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A vehicles supply voltage varies considerably depending the state of charge of the battery, the engine speed and the loads, such as any lamps and motors, that are operating. The Fuel, Temperature and Oil gauges all work by measuring a voltage put out by a sender that responds to fuel level, water temperature or oil pressure respectively. Hence, any variation in the supply voltage will cause changes in the readings of the gauges. In order to overcome this problem the voltage needs to be established.

A standard voltage stabiliser used in many vehicles is intended to regulate the voltage to the gauges at about 10 Volts. It does this by monitoring the input voltage and should it find it to be higher than 10 Volts, as it normally would be, then it temporarily removes the supply to the gauges. The gauges are so highly damped, that is slow in responding, that the voltage supply can be interrupted for a few seconds without noticeable effect.

For example, if a car is running with a good battery at reasonable speed, a voltage of 13.0 Volts might well be seen at the stabiliser. The stabiliser should therefore switch off for a few seconds and then switch on again so that the on to off ratio is about 77%:23% and the average voltage the gauges see is 77% of 13.0 Volts = 10 Volts. Similarly, the stabiliser should respond to a 11.5 Volt input, say when the vehicle is stationary with the ignition on, by being on for about 87% of the time and off for the remainder.


Two terminal voltage stabiliser with mounting/earth tab

Three terminal voltage stabiliser.


The pictures above show 2 typical Smiths style voltage stabilisers these are required when using Smiths style gauges and possibly other after market gauges that need a a 10 volt supply.

Voltage stabilisers are often attached to the back of instrument panels in production cars. If you are the type that owns a Maplin catalogue this voltage drop can also be obtained with a small resistance in the circuit, you will have to experiment to find the correct resistance.

The terminals of these devices are usually labelled as follows:
B - Battery/unregulated supply (Vehicles fused ignition switch supply).
I - Instrumentation (stabilized 10v supply).
E - Earth/Chassis Ground.

Typical voltage stabiliser circuit


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