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Re: volts and amps



John D mentioned an exceptional case earlier -
the car headlamp on the dry cell supply.

That's a case of the source impedance being too high. (BTW I don't
consider it exceptional.)

At 12:13 PM 7/14/00 -0500, brian whatcott wrote:

Here are one or two more:

Actually these are the other side of the coin: source impedance too low:

The starter motor used on 12 volt cars will likely burn out if
driven with a stiff 12 volt supply.

Good point. Motors are tricky. Depending on what the motor shaft is
driving, you can very easily get the following behavior:
a) If the voltage is really low, nothing bad happens. (Of course
nothing good happens either.)
b) At a slightly higher voltage, the motor burns out.
c) At a yet higher voltage, everything is baby-bear just right. The
motor does useful work and doesn't overheat.
d) At a very high voltage, the motor burns out.

Many people are surprised by the contrast between (b) and (c). How can a
motor that works OK at one voltage burn at a _lower_
voltage? Answer: back EMF. If the motor isn't spinning fast enough, it
can't make enough back EMF.

That's something to worry about, now that the summer brownout season is
here (brought to you by your local short-sighted utility company).

The coil on some older cars (Fords in particular) would also burn out
if their dropper resistor were somehow shorted for long
(for imagined spark boosting?).

That's an example of a circuit designed for a special purpose. It is (to a
salable but not very good approximation) trying to be a constant-current
source.

Another example in this category is a fluorescent light fixture. If you
replace the ballast by a lower-impedance object, things do _not_ work better!

There is a move to 42Volt auto systems, in progress now.
This is happening for much the same reason cars moved to 12V
from 6 V systems mid century.

42V seems like a strange choice. Obvious choices would be
stay at 12V (actually more like 14V in normal operation)
go to 28V DC (widely used in airplanes)
go to 115V 50/60 Hz (widely used)
go to 115V 400Hz (widely used in airplanes)