Trevor Boicey <tboicey@brit.ca> wrote:
" Without knowing the car in question, the ammeter could function
perfectly well without having all the current flowing through
the dashboard.
Remember, an ammeter is simply a voltmeter in a special
application. Current flowing across a resistance creates
a voltage drop across the device.
An ammeter involves two devices, one is a fairly well known
resistance, the other is the voltmeter. Flow the current you
want to measure accross the resistance (known as the shunt) and
knowing the voltage, compute the current.
By keeping the resistance very small (much less than one ohm)
the resistance doesn't affect the circuit being measured to
any real degree.
You COULD run an ammeter that had all the current going to
the dash. However, this is based on the notion that the ammeter
has to be a unit, shunt and meter, together. It doesn't.
The shunt could be placed anywhere, or an existing high current
line like a battery cable could be used as the shunt. Then simply
place a sensitive voltmeter in parallel with the cable, calibrate
it with some simple math, and put the meter in the dashboard. Only
the tiny wires to the meter need to enter the cabin."
You are absolutely right. From a theoretical standpoint, I agree 100%.
However, from a practical standpoint, I have a few concerns:
a) Even though the procedure you describe is fairly simple, it may be beyond
the scope of many of us on this list. At least to the point of feeling
comfortable modifying the wiring.
b) The shunt resistance would have to be MUCH less than one ohm. Even a 0.1
ohm resister, at an alternator output of 20 amp, will drop 2 volts, and
dissipate 40 watts. That would be quite a heater under the hood!
c) It will be extremely difficult to get a voltmeter calibrated and labeled
appropriately for this application, and have it look good in the dash. You
could use a felt tip pen and mark it up yourself, but I don't think most of
us have the skills to do a professional job. Of course, if there is a
commercial kit available for this, ALL my concerns go out the window. Does
anyone know how the ammeters sold by VDO, SW, AutoMeter, etc are connected?
Even though most ammeters operate as you stated, I don't think (but I'm not
100% sure) the ammeters used in Triumphs work that way. If I recall
correctly, and it has been a while since I looked into a Triumph ammeter,
they work on magnetic principle. Current flowing through a coil of wire
(large gauge wire with only a few turns) creates a magnetic field which moves
the needle. Can anyone confirm or disprove this? I do know that all the
current is routed through the ammeter on Triumphs (with the exceptions
previously noted). This would explain the instant response of an ammeter
compared to a voltmeter. Most volt meters for automotive use operate via a
coiled bi-metal strip connected to the needle. Current flow through a heater
element causes the bi-metal strip to expand, moving the needle. It takes a
while for the heater element to change temperature. The standard d'arsenval
movement (hope I spelled that right!) is too fragile for use in a car.
Interesting comments! Thanks for the input.
Dan Masters,
Alcoa, TN
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