Boiler Fault Finder

Potentiometers Intro

Potentiometers (potential dividers) are a variable resistor which, depending on the function it is used for, can be adjusted either by the end user (electronic thermostat knob on front fascia numbered 1-6 providing the set point, which could be between 54 degrees C and 82 degrees C), or by the engineer for onsite technical adjustments (within the inner face of the printed circuit board to adjust fan speeds, gas pressures / range rating, maximum operational temperatures, service mode and time delay periods). Potentiometers work by introducing a voltage across a fixed top and bottom connection on a metallic plate or coil of wire. A third connection, called the wiper connection, is positioned between the top and bottom connections and attached to a rotating shaft. The wiper rotates along the length of the plate or coil. If the wiper is positioned close to the top connection, almost all of the supply voltage is available at this connection. As the rotating shaft is turned, the voltage supplied to the wiper is moved from the top connection. A small measured direct current (DC) is passed from the printed circuit board into the potentiometers top connection. This current travels along the metallic plate or wire coil and returns to the printed circuit board via the bottom connection. The wiper connection voltage is governed by its position in relation to the top and bottom connection. The values of the top and wiper connections are then compared by the printed circuit board. The difference between these two voltages is then interpreted by the printed circuit board and used to determine the required temperature. When a potentiometer is used to control the primary or secondary water temperatures, it will dictate the parameters in which the boiler will work. As the temperature being measured approaches the potentiometers set point, the gas rate / gas pressure will adjust to maintain the correct temperature. The potentiometer will normally work in conjunction with a thermistor, p.c.b and gas valve modureg or fan speed control. When potentiometers fail, the information it sends to the printed circuit board may indicate no demand for heat or gives a demand for the boiler to be raised to maximum temperature. Rotating the potentiometers control dial will not change the information to the printed circuit board, symptoms such as no heating, boiler shutting down at low temperatures or overheating are signs of failure….

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