Automotive Voltage Stabilizers

Mechanical Stabilizer - Operation

by Roger Garnett

The "Voltage" Stabilizer as found in the fuel, temperature and other guage circuits of many older automobiles is really a type of current regulator. They are a mechanical device, using contacts on "bi-metalic" arms, similar to a turn signal flasher. This regulation serves 2 purposes-

  • reduces guage flucuation due to battery & charging voltage changes.
  • averages the response of guages, to avoid transient readings, such as gas tank sloshing, etc.

    The bi-metalic arms are made from strips of 2 carefully selected, dissimilar metals, laminated together. When heated, the 2 metals expand at different rates, which makes the arm bend in the direction of the metal which expands the least.

    When cool, the contacts are closed, and electricity flows. This current flow through the arms generates enough heat warm the bi-metal arms, and they bend away from each other, thus breaking the circuit (and stopping the generation of heat). Once the metal cools, the contacts close, and the cycle begins again. By changing the material and size of the metals, a duty cycle (on & off time) can be established, based on expected current draw in the circuit.

    Failure modes-

  • Contacts stick closed- guages will read very high
  • Arms no longer close- guages will have no readings
  • Dirty contacts, or tired arms- all regulated guages readings may tend to drift, or read high or low.

    As these stablizers were common in many cars through the 70's, even if an exact replacement for your car is not available, you may be able to find a good substitute from your local parts store or salvage yard. The modern alternative is to replace it with an electronic version.

    Electronic Voltage Stabilizer

    By Pat Vilbrandt September 1993

    From the TR3-TR4A Bently Manual:

    What I saw, looking at the output of the stabilizer with an oscilloscope, with the battery fully charged (12.6V at 50 degrees F) and the engine off, was a square wave swinging from 0 volts to -12 volts (positive "earth" you know), with a frequency varying widely from about 3 Hz to less than 1 Hz. The duty cycle was a bit more consistent at just under 50%. This would correspond to, not surprisingly, an average voltage value of right around 6 volts.

    To double check this, I built a low frequency "averaging" filter out of a 10,000 uF capacitor and a 1 kohm resistor (which gives a time constant similar to that of the gauges), and measured the filtered output, which was 5.9 volts, close to the value estimated above.

    The fuel and temperature gauges in my TR4 are accurate and stable, so I assume that the 10 volts mentioned in the Bently is incorrect, and that the value of 6 volts I measured is the correct value, at least for my car.

    So, for those of you that have considered making a "stabilizer" out of more modern components, it appears that an LM7906 3-terminal regulator, with a 10 uF or so capacitor at the output, and a 1N4001 diode in series with the input from the battery to block opposite-polarity transients, would make a suitable substitute. The current requirements of the gauges are rather modest (around 100 mA with a cold engine and a 1/4 tank of gas), so it doesn't even look like a heat sink would be necessary for the LM7906.

    Keep in mind, tho, that the mechanical voltage stabilizer/gauge combination may have inherent temperature compensation built in as a _system_. I seem to recall someone mentioning that after they had built a solid-state equivalent, their gauges showed some variability as the ambient temperature changed.

    Pat Vilbrandt Fluke Corporation Everett, Washington USA
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