When the control system is designed by using the decoupled digital signal, the transmission characteristics of the photoelectric coupler, namely the transmission rate, often becoming the determinants of the maximum data transmission rate of the system. In a lot of bus-oriented structure of industrial measurement and control system. In order to prevent the mutual interference among the various modules, not reduce the baud rate of communication at the same time. We have to adopt high-speed optocoupler to achieve mutual isolation between modules. The commonly used high-speed optical coupling is 6N135/6N136, 6n136/6n138. However, high - speed optocoupler price is high, resulting in higher design cost.
Two methods are introduced to improve the switching speed of common optical coupler
. Due to the distributed capacitance of the optical coupling itself, the transmission rate is affected, and the distribution capacitance Cbe and Cce are present in the photosensitive triode. When the current transmission rate is low, the load resistance of the collector is not too small, or the output voltage is limited. However, the load resistance should not be too large, and the larger the load resistor RL, the worse the frequency characteristics of the photoelectric coupler, due to the existence of the distributed capacitor, the longer the transmission delay.
With two photocouplers T1 and T2 complementary push-pull circuits, the switching speed of the optical coupling can be improved. When the pulse rises to "1", turn-off T1, turn-on T2.
In contrast, when the pulse is "0", turn on T1, turn off T2. The frequency characteristics of this complementary push-pull circuit are greatly superior to the frequency characteristics of a single photoelectric coupler.
In addition, a positive feedback circuit is added to the photosensitive base of photosensitive triode, which can greatly improve the switching speed of photoelectric coupler.
Add a transistor, four resistors and a capacitor. The experiment proves that the circuit can increase the maximum data transmission rate of the optoelectronic coupler by about 10 times.