What Are Analog Optical Isolators?
The table illustrates the fact that the resistance of a photocell can
Light History Considerations
increase substantially as it transitions from dark 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 Ø, 2, and 3.
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.
Each specific material type represents a tradeoff between several
characteristics. Selecting the best material is a process of determining
what characteristics are most important in the application. The chart
gives some appreciation for the general interrelationships between the
material types and their properties.
The light levels that are seen by the photocell in many analog
optoisolator applications are quite low, ranging from 0.1 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 determined by dividing the resistance of a given cell,
following “infinite” light history (R ), by the resistance of the same cell
LH
following infinite dark history (R ). For practical purposes, 24 hours in
DH
the dark will achieve R or 24 at approximately 30 fc will achieve R .
DH
LH
Variation of Resistance with Light History Expressed as a
Ratio R /R at Various Test Illumination Levels
LH DH
Illumination (fc)
Material
Type
0.01
0.1
1.0
10
100
Type Ø
Type 1
Type 2
Type 3
Type 4
Type 7
1.60
5.50
1.50
1.50
4.50
1.87
1.40
3.10
1.30
1.30
3.00
1.50
1.20
1.50
1.20
1.20
1.70
1.25
1.10
1.10
1.10
1.10
1.10
1.15
1.10
1.05
1.10
1.10
1.10
1.08
29
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
