I copied the op amp circuit, but I don't understand the function of one component.

王恒昌

I copied the op amp circuit, but I don't understand the function of one component. [Copy link]

What is the function of these two diodes? Can they be omitted?

maychang

I can't see the current path on the photodiode side of the optocoupler. In any case, the optocoupler cannot operate with a load resistance greater than 1 megohm.

R24, R25 and R30, R31 are two identical resistors connected in series, and I don't see any use for them.

I suspect the circuit is copied incorrectly.

王恒昌

maychang posted on 2021-1-23 10:45 I can't see the current path on the optocoupler phototube side. In any case, the optocoupler cannot work with a load resistance exceeding 1 megohm. R24, R25 and R30, ...

The picture is not standard. This is a Japanese circuit. The optocoupler load resistance is 4.7k. The 0.95v is equivalent to the ground. I don't know why the Japanese raised the circuit ground to 0.95v.

maychang

Wang Hengchang posted on 2021-1-23 10:50 The picture is not standard. This is a Japanese circuit. The optocoupler load resistance is 4.7k, and the 0.95v is equivalent to the ground. I don’t know why the Japanese put the circuit...

The optocoupler shares the same power supply on both sides, and I have no idea what this is for. It's the same as R24, R25 and R30, R31 are two identical resistors in series, which is inexplicable.

王恒昌

maychang posted on 2021-1-23 11:53 The two sides of the optocoupler share the same power supply. I don’t know what this is for. Just like R24, R25 and R30, R31 are two identical resistors in series, Mo...

This optocoupler is not used for isolation, but is a pair of infrared transmitters and receivers, equivalent to a photoelectric switch. I don't know why two identical resistors are connected in series. When I designed it, I used only one resistor. The diode can work without the circuit. I just don't know what the hidden dangers are.

maychang

Wang Hengchang published on 2021-1-23 12:12 This optocoupler is not used for isolation, but for infrared emission and reception, which is equivalent to a photoelectric switch. I don't know why two identical resistors are connected in series...

"This optocoupler is not used for isolation, it is a pair of infrared transmitters and receivers, equivalent to a photoelectric switch."

The infrared transmitter and receiver tubes share a power supply, so there must be at least two wires between the transmitter and the receiver: the ground wire and the power wire. Adding one more wire can transmit the signal. If you use differential signals to avoid interference, you need to add two wires. Why do you need "infrared transmitter and receiver tubes"?

王恒昌

maychang published on 2021-1-23 12:35 "This optocoupler is not used for isolation, it is a pair of infrared transmitters and receivers, equivalent to a photoelectric switch." The infrared transmitter and receiver share a...

Thanks for your reply. I am a newbie and I don't quite understand what you said. This infrared pair tube is also used in obstacle avoidance cars. The light of the infrared transmitting tube will be emitted to the receiving tube only when there is an obstruction. My application here is the key. If the finger blocks the infrared pair tube, it will be considered that there is a key, otherwise there is no key. I copied the circuit, please help me with it

王恒昌

maychang published on 2021-1-23 12:35 "This optocoupler is not used for isolation, it is a pair of infrared transmitters and receivers, equivalent to a photoelectric switch." The infrared transmitter and receiver share a...

You can see the actual picture I posted above. The black thing is the infrared tube.

chunyang

These two diodes provide a low impedance path for discharging C22 and C24 respectively and cannot be omitted, otherwise the output waveform will change slightly. You can see the difference by comparing it with an oscilloscope.

王恒昌

chunyang published on 2021-1-23 17:44 These two diodes provide a low impedance path for C22 and C24 to discharge respectively. They cannot be omitted, otherwise the output waveform will change slightly. Use an oscilloscope to check...

Thanks for the tip, I'll try it out later.