No, it will result in a 50% duty cycle full-voltage waveform. It's a
decent start, but given how non-linear an incandescent filament is, I'm
not sure you'll get what you want.
On the other hand, most of the non-linearities are due to the
resistivity of tungsten varying with temperature. The power cycling in
a 50% duty cycle 60 Hz wave is probably fast enough that the temperature
stays pretty steady.
Still, I suspect Greg's right; as a practical matter, it's probably the
simpliest and cheapest solution to the problem.
There is no need to speculate.
It works. I'd had a problem with a yard light dying whenever it was hit
(lawn mower wheel, paper delivery bag ... didn't matter). I installed the
diode, got about 1/2 the light (I knew this) but the bulb was still going
strong 18 months later when I sold the house.
Another example was the short lifespan on an emergency exit lamp. Think it
was high ambient temperature coupled with building vibration, but installed
a diode there ... same bulb was going strong two years later when I
Remember those "energy saver" buttons sold in the 1980s? They were nothing
more than a cardboard disk with a chip diode installed in the middle. Same
result (though you'll be hard pressed to find them these days). Easy to
install ... usually ended up soldering themselves to the lamp base and
getting tossed out when the lamp eventually died. Use 'em if you got 'em.
Remember that you're going to need to increase the wattage of the bulb a bit
more than 2x to get the same amount of visible light because at the lower
temperature, the lamp is putting quite a bit of energy into the infrared
region. Gives everything a reddish yellow cast instead of the tungsten-white
we're used to.
Again, no need to speculate ... it works.
Radio Shack part number 276-1104 is a 1N4005 diode rated at 1 amp
continuous, 600 peak inverse volts (PV or PIV). This should be good up to
200 watts (www.radioshack.com). Your local electronics parts supplier
(assuming you don't have a RS near you) can offer you their part number for
a 1N4005 ... you'll just have to ask.
Oh, and they are cheap ... think the RS price was $0.89 ... about the cost
of one 100 watt light bulb. If you want it cheaper, you can get them for
$0.017 each (in 5000 qty) from Digikey (www.digikey.com).
Who seriously thinks this thread has been thrashed about enough and deserves
to die a quiet, dignified death. RIP
"Roy Smith" wrote
... snip ...
Thanks for the support. <g> But...
Not to be pedantic, but actually the 'energy saver disks' were a
negative temperature coefficient thermistor. High resistance when
cold, but when current started to flow, they heated up, causing a drop
in resistance, giving the bulb near full brightness. Eliminated the
high current surge through a cold filament - which has a very low
Not necessary when using on 240v, however. The 120v bulb will glow at
full brightness. At 120v, of course, this WOULD apply.
There is a difference between 120 to neutral and 240 across two
opposite (or 120 degree/3p) hots. Or something like that... <g>
How does that save energy? The bulb reaches thermal equilibrium in a
small fraction of a second. Much too fast for the high inrush current
to be a significant factor in overall energy use. I could certainly see
how it might increase bulb life, by reducing thermal shock to the
filament, but save energy? I don't see how.
If you want to save energy, use a smaller bulb. Anything along the
lines of dimmers/diodes/thermistors just makes the bulb operate at a
voltage/current/temperature point different from it's design point, and
thus less efficient. Sure, you can make a 100W bulb draw 75W, but it'll
put out less light than a 75W bulb running at full power.
Hell, I didn't design them, sell them, or recommend them.
They were marketed to extend the bulbs life, and the small residual
resistance made them draw slightly less current. And yea, it made the
I agree, but morons will buy anything if your dress it up in a
colorful blister pack and promise the stars.
