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I thought it would be nice to
include an AC power supply in the Bench LENi, so a small board was made to
hold the rectifier and regulator circuitry.
Surplus 9V AC wall transformers
are readily available and any similar AC transformer will work fine. If
internal transformer is desired, a 6.3V filament tranny would work fine
too.
First the AC is rectified using
a 4 terminal bridge rectifier block, then filtered with a large
electrolytic. Value of the cap is non critical, I had some 3300's at 25V
and they worked fine.
An LM 317 regulator chip was
chosen for this project and wired according to the design sheets. It is a
3 terminal regulator with very good specs, capable of 1.5 Amps output and
has internal short circuit protection.
Wiring is a little different
than the normal 78- xx series in that the pin assignment is Adjust-
Output-Input , where the 78-xx are In- Ground- Out.
The 9 V DC goes to the input
terminal, shunted to ground by a .1 uF cap ( bypass).
From the ADJUST terminal, a 75
Ohm resistor in series with a 1K pot to ground. Also a 240 Ohm resistor
goes from here to the output terminal, where another filter is needed, I
used a 470 uF.
That's really all there is to it. Run the (+) to the (+) wire
from the LENi, the (-) to the (-) wire. Since the '317 is a positive
regulator
only, we float the ground ( LENi's are positive ground).
Fig. 1 - Battery wiring points (set is POSITIVE
GROUND):
 During the LENi conversion, we
eliminate 2 of the 4 flashlight batteries, plus their holder, by sawing
the 4 battery holder in half. This process leaves a small cavity at the
end of the remaining battery holder into which the soundboard is mounted.
By salvaging the other holder pieces, and sawing them down further, a nice
holder for the power supply board can be fashioned.
The circuit board is made from
a blank piece of fiberglass substrate with no copper on it. Holes are
drilled at the appropriate positions and tiny brass eyelets are installed.
Into these the component leads are soldered.
Method to make circuit
boards:
Fig. 2 - Battery wiring points (set is POSITIVE
GROUND):
 Taking the power supply ideas but using a standard 3 terminal
7805, you would wind up with 5 Volts regulated. This can be knocked down
to 2.9 Volts by using 3 series silicon diodes.
LENI's and 705 clones seem to be rather non tolerant of over
Voltages, so keep any supply at 3 or slightly les
A toggle switch is used to select AC or DC.
The next step is a vehicle power supply or mod that will
allow the set to be powered continuously in your car. This would be a
great application for the alarm hack we described earlier.
Have Fun
Geo |