Power supply fuse keeps blowing

[areebaa]

Hi,
i have small portable Humidifier which was working fine. one day i cleaned it and before drying i connected to mains by mistake. a big spark came out. so i opened its pcb where i saw blown fuse. after repairing the fuse i connected it after drying.but again the fuse was blown with big spark.

there are two PCB'S in the humidier. (please see attachment)

1- power supply part (dual output) input =220V , output label (V1 ,V2, Gnd) level not known
2- control part for ultrasonic transducer and fan

so i separated suuply stage and powered from a cell phone charger (5V,300mA), fan part is working with slow speed , but i dont to how much voltage is neeeded for ultrasonic part , how can we verify ?
how we can make dual supply easily at home ?
Thank You

 

[BradtheRad]

Low voltage is not likely to conduct through water, but high voltage is. Did you wash the transformer? Were the windings wet when you plugged it in? There is a likelihood that arcing occurred among the windings.

Maybe it needs more time to dry out. Then it might not blow a fuse next time.

 

[areebaa]

thank you for your suggestion , no i didnt wash any thing i cleaned only reservior part..
i checked the high voltage part and found 3 diodes (IN4007) wasted out of 4, by checking from multimeter 3 conducts both sides while one is healthy. so shud i further dig to next circuit or just replacing diodes is enough.
Thanks

 

[BradtheRad]

1N4007 is 1000V rating. This implies they are in a high voltage circuit. From your photograph, mains AC goes through the diodes to what must be a switching power supply (step-down).

Almost certainly the diodes burnt because of overmuch current being drawn. Three diodes shorted, then the fuse went. This suggests a lot of excess Amperes. 1N4007 family are 1A rating. The fourth diode is probably halfway gone too.

At the minimum you have to replace the diodes. That type is commonly available.

You must allow time for the transformer to dry internally. This might require heating it in an oven for an hour at, say, 120 degrees.

It is difficult to check every component. After replacing the diodes, and before you apply full 220VAC, can you apply a lesser voltage for a few seconds? Then even if the fuse blows, that may save other components. The reason to try it for a few seconds, is because there is no way to be sure the smps can safely operate at half voltage.

If you cannot fix the power supply, you may need to fabricate a substitute.

 

[areebaa]

Thanks .. yes i will use heating with oven or hair dryer.. good idea .. .
now i have diode "6A10" at home .. can i replce instead of "IN4007" i checked specs closly match but not exactly .. any suggestion ?
thank you

 

[FvM]

so shud i further dig to next circuit or just replacing diodes is enough.

I'd say, there's a 95 percent likelihood that more components are blown, first candidate is the switch transistor/IC mounted on a small heatsink, possibly others too. Repairing the power supply in this situation is probably beyond your skills, sorry.

 

[areebaa]

i changed the diodes, but same spark and takhhhhh..

 

[boylesg]

thank you for your suggestion , no i didnt wash any thing i cleaned only reservior part..
i checked the high voltage part and found 3 diodes (IN4007) wasted out of 4, by checking from multimeter 3 conducts both sides while one is healthy. so shud i further dig to next circuit or just replacing diodes is enough.
Thanks

In my experience it is not very reliable checking diodes in situ due to other 'routes', through the rest of the circuit, also forming a connection between your multimeter probes.

Really the only totally reliable way of checking diodes is to remove them from the circuit board.

I guess if they are completely blown then you could use the continuity checker across the diode in question.

If it is blown then you will get the beep where as the current from the multimeter through the rest of the circuit will not be sufficient to cause the beep.

But if you get a diode reading, but no beep, when you connect the probes across the diode both ways then you can't assume it is blown.