Two transformers blown - reverse polarity?

[Ric]

Hello,

I have on my boat a 12v drill and 12v vacuum cleaner, which are both charged from 240v when on shore supply. Suddenly both of the battery chargers have blown. Could it be that I accidentally connected up to reverse polarity on the shore power?

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[sailorman]

wouldnt have though so as the continental type of 2 pin plug can go either way into the socket

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[Bergman]

Unlikely

Basically AC equipment is not polarity conscious.

Have you had thunderstorms, could have been a voltage spike on the mains although it would be unusual to take out 2 chargers.

Is anything else broken?

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[William_H]

Both of the chargers have "blown" If they really blew you would notice a lot of heat damage. The AC connection is not able to cause that in fact probably the only way to "blow " a transformer is to short out the load causing overheating. Anyway I will take it that they have both failed and it is probably coincidental. I assume you are talking the plug pack type with the transformer housing forming the body of the plug. Thes can be cut open and repaired if you have a mind to do so. The most common problem is a break of the low voltage wiring out of the pack usually near the pack otr near the plug. Sop a first check is to measure with an ohm meter (multimeter) that you have resistance across the active plug pins. expect around 1 to5 thousand ohms) A very high resistance indicates an open primary winding or broken wire from thye pin to the transformer. ( this might be a good time to throw it away. If the primary is ok then check that the pack is indeed faulty, cut the outlet wiring to check if you have any voltage out of the wires themselves. That is after confirming there is no voltage out of the plug. If there is no voltage at the wires ( it may be AC or DC) then open up the pack by cutting around the join line with a hack saw. Don't cut deep as you may cut the coponents inside. When it is open check the continuity and condition of the outlet wires the resistance of the secondary about 10 ohms (1 to 100). Obviously at this point any sign of burning will tell it's own story. If you finda fault you can fix then glue it back together with epoxy. As you can see this exercise is for the crazy DIY but if you are bloody minded it can be done or at least many faults can be fixed. If it is the switch mode power supply type typified by light weight and have an input voltage capability of 120 to 250 volts then the primary winding resistance check doesn't work and you can almost forget repair unless it is a broken output wire.
Good luck and give it a try before you throw it away. will

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[oldharry]

These chargers (and many like them) often have a thermal fuse which melts and cuts off the power if the unit gets too hot. They are very cheap at Maplins, and can be easily replaced. If therefore you have open circuit on the mains side, the chances are the thermal link has blown, and there is nothing else wrong, as they are often set to go at unrealistically low tempratures, and simply leaving the battery on charge too long can blow them.

Normally they are a small silvery tube about the size of a 1/2w resistor, and you can only test them with a meter across them as they have no 'blown' indication.

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[Ric]

Thanks for these tips. I've fiddled around with the multimeter, and have found that the fault is downstream of the 240/12 transformer and rectification diodes.

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