seems to be a common design. I thought it wouldn't pass muster from a safety perspective. The warning of AC mains being potentially exposed on the lower voltage side of the circuit made me stop and take notice.

 

After some napkin drawing and initial troubleshooting I thought maybe someone in the E14 Community has experience and can offer advice.

 

There is no DC voltage. With 120VAC mains applied there is a reading through fuse F1 and nothing any point further. From observation the printed circuit board areas around A, B, C & D are darkened. A resistance measurement of R1 indicates at open. The surface of the component is scorched such that I am assuming its value is the same as R2. R3 & R4 read slightly less than their colour code, no shorts or opens. Diodes D5-D8 appear to test OK. The schematic component type is not correct. I was unable to read the markings and the diode was a fritzing listing. No shorts or opens are measured on the diodes. The same goes for the zener diodes.

 

I don't measure a short across the 12VDC rails. My first thought was to connect a supply at that point to see if the device still works. I'm not sure what took out the front end but based on what they say about currents for this type of PS design it being caused by the load. Then I recalled reading about having no load on this type of supply. Not being familiar with the design and all the warnings I figured I would post to this life line for some advice.

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  • That is a classic capacitive dropper power supply which is non-isolated (so be careful while probing). As long as the casing offers double-insulation and humans aren't touching anything metal, it seems to be safe enough.

     

    I suspect that it may be likely that something bad happened for R1 to go open, perhaps C1 had failed short in some way. R1 and R2 are likely to be same value - they are likely there as inrush current protection as the secondary capacitor can cause high current flow at initial plug-in.

     

    If not, then it seems likely to me that U3 zener has gone bad over time - the zener forms a shunt-regulator on the output which burns energy and there are a good many Chinese wireless power switches with similar designs where the zener has overheated and cooked itself. This would then let the secondary side voltage "fly up" to sometimes in excess of 20V, killing everything downstream and consuming enough power to burn something. If it's marginal, there could be a chance that C2 has experienced overvoltages and is consuming excessive current because it is damaged. Usually it will bulge if this is the case.

     

    Another common failure mode is with the C1 capacitor is probably a polyester film or X2 style capacitor - the film inside may have burned or vaporised through accumulated voltage transients over the years. If the capacitance value is significantly less than the nameplate rating, then the current it will let through will be too little to run the doorbell, thus the output voltage rail would be below the rating. I've had a number of capacitive dropper supplies fail in this manner, but it would not explain why R1 would have failed open

     

    I would agree that the best way to test is to "break" things into sections, but such a supply would be unhappy without load. I would say check that the load works with a power supply attached - if it doesn't, then whatever catastrophic event happened may have caused collateral damage. If the load side is fine, then use a current limited supply set to some conservative value just above the ordinary consumption and push it above 12V. The zener stack should clamp the voltage at some value a little above the nominal operating value - if that works, then it's time to look at the capacitive dropper. I would test the full bridge rectifier diodes and if they work fine, I would test the capacitor (or replace it entirely).

     

    - Gough

Reply

  • That is a classic capacitive dropper power supply which is non-isolated (so be careful while probing). As long as the casing offers double-insulation and humans aren't touching anything metal, it seems to be safe enough.

     

    I suspect that it may be likely that something bad happened for R1 to go open, perhaps C1 had failed short in some way. R1 and R2 are likely to be same value - they are likely there as inrush current protection as the secondary capacitor can cause high current flow at initial plug-in.

     

    If not, then it seems likely to me that U3 zener has gone bad over time - the zener forms a shunt-regulator on the output which burns energy and there are a good many Chinese wireless power switches with similar designs where the zener has overheated and cooked itself. This would then let the secondary side voltage "fly up" to sometimes in excess of 20V, killing everything downstream and consuming enough power to burn something. If it's marginal, there could be a chance that C2 has experienced overvoltages and is consuming excessive current because it is damaged. Usually it will bulge if this is the case.

     

    Another common failure mode is with the C1 capacitor is probably a polyester film or X2 style capacitor - the film inside may have burned or vaporised through accumulated voltage transients over the years. If the capacitance value is significantly less than the nameplate rating, then the current it will let through will be too little to run the doorbell, thus the output voltage rail would be below the rating. I've had a number of capacitive dropper supplies fail in this manner, but it would not explain why R1 would have failed open

     

    I would agree that the best way to test is to "break" things into sections, but such a supply would be unhappy without load. I would say check that the load works with a power supply attached - if it doesn't, then whatever catastrophic event happened may have caused collateral damage. If the load side is fine, then use a current limited supply set to some conservative value just above the ordinary consumption and push it above 12V. The zener stack should clamp the voltage at some value a little above the nominal operating value - if that works, then it's time to look at the capacitive dropper. I would test the full bridge rectifier diodes and if they work fine, I would test the capacitor (or replace it entirely).

     

    - Gough

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