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參數(shù)資料
| 型號： | VT900 |
| 廠商： | PerkinElmer Inc. |
| 英文描述： | Photoconductive Cells and Analog Optoisolators (Vactrols) |
| 中文描述： | 光導(dǎo)細胞和模擬光隔離器（Vactrols） |
| 文件頁數(shù)： | 34/76頁 |
| 文件大小： | 1240K |
| 代理商： | VT900 |
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29
What Are Analog Optical Isolators
Light History Considerations
Photoconductive cells exhibit a phenomenon knows as hysteresis, light
memory, or light history effect. Special consideration must be given to
this characteristic in the analog optoisolator because the
photoconductive element is normally in the dark. This will lead to
having the photocell initially in a “dark adapted” state in many
conditions.
The light levels that are seen by the photocell in many analog
optoisolator applications are quite low ranging from0.1 to 1.0 fc. The
effect of this combination of dark adapt and low light levels will be seen
in the following table.
The table shows the relationship between light history and light
resistance at various light levels for different material types. The values
shown were determned by dividing the resistance of a given cell,
following “infinite” light history (R
LH
), by the resistance of the same cell
following infinite dark history (R
DH
). For practical purposes, 24 hours in
the dark will achieve R
DH
or 24 at approximately 30 fc will achieve R
LH
.
Variation of Resistance with Light History Expressed as a
Ratio R
LH
/R
DH
at Various Test Illumnation Levels
The table illustrates the fact that the resistance of a photocell can
increase substantially as it transitions fromdark adapted state to a light
adapted state. The table shows that the Type 1 photocell can increase
resistance by a factor of more than three times as it light adapts up to
0.1 fc. In some applications, this can be an important consideration. In
general, the magnitude of this effect is larger for types 1, 4, and 7 than
for types , 2, and 3.
Each specific material type represents a tradeoff between several
characteristics. Selecting the best material is a process of determning
what characteristics are most important in the application. The chart
gives some appreciation for the general interrelationships between the
material types and their properties.
Material
Type
Illumination (fc)
0.01
0.1
1.0
10
100
Type
1.60
1.40
1.20
1.10
1.10
Type 1
5.50
3.10
1.50
1.10
1.05
Type 2
1.50
1.30
1.20
1.10
1.10
Type 3
1.50
1.30
1.20
1.10
1.10
Type 4
4.50
3.00
1.70
1.10
1.10
Type 7
1.87
1.50
1.25
1.15
1.08
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