John Williamson expressed precisely :
Dave Plowman (News) wrote:
In article ,
Rick Hughes wrote:
On 29/12/2012 00:11, Dave Plowman (News) wrote:
In article ,
Rick Hughes wrote:
I have a relatively new Car Battery Charger (twas a gift) claimed to
be electronic 'smart' charger.
Took the cover off and found it to be a straight transformer to full
wave rectifier set of diodes ... and out to croc leads.
Then it will blow up when you try and use it.

Why ?

220V AC step down transformer full wave rectifier giving DC output to
a pair of croc clip ended leads ..

Display gives some LED indication, and there is an in-line fuse
........... why would it blow up ?

Because a lead acid battery has a very low internal resistance. To a power
supply like that virtually a dead short. A crude charger would have some
form of series resistance to limit the maximum current.

Unless they've cunningly used the transformer's impedance to give a sagging
output with increasing load. Saves the regulator and means the transformer's
cheaper, too.

Is the correct answer.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk