Re: about those rheostats...

(mrdarrett@xxxxxxxxx) writes:
Tom Biasi wrote:
<mrdarrett@xxxxxxxxx> wrote in message
news:1168765651.992070.117290@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Let's say an incandescent light bulb is controlled by a rheostat. Does
a rheostat truly save power when set to low power? Or, is the same
amount of current going through the rheostat regardless of the power
setting?

Michael


Hi Michael,

A rheostat is the name for a variable resistor that is being used in series
with a device to control the total current in the circuit.
It does this by varying the amount of resistance in the circuit.In your
example, the bulb and the resistance will share the voltage applied.
The bulbs power dissipation will depend on the voltage across it and the
current going through it.
The power dissipated in the rheostat will also depend on these factors.
Since it is a series circuit, the current through both devices will be the
same and the voltage will be divided between the two.
Whatever part of the applied voltage is not across the bulb will be across
the rheostat.
So, whatever power does not get dissipated by the bulb will get dissipated
by the rheostat.
Now answer your own question.

Regards,
Tom


So, if the current is constant, and the voltage is shared between the
rheostat and light bulb, then exactly the same power will be
dissipated.

Bear with me, it's 3:20am here, and I've had three shots of whiskey.
;-0

So, I'm not quite understanding how a rheostat saves any power
whatsoever...

Maybe because what you are thinking of is being misnamed?

YOu are asking about rheostats, and people have given the answer.

But while rheostats can dim lights, there are other means of doing
the same task.

The common wall light dimmer as we know them today, those aren't rheostats.
They are using a triac to vary how much of the cycle power can get to
the light. INstead of being a variable resistance method, like the rheostat,
the triac either lets all the voltage through, or none at all. But the
dimmers work by varying how much of the time the triac is on. It's like
turning the light bulb on and off really fast, the bulb can never get as
bright when it's only receiving power part of the time.

MIchael
.

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