[ARC5] Receiver AC Power Supplies

[AKLDGUY .]

> Something that was bugging me today....Ohmic copper loss increases as the
square of the increase in current (power
> equals I-squared x R). Windings carrying 4amps will dissipate 4 times the
heat they did at 2amps. So, wouldn't a
> transformer rated 25vct 2amps (delivering 50watts output) run even hotter
if you used it to obtain 12.5volts at 4amps
> (still delivering 50watts)? Might spell eventual trouble for the
insulation around them, if not for the wire itself.

There's another factor involved. Time.
The transformer delivering 25V to a bridge rectifier at 2Amps is supplying
50W in each half-cycle. The heating effect is therefore 50W continuous for
that complete cycle.

If the same transformer is now connected to a 2-diode full-wave rectifier
with the center tap brought out as the common terminal in the usual way, it
will deliver 12.5V and the current drawn can in theory be increased to
4Amps in each half-cycle for the same heating effect per complete cycle.

In practice, the wire for the windings is usually chosen for the lower
rating (2Amp full winding) and heating losses in the wire would not
allow the full doubling to 4A.

In earlier days when 2-diode centertapped operation was the norm,
transformers were probably rated accordingly, but for some time I think the
tendency has been to rate them for current flow in the entire winding on
each half-cycle, ie. as for bridge rectification.

73 de Neil ZL1ANM


On Sat, Dec 13, 2014 at 3:31 PM, <hwhall at compuserve.com> wrote:

>
>  Something that was bugging me today....Ohmic copper loss increases as the
> square of the increase in current (power equals I-squared x R). Windings
> carrying 4amps will dissipate 4 times the heat they did at 2amps. So,
> wouldn't a transformer rated 25vct 2amps (delivering 50watts output) run
> even hotter if you used it to obtain 12.5volts at 4amps (still delivering
> 50watts)? Might spell eventual trouble for the insulation around them, if
> not for the wire itself.
>
> Wayne
> WB4OGM
>