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[Debs.]

I`ve acquired a (supposedly) 'dead', high-output 12 volt battery charger......
The transformer`s fine, as is the rectifier; what`s actually 'dead' is the bimetallic (thermal/auto reset) overload-breaker on the output side...in fact the breaker has disintegrated and hence the want of output!

Yes, I could bridge the (failed) breaker with heavy gauge copper wire and have an instantly working charger: but is there a 'safer' fix with respect to the tranny/diode.....fuses seem out, as the output is high enough for (some) cranking/starting etc.
Any ideas?

[Kevin Wood]

I would replace it with a thermal breaker that takes out the mains input - so doesn't have to switch the full output current. Mount it in good thermal contact with the transformer or rectifier, whichever gets hottest under duress, and it will at least keep it safe. I suspect it's there so that it it's left charging a flat battery that's pulling a lot of current it limits the temperature of the charger.

If the mains input isn't fused I would fuse it or fit a circuit breaker to protect against short-term abuse too.

Kevin

[Debs.]

Thanks Kevin; I`m now browsing Maplin for the thermal fuse and breaker.

[partsman]

First question has to be................what is the output current.

If within the range of standard domestic MCB's then you could use one of these as they are rated at 60VDC

[partsman]

I would replace it with a thermal breaker that takes out the mains input - so doesn't have to switch the full output current. Mount it in good thermal contact with the transformer or rectifier, whichever gets hottest under duress, and it will at least keep it safe. I suspect it's there so that it it's left charging a flat battery that's pulling a lot of current it limits the temperature of the charger.

If the mains input isn't fused I would fuse it or fit a circuit breaker to protect against short-term abuse too.

Kevin

Problem here is that a shorted output would destroy a rectifier diode for faster than it takes for the thermal mass of the diode body to get to a point where the trip would kick in.

This method does work well for transformer protection though!

[Kevin Wood]

a shorted output would destroy a rectifier diode[/highlight] for faster than it takes for the thermal mass of the diode body to get to a point where the trip would kick in.

This method does work well for transformer protection though!

That's why I suggested a fuse/ breaker on the mains input but as you say depends on rated output current as to whether it's better on the output. Mains breakers are pretty cheap and available in a ride range of ratings so maybe a suitable rating for the output exists.

Kevin

[Debs.]

First question has to be................what is the output current.

If within the range of standard domestic MCB's then you could use one of these as they are rated at 60VDC

The charger is rated at 35 A. continuous and 120 A. for short periods (on a duty cycle) for cranking or initial connection to a completely-dead battery.
I doubt if I`d use it for cranking however; that`s really an abuse for the modern car`s electronics.
I have an old 32A MCB to hand so will install that on the L.T output side and the thermal fuse that I sourced today from Maplin will attach next to the diode on to the (huge) aluminium-plate heatsink and protect the primary circuit.
Excellent suggestions; `thanks guys.....

It`s a really good quality Sealey battery charger that I rescued from the bin: the things folks throw-away.

[Kevin Wood]

Sounds like a plan. Always nice to have "recycled" something useful.

Kevin

[Marks DTM Calib]

First question has to be................what is the output current.

If within the range of standard domestic MCB's then you could use one of these as they are rated at 60VDC

The charger is rated at 35 A. continuous and 120 A. for short periods (on a duty cycle) for cranking or initial connection to a completely-dead battery.
I doubt if I`d use it for cranking however; that`s really an abuse for the modern car`s electronics.
I have an old 32A MCB to hand so will install that on the L.T output side and the thermal fuse that I sourced today from Maplin will attach next to the diode on to the (huge) aluminium-plate heatsink and protect the primary circuit.
Excellent suggestions; `thanks guys.....

It`s a really good quality Sealey battery charger that I rescued from the bin: the things folks throw-away.

Sounds fine (note Partsman was me logged in at my parents!).

Idealy the breaker will be a type B and not an A varient as found in some industrial applications