June 2011 Archives

RV 12V Converter repair - redundant parts

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My neighbour accidentally connected the 12V RV battery backwards to his tent trailer. The brief, but mildly spectacular flash was a premonition that something bad had happened. The battery was quickly disconnected, but not before the converter / charger was damaged. Having previously rescued a power amp that suffered the same reverse of fate, I decided to have a go at diagnosis and repair. First problem was the fuse. In troubleshooting the lack of 12V, my neighbour started with the fuses - good idea, poor execution. The main barrel style fuse holder was a bit different than most 1/4" fuses - instead of a twist-n-lock, it was threaded. Misunderstanding this resulted in the cap being damaged. 

Broken threaded 1/4" fuse holder.
That part is now on order.

We removed the unit and I brought it home. On my bench, I started by looking for any traces of the magic smoke that engineers carefully inject into delicate components. Not finding any, I decided to test the unit. I by passed the fuse holder and upon apply power heard a distinct clicking sound. Further investigation revealed a curious part common in switching power supplies - a double diode - common cathode. This one's part number(s) proved difficult to source 6N24 - any ideas?

6N24 - double diode, common cathode

Any way, I noted an interesting form of installation. Seems the engineer only needed a single diode, and so the part was soldered in with both an anode and the cathode on the + rail and the other anode on the negative rail (presumably to provide some back current protection). A quick check with the meter showed that the active half has internally shorted when the battery was misconnected - hence the clicking sound upon powering up. I removed the part, flipped it around to make use of the convenient spare diode, soldered it back in and powered it up.

No more clicking! 13.4V on the output as expected!

Yeah! No to wait for the replacement fuse holder to arrive. By the way - any idea why the manufacturer would have installed the fuse in line with the Neutral and not the Main / Hot?