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Ask about the role of this diode.

Hardware design
November 04, 2020 by Kris 1113

The model of the diode is 1N914. At first I thought it was used for reverse freewheeling, but if you look closely, it is placed on the input side of the optocoupler. What is the function of it here?

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Terri Posted on November 4, 2020

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 under reverse voltage. Because of its low reverse withstand voltage capability, it is easily broken down.

The clamping effect of the anti-parallel diode limits the reverse voltage across the optocoupler input diode to a few tenths of a volt (no more than one junction voltage at most) to prevent the optocoupler input diode from being reversed breakdown, which effectively protects 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.

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Aamir Posted on November 4, 2020

Clamp so that the maximum input amplitude does not exceed VCC+0.7.

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Pauline Posted on November 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.

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