[Vwdiesel] voltage regulator and hot wire instruments

Sandy Cameron scameron at storm.ca
Sat Mar 29 08:25:18 PDT 2008

I may have published this here before.


The  gauges are "hot wire" instruments that operate by the expansion or
contraction of a metalic element, wire, or bi-metal wound with resistance wire.

Good way to average out rapid fluctuations caused by fuel sloshing in the
tank, etc.
They have an inherant  time delay rated in many seconds, up to a minute.

Such instruments, more current operated than voltage, are sensitive to
changes of both.

Therefore, the need to keep the suppply constant, notwithstanding alternator
variation, wiring harness losses load variations, etc, a regulated voltage
is desireable.

Many years ago, before silicon, some American cars (Ford was one) had an
electro-mechanical regulator, using the same "hot wire" principle , to
switch the gauge power on and off rapidly, like a high speed signal flasher,
to provide a constant AVERAGE voltage for the gauges.

[ The voltage goes higher, the off to on time increases], duty cycle variation.

The on versus off rate is called "Hysteresis", and the hotwire gauges
average it out due to their time lag.

Modern solid state alternator voltage regulators work the same way, and
except for the hysteresis of the rotor's magnetic circuit, would "chop"at a
high speed.

Chopping the current on and off eliminates the need for huge heat sinks that
would be required if the regulator were to try to control the field current
in analog mode, wasting 10 to 20 watts as heat.

The instrument cluster,(according to Bentley) is a 10 volt regulator. Any
voltage more than about 11 volts fed in to it, will come out firmly clamped
to 10 volts.
It is probably a generic 7810 device, it is a TO5 package.

Curiously, it too, works in the on-off mode, but at a very high frequency
(for all of the reasons mentioned above) but in electronic circuits, a very
small capacitor will smooth out the "buzz" (the hysteresis element).

The old electro-mechanical regulators used on auto generators up to the
middle of the last century worked the same way, using a voltage sensitive
relay to turn the generator field winding on and of rapidly,  (sometimes not
so rapidly I could see the lights blinking at times) and again, it was the
magnetic hysteresis of the generator that smoothed it out.

The later ones, post WW2, ( just before alternators appeared) had 3
different charge levels, where the relay could switch resistors in and out
of the field circuit to smooth the on- off variations a bit.

Here endeth the lesson. (and the hysterics :^)


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