Toaster is a pure resistive load?
Srikanth Viswanathan
DC voltage since it is a purely resistive load. Could someone please
explain this?
Thanks.
Srikanth
Joel Kolstad
>On a another network, I read that a toaster can use either AC or
>DC voltage since it is a purely resistive load. Could someone please
>explain this?
Resistors take electrical power, and turn them into heat. That's all they
do. They don't care at all what the power "looks like" (AC or DC) -- they
just convert it into more or less heat. (Of course, there is no such thing
as a perfect resistor, but a toaster comes pretty close.)
Non resistive loads (i.e., inductive or capacitive) don't turn all
of the electrical power into heat. Instead, some of it is can be stored in
electromagnetic fields. Because of the way electromagnetic fields "work"
(see any physics text...), they care very much about the shape of the
electricity. Therefore, the amount of electrical power drawn from a power
source can vary considerably in a non-resistive load, depending on what the
input power looks like (AC or DC). An inductive or capacitive load
designed to work at 60Hz AC probably won't work at DC. Some simple
examples: A capacitor acts like an open circuit at DC, but allows some
current to flow through it for AC. An inductor acts like a short circuit
at DC, but only allows a some current to flow through it for AC.
Some resistive loads around your house: Toasters, hair dryers, ovens, other
variations of the basic heater idea.
Some inductive loads around your house: Anything with a transformer in it.
Many things with motors in them. Probably your computer and stereo and TV.
One final note: For resistive loads, you need to adjust the DC voltage to give
the same power input as an AC power source. The AC coming out of the wall
has a peak voltage of about 177 volts, at 60 Hz. To deliver the same
amount of power from a DC source, you need to use 120 volts DC. This is
why the AC line is called "120 volts RMS". The RMS voltage tells you the
equivalent DC voltage needed for a resisitve load to deliver the same
amount of power.
---Joel Kolstad
Michael Covington
>DC voltage since it is a purely resistive load. Could someone please
>explain this?
It converts electricity into heat. There is no motor or transformer, and
hence nothing in it that would respond to DC differently than AC.
--
:- Michael A. Covington, Associate Research Scientist : *****
:- Artificial Intelligence Programs mcov...@ai.uga.edu : *********
:- The University of Georgia phone 706 542-0358 : * * *
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John Nagle
>In article <25MAY199...@vmsb.is.csupomona.edu> cvaf...@vmsb.is.csupomona.edu (Srikanth Viswanathan) writes:
>>On a another network, I read that a toaster can use either AC or
>>DC voltage since it is a purely resistive load. Could someone please
>>explain this?
>It converts electricity into heat. There is no motor or transformer, and
>hence nothing in it that would respond to DC differently than AC.
Dumb toasters will run on DC, but today, I'd hate to assume a
toaster had no electronic components. Some have motors, some have
electronic timers, some have photoelectric toast sensors.
Side issue: are toasters a roughly constant resistance, or do they
draw more current cold, like lightbulbs?
John Nagle
Dave Turner
>
>Some resistive loads around your house: Toasters, hair dryers, ovens, other
My hair dryers contain motors as well as the heating elements.
The motor will contribute an inductive component to the load.
>
>Some inductive loads around your house: Anything with a transformer in it.
>Many things with motors in them. Probably your computer and stereo and TV.
--
Dave Turner (510) 823-2001 {att,bellcore,sun,ames,decwrl}!pacbell!dmturne
Joel Kolstad
>In article <1993May26.0...@doug.cae.wisc.edu> kol...@cae.wisc.edu (Joel Kolstad) writes:
>>
>>Some resistive loads around your house: Toasters, hair dryers, ovens, other
> ^^^^^^^^^^^
>My hair dryers contain motors as well as the heating elements.
>The motor will contribute an inductive component to the load.
Good point. Sorry, I forgot about what pushes the heat out...
LE...@qucdn.queensu.ca
(Dave Turner) says:
>
>My hair dryers contain motors as well as the heating elements.
>The motor will contribute an inductive component to the load.
Most of the hair dryers I have seen uses a small DC motor and rectify
the voltage from part of the heater network (ie uses the heater as some
sort of voltage dividers to provide a few volts for the DC motor).
There are no filtering caps on the rectifier and the motor uses a very
small percentage of power (vs the heater). I would speculate the load is
very close to unity.
I think my parents might still have an old hair dryer that uses an AC motor
(single phase Universal motor) lying around wait for me to fix. :) Now
that one is definitely the inductive load type you mention.
The other poster said:
>>Many things with motors in them. Probably your computer and stereo and TV.
Computer is more like a capacitive load. Modern TV probably capacitive.
>Dave Turner (510) 823-2001 {att,bellcore,sun,ames,decwrl}!pacbell!dmturne
K. C. Lee
Elec. Eng. Grad. Student
Jeroen Belleman
>Computer is more like a capacitive load. Modern TV probably capacitive.
Computers and TV sets mostly use switching power supplies (SPSs)
these days. These things present a very odd load to the mains.
First off, they're generally neither particularly capacitive nor
inductive. They draw current near the peaks on the sinewave, which
makes them produce a strong third harmonic current. We have lots of
SPSs around here, and we've seen overloads on the neutral line of
three-phase AC nets because of this. (The third harmonic doesn't
cancel on the neutral line; it adds.)
There are chips on the market now which address this problem.
Secondly, as the mains voltage drops, the current tends to increase,
giving the supply a definite negative resistance character. I've
been told this can lead to funny problems too, but I have yet to
encounter any.
Best regards,
Jeroen Belleman
jer...@dxcern.cern.ch
Bill Shymanski
A toaster is a nearly pure resistive load; any circuit of finite
dimensions will have some inductance and capacitance, of course, but in
the world of power systems we'd call a toaster R, not L or C.
However... you wouldn't really want to run your toaster on DC for a long
time. Toasters, being AC appliances, rely on the AC current going to
zero at regular intervals. The contacts that operate to open the heating
circuit would rapidly be destroyed if they had to open the same current
on 120 volts DC as they can handle for years on 120 volts AC. Its
possible to design contacts for DC service but they generally use
tiny "arc chutes", blowout coils, or permanent magnets to assist in
extinguishing the arc - something that AC contacts don't need help for.
In typical industrial control use, a pair of contacts rated for
say 20 amps at 120 volts AC might only get a 2 amp rating at DC,
or less.
Bill
bi...@inqmind.bison.mb.ca
The Inquiring Mind BBS, Winnipeg, Manitoba 204 488-1607
Mark Olson
Most handheld "blow driers" use a small DC motor with
a bridge rectifier. The drier works fine with DC power.
I have verified this...
However, some older units used synchronous AC motors.
Needless to say, it would be disasterous to power one
of these with DC...
Mark Olson