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Controlling Light

Nov. 15, 2002
Operational supplies are frequently used to regulate the intensity of heavy-duty light sources such as xenon or arc lamps.

Operational supplies are frequently used to regulate the intensity of heavy-duty light sources such as xenon or arc lamps. The uniform illumination of a monochronometer is a typical objective. Light sensors typically provide the regulating feedback. These sensors include photomultiplier tubes and photovoltaic or photoresistive semiconductors.

Photovoltaic silicon photocells are the most convenient sensors. Their output, however, usually requires some voltage amplification before use as a command signal. A small dc amplifier is generally used to increase the sensor signal level. This amplification also contributes to loop gain and, thus, improves light intensity regulation. Photoresistive cells can also sense light intensity. Such cells generate a variable terminal resistance as a function of incident illumination. The process is one of resistance-to-voltage translation, for which programmable power supplies are well suited. This translation procedure depends on a constant bridge current producing a linear voltage with changes in sensor resistance.

For example, the power supply is said to be controlled at an Ω/V ratio equal to the reciprocal of the bridge current. At a bridge current of 1 mA, the control ratio is 1,000Ω /V. Thus, for every 1,000Ω change in photoresistance, the supply output changes 1 V.

A similar technique provides temperature control. Here, the output of a programmable supply can generate heat directly by powering resistive heaters. Use of temperature-sensing feedback elements, then, produces precise proportional heating.

One or several thermistors form the temperature feedback sensor. These sensors program the output of a small dc supply which, in turn, controls the main operational power source. Feedback voltage to this source has the sensitivity of the thermistor in Ω/°C divided by the Ω/V ratio of the translating power supply.

Typical temperature controllers handled with operational supplies are in the 1,000-W category, sufficient to heat a 27-ft3 temperature chamber. A typical application might use one supply to heat radiators, and two other smaller supplies in opposition to provide a differential programming input.

One of these small supplies monitors temperature from thermistors placed around the temperature chamber. The other small supply provides a reference voltage serving as the adjustable temperature-control command. With a forward gain of between 150 and 200 in the main supply, such an arrangement can control temperature to better than 0.1°C over long periods.

A similar technique often controls electrochemical processes. Many electrolytic processes proceed at rates governed by the voltage appearing at an immersed reference electrode. This voltage level frequently determines which of two electrolytic reactions take place.

Such a process is known as controlled-potential electrolysis. The devices used in the procedure are called potentiostats. An operational supply and simple feedback circuit can comprise a potentiostat. Here, the reference electrode is in the feedback loop, and the supply powers a working electrode. The voltage appearing between the reference and working electrodes is compared to the output of a small command supply. The difference signal then controls the output of the main supply.

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