Re: Adapting mismatched fuel gauge and sending unit

Charles <nospam@xxxxxxxxxx> wrote:
AZ Nomad wrote:

On Wed, 31 Oct 2007 22:06:51 -0500, Bob Urz <sound@xxxxxxxxxxxx>
wrote:
Is there any possible way to make a little circuit to adapt a fuel
gauge to a sending unit with the wrong ohm range? Specifically, I
want >> to use a particular fuel guage that expects a 30-240 ohm
range, and my >> existing sending unit is 10-73. This is a factor of
3 across the range >> (close enough), so I'm really wondering if I
can somehow make it work. >> I expect that classic car folks must
run into this all the time when >> they want to switch to modern
gauges. >>
If you could get inside the gauge cluster and see if there is a
series resistor in line with the meter movement, you might be able
to change the resistor value to get a wider range.

Other wise, a electronic impedance convertor would have to be built.

Sounds like a resister ladder problem for an EE major.

Looks to me like I could put a resistor in parallel with the gauge and
get it to work possibly. The resistor value would be about one half
that of the gauge internal resistance. I'd need to really do it to be
totally convinced though. The downside is additional current going
through the sender, and I have no idea how much they can tolerate
(before they get hot and cause my entire car to explode).

Okay, resistors in parallel add reciprocals. So if you put a 50 ohm
resistor in parallel with your sending unit:

1/10 + 1/50 = 1/8.3 when the sending unit is at 10 ohms, the meter
sees 8.3 ohms

1/73 + 1/50 = 1/29.6 when the sending unit is at 73 ohms, the meter
sees 29.6 ohms

That is, by putting a resistor in parallel with the sending unit, you
can REDUCE the span of resistances that the meter sees, but you cannot
increase it. (You also REDUCE the current going through the sending
unit since some of it is shunted through the resistor).

Now, resistances in series add. So you could take a 20 ohm resistor,
put it in series. Then, when the sending unit is at 10 ohms, the
meter sees 30 ohms. And when the sending unit is at 73 ohms, the
meter sees 103 ohms. If you do this, the meter will only move half
the total span, but the top of the scale will be in the right place.
This also reduces the total current going through the sending unit also.

In order to INCREASE the span, you need an amplifier circuit with gain,
but to reduce it you can use a shunt resistor. To move the low point
on the meter up and move the high point up the same amount, you can
use a series resistor.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
.

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