On 29/12/2012 16:02, John Williamson wrote:
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.

No in-line resistor
Only resistors are around the LED's and are typical low current items.

My previous charger was the same .... transformer rectifier output.

I increased its performance by putting large value capacitor on output
.... raised o/p voltage sufficient to make charging more effective ....
no good for float charge though.