Bruce EVangel Parmenter wrote:
Jerry's idea is effectively doubling the power demanded from the
original design. You would need to see if you are also overheating the
transformer. And if so, you could stick a fan on it to keep it cool.
That's not quite the case. The amount of power a given transformer can
supply without overheating depends on the load, the type of rectifier,
and filter (if any). Here's a quick summary of it:
http://www.powervolt.com/techan01.html
The calculations are tedious, but there is a chart toward the bottom of
this page with some "rules of thumb" for estimating how much power you
can get out of a given transformer with each of these circuits. A
battery charger is a "capacitor input filter" type of supply, with the
battery acting as the capacitor.
You might think that changing from a 2-diode center-tapped circuit to a
4-diode bridge would double the voltage, and halve the current. But the
output capacitance seriously "screws up" this intuitive guess. The
capacitance causes very large peak currents, and thus higher losses
(extra heating in the diodes and transformer).
For example, the web page selects a transformer to supply 24v at 2.4a DC:
- 2-diode center-tapped transformer secondary: 59vct @ 2.88 amps 170VA
- 4-diode bridge transformer secondary: 30.5v @ 4.32a 132VA
Notice that a lower VA capacity (smaller) transformer can be used. This
is because the 4-diode bridge circuit uses *all* of the transformer
secondary on both half-cycles. This reduces the resistive losses in the
transformer, reducing heat.
Stated another way, this also means you can get more power out of the
same transformer by using it with a 4-diode full-wave bridge. You won't
get *double* the power; but you will get about 170va/132va = 30% more
power for the same transformer temperature rise. (Thus the need for the
fan in Bruce's case).
But the problem with a old fashion transformer type (dumb) charger is
that it is fast charging at the beginning, but tapers to a crawl toward
the end of the charge. You end up paying a 'charging time' cost for
going cheap.
Yes, that's a fundamental problem with any unregulated charger. A "20
amp" charger actually averages 10 amps over a full charge cycle. These
are called "taper" chargers, since the current continuously tapers down
as the battery goes from empty to full.
However, some golf cart chargers (notably the ones made by Lester) uses
constant-voltage transformers. These are *regulated* chargers, and thus
deliver closer to the same current throughout the bulk charging phase.
--
Failure is only the opportunity to begin again more intelligently.
-- Henry Ford
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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