You could put a second bulb and run them in series, using standard 120v
Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD"
>>> To reply, remove the TRABoD! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf
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Save Model Rocketry from the HSA! http://www.space-rockets.com/congress.html
An alternative is to install a rectifier (diode) in series with one of the
power leads. This will reduce the effective power to the bulb by 1/2. You
will need a diode with minimum current rating of 2 amps or so to be "safe"
rated at 400 volts or better. An alternative is to install a dimmer (most are
rated at 600 volts) and only turn it up half way 8^)
Well thanks to the cool heads in this conference and the stearn chastisement
about dangerous wiring, I fixed this for good. I ran a separate cord for
the worklight and spliced it in.... piece of cake, legal, safe. The only
part that urks me is that my new lamp cord is brown and does not match my
black power cord. <grin> Maybe there's an aftermarket for worklight upgrade
You've gotten so many good answers, but what I haven't seen is the
question of "why". How big of a drill press do you have that 220 is
needed or even beneficial? I always thought that until the motor gets
pretty big (2 hp or larger?) that 220 doesn't provide any benefits
over 110. Sticking on external work lights, rigging the wiring on a
major tool, and/or installing various combinations of lights and other
electric gizmos to get around a problem caused by the switch-over
wouldn't be my choice unless I received some significant benefit from
running my DP on 220. Have you considered switching it back?
I'm pleased to ponder that question and give a reasoned answer. Actually I
did ponder it before switching. Its not about the benefit for the drill
press. Its about the flexibility of my 110 v power supply in the shop. I
spent years working with barebones in the garage with one 20 amp circuit.
Actually it was a 15 amp circuit (14 gauge) which the original builder
mistakenly fed with a 20 amp breaker. My standard mode of operation was to
switch off the air compressor when I wanted to run the bench grinder or shop
vacuum. There was no such thing as running things simultaneously.
So finally, I did the work and ran power with separate breaker panel for the
shop. I have the shop physically partitioned into three 110 volt 20 amp
circuits. During the winter I occasionally use an electric space heater (13
amps). The drill press is 9 amps at 110volt when loaded. So that means I
could not run the drill press and the space heater at the same time on the
same circuit. Yeah, I know I could pay attention to which circuit things
are plugged in, but that's a royal pain in the butt. After all that work
wiring this place properly, I want to be able to run anything at any time
within reason. My strategy from the get-go was to have three 220 circuits
and sprinkle 220 sockets around liberally for the stationary shop tools.
Yep, it was a hassle to fix the work light, but its a one shot hassle and I
get to retain my totally flexible power situation. I knew I would change my
mind about the shop layout as I learned more and acquired more tools. I
also knew that I would need to have a mobile shop, since I'm not blessed
with a 100x80 foot area. The one thing I could do that did not cost much
but greatly enhanced my flexibility was to put in ample power and have
outlets everywhere. I have a 220 volt outlet every three feet in my shop.
Dispite all the good answer, I read thru this entire thread waiting to
see if someone would post the MOST logical answer (in my mind) to this
problem and did not see it.
Think about the motor: in order for it to be usable at either 110 or 220
volts, it must havew 2 sets of windings. The windings connected in
parallel for low voltage operation, or in series for high voltage
operation. You currently have the latter configuration. If you connect
the lamp across one set of windings (ie from line to "center-tap"), the
lamp will "see" 110v. In effect you're using the motor as a transformer
(it is after all just windings on a magnetic core).
Since you've already implemented a fix, you may choose not to do this,
on the other hand you can get rid of that brown wire.
I had a Sears radial-arm saw, that's exactly how it was wired.
Your description is fascinating. At first, I did not understand how this
would work. Then I realized that what you propose is an approximation, not
a true tranformation splitting the voltage. Putting the light bulb across
one of the windings means that you have two equal reactive loads in series,
one of them wired in parallel with a resistive load (the bulb). The light
bulb load is small compared to the motor windings. The net result is that
the voltage drop across the winding-bulb will approximate half of the 220
volt. My guess is that it will actually be less than half because you've
reduced the overall impedence in that half of the circuit.
The net result - the bulb will see something that it can run on, but the
motor will see an imbalance in voltage across the two windings. It may be
academic, but I'd rather not intentionally introduce an imbalance across the
True enough there will be an imbalance between the current in the motor
windings. With a 60 watt bulb that will be about 1/2 amp greater in one
winding. That represents about a 5 - 10% deviation, or about the same
deviation that would result just from the maufacturing variables in
building the motor.
While I understand your reluctance, as I said I owned a Sears radial arm
saw for many years. It was wired at the factory so that when the user
(me) decided to change from 120 to 240 volt operation I did not have to
concern myself with rewiring the bulb. I owned that machine for about 15
years, ran it at 120 for the first 7 or 8 years. My son now continues to
Good luck with whatever you decide. I just wanted to expose one more
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