Not Strictly Boating but 12-volt problem

johna

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A friend has a Hymer (German) motor home with a central electrical control unit that acts as battery charger as well as a 12-volt distribution board. Recently the 1.6amp fuse located inside the unit on the 230v side of the transformer blew when he connected to a site supply. We took the unit apart and after some basic tests replaced the fuse. It blew again, not immediately but after a short time. Inspection of the two 12-volt domestic batteries located under the bolted down driver and passenger seats, indicated a need for extensive topping up, 2 litres per battery!!!

Basic continuity tests of the transformer wiring do not show anything shorting out but a live bench test, no 12-volt connections, blew the fuse again. Use of 5-amp fuse wire with all wires disconnected from the low voltage side of the transformer resulted in successful operation with 12 and 25 AC outputs from the transformer. A re-test with the low voltage transformer connections re-made was also successful. Replacing the 5-amp wire with a 1.5 amp fuse is also working but only time will tell if this is permanent or not. It is now re-installed in the vehicle awaiting new batteries.

As the fuse has blown on the bench I have discounted the poorly maintained batteries as the direct cause but might they have contributed to some failure in the circuitry of the transformer output rectifier? Can anyone help on reasons why the 230-volt side should fail?
 

VicS

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Clearly if there was no load the fuse should not have blown on a bench test although you do not say if you had disconnected the rectifiers and any associated circuitry but you seem to indicate you had. If it's not what you have done then you should bench test the transformer with absolutely nothing else connected to it. Are there any capacitors on the mains side of the transformer that could have partially failed or any on the low voltage side for that matter.

If the transformer, and the transformer alone, causes the fuse to blow then it'll be curtains for it I fear.
 

misterg

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The main rectifier may have (partially) failed, the main smoothing capacitors are getting leaky, shorted turn on the transformer (for example). Just examples that could give the symptoms you describe. Either way, it needs to be fixed - leaving the higher rated fuse in place is a bad idea. If it's a fairly high-tech charger with a "switch mode" type power supply (little ferrite transformer(s) rather than a whopping great big iron one) - be very wary of the high voltage capacitors - a shock from them could send you into next week, if not worse.

Another thought: if it's a switch mode supply, the input voltage may be too low - bad connections? (Does it have an input voltage selector?)

Just thoughts.....

Andy
 

halcyon

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Sounds like you could have the start of insulation breakdown on the primary winding. This a low resistance winding and hard to spot, normally if it blows the fuse with secondary disconnected I assume a primary winding failure. They are a very low quality units normally, and if you have been pulling high output in may have been the cause. You may find a transformer in Maplin, CPC etc.


Brian
 

VicMallows

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I would go first for any suppresion capacitors on the input as VicS suggests. What is the output rating of the charger? .... the 1.5A fuse does sounds a little low when you consider the inrush current, though it sounds from what you say that it was the original fuse? If nearly on their limit, fuses do age and eventually fail. New fuse might have had a different characteristic ... you can get 'slow-blow' fuses if surges are involved.

Vic
 

johna

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Thank you for your answers and helpful comments. We are waiting delivery of the batteries before we can be sure of it all working again. Just to clarify points raised todate I have answered three posts below. Will give some follow up once the matter is resolved.

VicS
Re tests
1st bench test was with 1.5 amp fuse with rectifier connected. Fuse Blew.
2nd test with 5 amp wire (running out of fuses) no rectifier. Fuse OK
3rd test with 1.5 amp fuse no rectifier. Fuse OK
4th test with 1.5 amp fuse with rectifier. Fuse OK
Re capacitors
None on 230 volt input side only the 1.6 amp fuse. The fuse appears to be connected to the mid point of the primary winding.
I have not investigated the rectifier circuits as I am more electrics than electronics if you know what I mean.

Misterg
Unit comprises heavy iron core transformer. There is no indication that it is a staged charger but it may well be as it is a specialised power unit for distribution as well.

VicMallows
Re output from memory the charger is 15 amp. Power distribution is limited to 10 amp.
 

VicS

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Your answers to my questions points a finger of suspicion at the rectifier as once disconnected the fuse didn't blow.

like Vic Mallows I wondered about the fuse rating but if the output is only 15 amps then it should be OK. Presumably if it is internal it was the original.

I don't understand "The fuse appears to be connected to the mid point of the primary winding." One would expect the fuse to be in the live feed to one end of the winding with the neutral connected to the other.

Is this by any chance a dual input voltage unit ie 120/240 volt if so is there any chance that it's wrongly set. (Must admit tho' that the only time I connected one up to 240 when set to 120 it blew it completely.)

A shorted turn in one of the windings will cause the fuse to bow immediately but Andy's other suggestion about leaky electrolytic condensers (coupled with flat batteries taking their maximum current) may be a possibility if there are any. Had the unit been left idle for a long time?
 

TrueBlue

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Possible cause - Damp??

I have known problems to be with condensation causing the high voltage windings to be wet and 240 tracking across the insulation.

Dry the unit and probs go away. Isn't that what happened to you?

Just a thought.
 
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