Re: quick question about watts.
From: Don Klipstein (don_at_manx.misty.com)
Date: 01/10/05
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Date: Mon, 10 Jan 2005 04:34:33 +0000 (UTC)
In article <1105321050.516507.286010@c13g2000cwb.googlegroups.com>,
ngdbud@hotmail.com wrote:
>if i pluged your average 100 watt bulb into an average outlet, it would
>draw just under 1 amp, right? well, if i pluged this same bulb into a
>10 volt 10 amp power supply (100 watts) would i get the same result as
>the regular outlet?
No.
The voltage applied to the lightbulb would be 10 volts. Ideally, that
is. Ideally, a power supply delivers its rated voltage to the load even
if the load when supplied this voltage draws an amount of current
different from what the power supply is rated for.
Having a load drawing less current than the power supply is rated for is
OK, while drawing more current than the supply is rated for can result in
voltage being less than the rating of the power supply and/or overheating
of the power supply.
Power supplies that lack regulators will produce slightly to somewhat
higher voltage when you draw less than rated current. Possibly
significantly more, but I have yet to see or hear about twice as much.
So what happens when you apply 10 volts to a 100 watt lightbulb:
If the lightbulb is an ideal resistor, it would draw 10/120 as much
current at 10 volts as it would at 120 volts.
At 120 volts, 100 watts means .833 amp. 10/120 of that is .0694 amp.
But incandescent lamps are famously not linear resistors, since the
resistance of most metals varies roughly proportionately with temperature
in degrees K. (Very roughly - tungsten at 2800 Kelvin has about 15 times
as much resistance as it does at 300 K, not all that close to proportional!)
Roughly very roughly and there are variations, but the resistance of an
incandescent lamp is usually roughly proportional to the square root of
the applied voltage (within the range of voltages at which the lamp
glows). Likewise the current is roughly proportional to the square root
of the applied voltage. this means that a 100 watt 120V lightbulb at 10
volts would draw about .24 amp.
(I just tried this and got .24 amp at 9.3 volts and about .255 amp at
11.2 volts, interpolating to about .245-.246 amp at 10 volts - plus or
minus meter reading tolerances. BEWARE - some lightbulbs will not follow
my above "rule of thumb" that closely, with deviations becoming more
severe with greater degree of undervoltage.)
Go ahead and conect your 120V 100 watt lightbulb to your 10 volt 10 amp
power supply. Expect the bulb to glow a dim orange or reddish orange,
possibly not visible in normal room light and fair to good chance not if
the bulb is frosted or "soft white". Even at 12 volts a 120V 100 watt
lightbulb is a little dim for use as a night light.
- Don Klipstein (don@misty.com)
- Next message: Danny T: "Re: MOSFET/circuit troubleshotting :-\"
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- In reply to: ngdbud_at_hotmail.com: "quick question about watts."
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