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January 22, 2014

Voltage Stabilizer

[Click the images for a bigger view]

The fuel gauge and the coolant temperature gauge on my TR6 are electrical instruments that respond to the current flowing through them.  The current is controlled by a varying resistance in the appropriate sensor--the fuel tank float in the case of the fuel gauge, and a resistive temperature sensor in the coolant stream for the temp gauge.  This arrangement works well, but it depends on a stable voltage for the gauges to work with.  A changing voltage will also vary the current through the gauges, and affect their readings.

Unfortunately for the gauges, the system voltage on this and most other cars can vary pretty widely--from less than 12 volts to nearly 15.  For this reason cars of this vintage have a voltage stabilizer included solely for use by the electrical gauges.  On my TR6, it was mounted on a bracket on the back of the speedometer.

This stabilizer is a pretty standard type used on many cars for decades before and after the TR6.  It works by putting the input voltage across a coil of resistance wire wound around a bimetallic spring.  Current through the coil heats the spring, causing it to flex.  The spring moves a contact that disconnects the input voltage form the coil and also from the output terminal.  When disconnected, the spring cools, eventually to the point where it again connects the input to the coil and to the output.  The result is that the output alternates between the input voltage (which is the system voltage) and zero volts.  The higher the input voltage, the faster the spring heats, so the faster the alternation between on and off.  

The graphs below show what the output waveform looks like for input voltages of 12 and 14 volts.  Each graph shows five seconds worth of time on the horizontal axis, and they show that the switching frequency of my device was between one and two cycles per second.

Even though it may seem a little funny to call something that does this a "stabilizer", it does do the job.  The fuel and temp gauges also work with a resistance coil around a bimetal spring, but they are designed to react very slowly to changes in current.  They are sluggish enough that they can't respond to the individual pulses from the stabilizer, but settle on a reading that represents the average current that they see.  In both of the graphs above, the average voltage coming from the stabilizer is about 10 volts.  When the input voltage is higher, the output has to spend relatively more time at the "off" level  to keep the average at 10 volts.

While I concede that this is a tried and true approach, it is ancient technology, pretty much unchanged since at least the thirties or forties.  Being a sort of a techie by nature, I'm not really able to leave this Neanderthal device alone.  An upgrade to a solid state regulator is cheap, simple, and quick.  There are multiple places to buy modern drop-in replacements, and even at least one web site that explains how to roll your own.  Since this is the kind of thing I do (did, actually) for a living, it seemed like a good place to apply some of that expensive education.

There are a number of three-terminal voltage regulators on the market that can be used for this application.  I chose an LM2940 device because it was the first one I came to in the catalog.  The manufacturer recommends a capacitor on the output to ensure stability.  What this means is that without the capacitor, it's possible under certain load conditions to get some high frequency oscillation of the output.  Oscillation wouldn't affect the gauges, but it could conceivably interfere with the radio, so I included the cap.  The circuit board is a hand-done home brew.

Made a little aluminum case to make mounting easier.

There are only two electronic components and a few terminals.  Unlike the original device, which was grounded through its case to the case of the speedometer, I included an explicit terminal in for ground.  Bad grounds are one of the most common causes of electrical problems in cars, especially older ones.

The regulator is surface mounted on the foil side of the board.  It is facing the board so that its back side can seat against the aluminum case for heat sinking (though at the voltages and currents involved, heat sinking might not be necessary).

Made a bracket to match the original.  I used a nylon screw on one side because of the proximity to the terminals.

The finished product.

Guess I should at least test it!

Installed it in it's new home.  I keep the original for nostalgia.

So is this an upgrade?  An improvement?  I don't know.  Reliability might be a little better, but I wouldn't expect to see any noticable difference in gauge performance.  I don't think I'd pay the $15-$20 for a commercial replacement, but for about $3 in parts and a pleasant afternoon in the shop, it seems like a good deal to me.

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