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VTL5C1 Datasheet(PDF) 11 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 11 Page - PerkinElmer Optoelectronics |
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11 / 76 page  6 Selecting a Photocell Slope Characteristics Plots of the resistance for the photocells listed in this catalog versus light intensity result in a series of curves with characteristically different slopes. This is an important characteristic of photocells because in many applications not only is the absolute value of resistance at a given light level of concern but also the value of the resistance as the light source is varied. One way to specify this relationship is by the use of parameter (gamma) which is defined as a straight line passing through two specific points on the resistance curve. The two points used by PerkinElmer to define γ are 10 lux (0.93 fc) and 100 lux (9.3 fc). Applications for photocells are of one of two categories: digital or analog. For the digital or ON-OFF types of applications such as flame detectors, cells with steep slopes to their resistance versus light intensity curves are appropriate. For analog or measurement types of applications such as exposure controls for cameras, cells with shallow slopes might be better suited. Resistance Tolerance The sensitivity of a photocell is defined as its resistance at a specific level of illumination. Since no two photocells are exactly alike, sensitivity is stated as a typical resistance value plus an allowable tolerance. Both the value of resistance and its tolerance are specified for only one light level. For moderate excursions from this specified light level the tolerance level remain more or less constant. However, when the light level the tolerance level remain more or less constant. However, when the light level is decades larger or smaller than the reference level the tolerance can differ considerably. As the light level decreases, the spread in the tolerance level increases. For increasing light levels the resistance tolerance will tighten. Likewise, for dual element photocells the matching factor, which is defined as the ratio of the resistance of between elements, will increase with decreasing light level. Dark Resistance As the name implies, the dark resistance is the resistance of the cell under zero illumination lighting conditions. In some applications this can be very important since the dark resistance defines what maximum “leakage current” can be expected when a given voltage is applied across the cell. Too high a leakage current could lead to false triggering in some applications. The dark resistance is often defined as the minimum resistance that can be expected 5 seconds after the cell has been removed from a light intensity of 2 fc. Typical values for dark resistance tend to be in the 500k ohm to 20M ohm range. Temperature Coefficient of Resistance. Each type of photoconductive material has its own resistance versus temperature characteristic. Additionally, the temperature coefficients of photoconductors are also dependent on the light level the cells are operating at. From the curves of the various types of materials it is apparent that the temperature coefficient is an inverse funstin of light level. Thus, in order to minimize temperature problems it is desirable to have the cell operating at the highest light level possible. Speed of Response Speed of response is a measure of the speed at which a photocell responds to a change from light-to-dark or from dark-to-light. The rise time is defined as the time necessary for the light conductance of the photocell to reach 1-1/e (or about 63%) of its final value. γ Log Ra Log Rb – Log a Lob b – ------------------------------------- = Log Ra Rb ⁄ () Log b a ⁄ () ------------------------------ = Dual Element Photocell Typical Matching Ratios 0.01 fc 0.1 fc 1.0 fc 10 fc 100 fc 0.63 – 1.39 0.74 – 1.27 0.75 – 1.25 0.76 – 1.20 0.77 – 1.23 |
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Similar Description - VTL5C1 |
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