There are two types of photoelectric couplers
: one is nonlinear optocouplers, the other is linear optocouplers. The current transmitting characteristic curve of nonlinear optocoupler is nonlinear, which is suitable for transmitting switching signal but not suitable for transmitting analog quantity. The common optocouplers of 4N series belong to nonlinear optocouplers.
The current transmitting characteristic curve of linear optocouplers is close to a straight line, and its property is better with a small signal and it can be in isolation control through linear property. The common linear optocoupler is PC817A-C series.
The common optocoupler in switching power supply is linear optocouplers. If we use nonlinear optocouplers, the oscillation wave may get broken. In severe cases, it will cause parasitic oscillation and then the oscillation frequency of thousand Hertz is modulated in turns by low-frequency oscillation of ten to hundred Hertz. As a result, it will cause interference in the image of color TVs, color display screens, VCDs and DCDs and so on. At the same time, the load capacity of power supply will decline. For maintenance of switching power supply like color TV and display screen, if the optocoupler is broken, it must be replaced by linear optocouplers. The common four-feet linear optocouplers are PC817A----C, PC111 and TLP521 and so on. The common six-feet linear optocouplers include LP632, TLP532, PC614, PC714 and PS2031, etc.. The common 4N25, 4N26, 4N35 and 4N36 are not suitable for switching power supply, because they are nonlinear optocouplers.
Because the types of photoelectric couplers are over thousand in photoelectron DATA handbook, they can be classified by the following methods:
(1) According to optical paths, it can be divided into reflector path photoelectric couplers ( photoelectric interrupted detector) and inner optical path photoelectric couplers. Reflector path photoelectric couplers also can be divided into transmission-type and reflection-type photoelectric couplers.
(2) According to output forms, it can be divided into:
a. Photosensor output-based, including photosensitive diode output-based, photosensitive audion output-based, photocell output-based and light-activated silicon output-based and so on.
b. NPN audion output-based, including alternating current input-based, direct current input-based and complementary output-based and so on.
c. Darlington audion output-based, including alternating current input-based, direct current input-based.
d. Logic gates output-based, including gate circuit output-based, Schmidt trigger output-based and three-state gate circuit output-based.
e. Low conducting output-based ( output low level at MV order of magnitudes).
f. Light switch output-based ( the on-resistance is lower than 10 Ω).
g. Power output-based (output like IGBT/MOSFET and so on).
(3) According to encapsulation modes, it can be divided into coaxial type, dual-in-line type, TO package type, flat package type, chip package type and optical fiber transmission type.
(4) According to transmission signal, it can be divided into numeric type photoelectric couplers ( OC gate output-based, totem-pole output-based and three-state gate output-based) and linear photoelectric couplers ( including low drifting type, high linearity type, broadband type, single power type and dual power type).
(5) According to speed, it can be divided into low speed photoelectric couplers (photosensitive audion and photocell output-based) and high speed photoelectric couplers ( photosensitive diode with the signal processing circuit and photosensitive integrated circuit).
(6) According to channel, it can be divided into single channel, two channel and multi-channel photoelectric couplers.
(7) According to isolation characteristics, it can be divided into ordinary isolated photoelectric couplers ( generally the optical cement embedment is lower than 5000V, and the dummy molded strip is lower than 2000V) and high-pressure isolated photoelectric couplers ( including 10V, 20V and 30V).
(8) According to working voltage, it can be divided into low supply voltage photoelectric couplers (5-15V) and high supply voltage photoelectric couplers ( higher than 30V).