In article ,
Richard Ferguson wrote:
The big issue is getting a transformer large enough for 7 amps. If you
can find a transformer that has a 2:1 ratio capabile of 7 amps, you are
in business. Even an old transformer that changes 220 to 110, or 440 to
200, would work fine, if it was large enough. Are you sure the 24V
circuit will provide 3.5 amps? (same wattage as 12V at 7 amps). Not
sure what gear you have lying around.
Why not check to see whether the transformer already there has a
center tap on the secondary? If so, just add another bridge rectifier
and filter capacitor to one half of the winding. (Or, to do a nicer
job, using the existing bridge rectifier, try a hookup like this):
| | |
| | | __________________________________________________ ___0 (~)
| | | E / \
| | | E (+24V)+--------------------------- /-+ \
| | | E | / | \
| | | E === / | \
| | | E | / | \
| | | E | / | \
| | | E----------------------+ (+12V) (-)+-0 +------0 (+)
| | | E | | \ /
| | | E | | \ Bridge /
| | | E === | \ Rect. /
| | | E | | \ /
| | | E (GND)+---------------------+ \ /
| | | E \ /
| | | E 0 (~)
| | | E_________________________________________________ ____|_
| | |
| | |
| | |
I can't make a good diode symbol in the bridge rectifier, so I
drew it without, just marking the terminals as '+', '-' and '~' (AC
input terminals).
So -- what is done above is to replace the filter capacitor with
two in series, with the common point also connected to the center tap of
the transformer's secondary. (I didn't bother drawing the primary, to
save some space.
The advantage of this over using an extra set of rectifiers to
develop the 12V from the center tap and one end is that the extra
current is shared between the two halves of the winding, so there is
less chance of overheating it. (This assumes that the transformer can
handle the extra load of a lower-voltaged pump.
Good Luck,
DoN.
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