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#1 As you know, a gas sending unit
is just a variable resistor acting as a
voltage divider. There are several
possible reasons for a jumpy reading.
One would be if the float is not well
placed in the tank and the gas sloshing
around just moves it up and down. If
that is the case, a filter may help.
Since the gas level should change
very slowly, a simple series connected
capacitor/resistor combination connected from the output of the sending
unit to ground would be a good filter.
A large electrolytic with a high enough
voltage rating to withstand any spikes
in the 12V system would be best.
You need a time constant that is
longer than the period of the gauge's
jumps; perhaps a couple of seconds.
You also need a resistance that is small
enough to allow enough current to
pass through it when needed to
maintain the dial reading. TC (Time
Constant) = R C or C = TC/R. R is the
value of the series resistor. I would
take it to be a few ohms, perhaps 10
as a first try.
It is a bit hard to calculate the
power dissipated in this resistor as it
depends on the frequency and severity of the fluctuations, but I would start
with at least a one watt size and if that
burns up, try five watts. With a one
second time constant, we have: C =
1/10 = 0.1 farads. That is 100,000 µF
which is probably impractical. I would
try a nice large electrolytic and see
what happens; 20,000 µF at 25 volts
would be a good first choice.
If that helps but is not enough, go
larger. The negative (-) terminal on the
capacitor would go to ground. It does
not matter which side of the capacitor
the resistor is on, as long as it is in
series with it. This will not affect the
accuracy of the readings, as long as
the added resistor is not in series with
the line to the gauge.
Another possible cause would be
a bad sending unit, but you say you
have already replaced it so that is not
likely. A third cause would be bad
wiring between the sending unit and
the gauge. Before you try anything
80 August 2013
else, you might run an extra temporary
wire to see if it helps. If it does, repair
Finally, it could be a bad gauge.
After trying the above filter and extra
wire fixes, you could temporarily rig a
100 ohm pot at the dashboard to
activate the meter. Disconnect the
wire from the sending unit and
observe the meter as you drive down
a bumpy street. If it still jumps, it is the
gauge itself and that must be replaced.
#2 On my old Dodge, (and this
applies to Fords and Chevys, too) the
instrument panel has a thermal voltage
regulator. The instruments operate at a
voltage of about 5-7 volts. The gauges
have built-in damping.
The thermal voltage regulator is
nothing more than a bi-medal strip
with a heating coil on it that opens
and closes a contact, thus maintaining
a poor — but semi constant — voltage.
I've replaced these devices with
simple voltage regulators like an
LM317 set to a voltage that approximates six volts. More reading is available at www.chevytrucks.org/tech
Mentioned in the above link is to
check for a good ground at the sender
in the tank.
[#7132 - July 2013]
I’m trying to get my 18 year old
interested in a technical career. I need
opinions as to what skills are the most
marketable and have the best chance
of employment in today’s reality. Are
online or off-campus trade schools
worth the money, or do companies
favor traditional college degrees?
Technical careers can vary far and
wide, and many of them do almost
require a degree of some kind. If
he/she is interested in engineering,
then a degree will be extremely helpful. An easy way to figure out whether
it's necessary is to take a look at local
technical job listings. Unfortunately,
the degree is often very arbitrary. It's
required to get hired, but most of
what you learned to get the degree is
Jersey Shore, PA