Author: John Mulindi
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The Instrumentation for Furnace Control
The figure below shows a typical basic furnace used to raise the temperature of a heat-transfer medium that is used on other equipment in a plant process. From the figure above, the temperature of the heat-transfer medium is the parameter that sets the demand on the heating system. The amount of heat required is manually…
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Null vs. Deflection Type Instruments
Null Instrument The null technique is one of the operational modes for a measuring instrument. A null instrument employs the null method for measurement. In this technique, the instrument exerts an influence on the measured system so as to oppose the effect of the measurand. The influence and the measurand are balanced until they are…
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Analog vs. Digital Sensors
Analog Sensors Analog sensors provide a signal that is continuous in both its magnitude and its temporal (time) or spatial (space) content. Most physical variables such as, current, pressure, temperature, displacement, acceleration, speed, light intensity and strain tend to be continuous in nature and are readily measured by an analog sensor and represented by an…
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Basic Features of a DC Potentiometer
A potentiometer is an instrument that is used for measurement of potential difference across a known resistance or between two terminals of a circuit or network of known characteristics. A potentiometer is also used for comparing the emf of two cells. A potentiometer is widely used in measurements where the precision required is higher than…
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Variable Capacitance Sensors
Several mechanical input sensors, as well as some sensors which measure humidity or temperature, operate by the transduced quantity causing a change in capacitance, which in turn, is converted to an analog output voltage by an ac bridge circuit or other electronic system. The capacitive sensors have been designed to measure force by means of…
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DC Motor Power Op-amp Speed Controller
Let’s consider the functional diagram below: The D/A converter outputs a voltage directly related to the potentiometer position. However, the D/A converter’s output current is limited and not enough to drive a motor. A power operational amplifier (op-amp) circuit, configured as a noninverting amplifier can drive the motor at the higher currents required. In effect,…
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Power MOSFET Motor Control
Because of the very high input gate resistance of MOSFETs, MOSFET’s fast switching speeds, and the ease at which they can be driven makes them ideal to interface with op-amps or standard logic gates. Nevertheless, care must be taken to ensure that the gate-source input voltage is correctly chosen because when using the MOSFET as…
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Metal Oxide Semiconductor FET (MOSFET)
The Metal oxide semiconductor FET (MOSFET) also known as the insulated-gate FET (IGFET) is similar to JFET but exhibits even larger resistive input impedance due to the thin layer of silicon dioxide that is used to insulate the gate from the semiconductor channel. This insulating layer forms a capacitive coupling between the gate and the…
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The Junction Field Effect Transistors (JFET)
The bipolar transistor has several disadvantages: it has low input impedance because of forward biased emitter junction, it has small high-frequency gain, it has considerable noise level and it is non-linear when |VCE| < 2V. Although low input impedance problem may be improved by careful design and use or more than one transistor, even so…
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Bipolar Junction Transistors (BJT)
By placing two PN junctions together we can create a bipolar junction transistor. In a PNP transistor the majority charger carriers are holes and typically germanium is favored for these devices. In NPN transistors the majority charger carriers are electrons; for this case, silicon is typically used. The thin and lightly doped central region is…