$5.00 variable voltage controller

[harvey]

Below is a "recipe" for a low-cost, low-tech variable voltage controller
for electric hammer irons, etc. Southern translations included where
applicable.

Disclaimer: The following is not and would not be approved by OSHA. For
that matter, once the components are modified, the original UL approval on
the components is also questionable, if not void. Builder/user assumes any
and all risks. If you want certified stuff, spend the $75 or so for a
commercial-quality unit.

My local supplier, Wally World, seems to be scaling back on the hardware
end of electrical parts. A visit to 'orphan alley' [Harveyism for where
they stash the blow-out stuff] last night unearthed the following items and
their discounted prices:

$2.50 GE single pole, push-on, full range dimmer (#D161ULM5)
   .50 Steel "handy box" (receptacle box)
   .50 Steel light switch box cover (one slit -- not two-hole receptacle type)

Considering these prices, I picked up the above items, and tonight I
assembled the controller. Additional discussion follows assembly details below.

COMPONENTS (Ingredients):
Dimmer switch
2-wire extension cord w/moulded ends [I used one of dozens that seem to
create clones of themselves over time -- think I've only purchased one in
my life]
Handy box
Box cover
Misc: Rubber grommets, wire nuts (or solder and electrical tape)

Leave knockouts in electrical box intact. Drill a 3/8" hole through
existing knockout in each end of box, and DE-BURR the holes. Install one
rubber chassis grommet into each drilled hole. (Otherwise, silicone sealant
following final assembly comes to mind, as a variant for the grommet impaired.)

You'll need the "business" ends of the extension cord, but not necessarily
all the wire in-between. So, cut a 6" length from the plug and extension
ends, respectively. (Wire itself should be 6", -excluding- moulded ends).

Insert cord through holes in grommets, and separate (pull apart) cord on
each end of the box into two pigtail wires (total 4 pigtails). Cord is
separated far enough to tie a single loop knot in pigtails, to serve as
strain relief. Not that it matters much with AC, observing original cord
polarity (tracer or raised edge on insulation -- style varies), reconnect
one leg of wire back to "original" condition by either soldering and taping
(my preference for portable use) or with a wire nut. Similarly attach each
of remaining pigtails to each wire of dimmer switch. Screw dimmer switch
into handy box, dressing wires out of the way of attachment screws, OR
shorten included screws, OR use alternate screws. Screw box cover (face
plate) onto dimmer switch, and attach dimmer knob.

Total construction time: less than 15 minutes.

SCALE CALIBRATION (optional):
None is okay, since trial-and-error will also work. Otherwise, use an
indelible marker to draw a vertical line on the side of the dimmer knob.
With the knob in maximum counter-clockwise position, index the drawn line
to the box cover. Making the mark adjacent to one of the cover plate screws
is a good "zero" point. Note: the reference is only important in case the
knob falls off or is removed at some time -- it makes placing the knob back
in the correct position a lot easier. From the zero point, connect a load
to the dimmer. I connected a work lamp with a 60-watt incandescent bulb to
the output, and paralleled a digital voltmeter across the lamp's plug
prongs to provide rough visual and voltage reference points. I then turned
on the dimmer, and advanced the control until the meter read 50v (the first
really viable reading), then 60, 70...and so on. At 50v and each 10v
increment, I used the marker to draw corresponding lines on the steel cover
plate, directly across from the line drawn on the knob. Again, really
low-tech, but it provides a quick dial scale for repeated settings.

ERRATA:
Although I don't know how it could be made less expensively than the
existing $5 or so, this device may certainly be improved upon and/or
otherwise "dressed out" with additional time, components, and cost. An LED
pilot lamp to indicate off/on status, a 2-wire w/ground "orange" appliance
cable instead of the cheap lamp cord, even a built-in voltage meter are
some things that come to mind. I purposely wanted short lengths on the plug
and socket(s) outside the box, since I'll be using it in conjunction with a
"normal" extension cord. You may wish to leave the original length on the
PLUG side. 

THEORY:
Someone in the know may explain this better (please), but trusting what
little electronics theory remains, and without disassembling the [sealed]
dimmer, I must assume this is what is called a 'half-wave' rectifier
(rectal fire in Southern). It likely has a small bridging capacitor to help
prevent RF interference and hum to TV's and radios. If this is the case,
the resulting waveform would be a spiked DC voltage (still containing an AC
component (Southern: half-assed DC). More expensive commercial controllers
probably feature full-wave rectifiers, more filtering, and perhaps voltage
stabilizing circuitry -- and hopefully a more linear (or is it
logarithmic?) voltage pattern across the voltage range.

However, the dimmer will support up to 600 watts before going thermal
(Southern: blows up), more than adequate capacity for controlling
incandescent lights, soldering (hammer) irons, old-style electric drills
and Dremel Moto-tools (without built-in speed controllers), and so on. IOW,
brush-type motors and resistive devices are okay, within the wattage
limits. Capacitor, shaded-pole, inductance, and motors that are already
DC-controlled do not qualify. Likewise devices requiring trigger or startup
voltages, such as florescent lamps are also on the no-no list.

Have a nice weekend!


Jim Harvey, RPT
Greenwood, SC
harvey@greenwood.net
________________________
 -- someone who's been in the field too long.