The model of the diode is 1N914. At first I thought it was used for reverse freewheeling, but when you look carefully, it is placed on the input side of the optocoupler. What is the function of it here?
Lucia Posted on August 4, 2020
The four diodes are connected in reverse and parallel to both ends of the optocoupler input diode. According to the screenshot of the host, the general four-channel input signal is about 0 ~ ExVCC. (Assuming that it is working in the switching state, when the level is 0, the optocoupler is on; when the level is ExVCC, the optocoupler is off.) In this range, the diode has no effect.
However, when the signal amplitude of the four input signal lines exceeds ExVCC due to strong interference, wrong wiring, or accidentally touching the higher voltage line, or when the amplitude of the input signal itself is much higher than ExVCC, the optocoupler The input diode will be at reverse voltage. Because of its low reverse withstand voltage capability, it is easily broken down.
The clamping effect of the anti-parallel diode restricts the reverse voltage across the optocoupler input diode to a level of a few tenths of a volt (no more than one junction voltage at most), avoiding the optocoupler input diode from being reversed breakdown, effectively protecting the light Coupling.
For the normal signal whose amplitude of the input signal itself may be higher than ExVCC, of course it can work normally.
Danny Posted on August 4, 2020
Clamp so that the maximum input amplitude does not exceed VCC+0.7
Dante Posted on August 4, 2020
The 1N914 here is a voltage clamp tube, which has the same function as the one connected to the relay. The 1N914 in the picture may be set to protect U10 from breakdown by high voltage.
