Anti-jamming weapon optocoupler

Anti-jamming weapon optocoupler

What is an optical isolator?

The optical isolator is an electronic component used for optical transmission of electrical signals between two isolated circuits. Also called optocoupler, optocoupler.

A common type of optical isolator consists of a light-emitting diode LED and a phototransistor in the same opaque package. Usually optoisolators transmit digital (on-off) signals, but some optocouplers can transmit analog signals.

The light couples the source-receiver end, which is the meaning of coupling.

In addition to coupling, there is another meaning of decoupling. Some noise signals cannot be transmitted to the receiving side through optical signals, which is decoupling.

Therefore, the general understanding of coupling often has two sides. On the one hand, the useful signal that the design cares about is transmitted to the receiving side; on the other hand, the unwanted noise signal is blocked at the source end.

Some optocouplers can transmit analog signals

How does optocoupler work?

The optocoupler consists of a light source, generally a near-infrared light-emitting diode (LED), a closed light channel and a photoelectric sensor.

The near-infrared light-emitting diode converts the electrical signal at the input terminal into a light signal. After passing through a closed optical channel, the photoelectric sensor detects the light signal from the LED and converts it into electrical energy or modulates the current flowing from the external power supply.

The photosensor can be a photoresistor, photodiode, phototransistor, silicon controlled rectifier (SCR) or triac. Since LEDs can sense light in addition to emitting light, a symmetrical bidirectional optical isolator can be constructed. The light-coupled solid-state relay contains a photodiode opto-isolator, which is used to drive the power switch, usually a pair of complementary MOSFETs.

Normal digital signals can be smoothly coupled and transmitted to the other side through the optocoupler, while various interference noise signals on the input side cannot be transmitted to the output side through the optocoupler.

Common isolation technologies include: photoelectric isolation, relay isolation, transformer isolation, etc. Optical media and magnetic media isolation are the main two technical means.

Why is the interference signal unable to pass through the optocoupler with a high probability?

The internal resistance of the interference source is generally very large. Although it can also output a relatively high interference voltage, the energy that can be output is generally small, and only a small interference current can be formed in the loop. Because these interference currents are very small, they cannot To excite the light-emitting diode to emit light, the light-emitting diode needs a certain large current to emit light energy. Because it does not have enough energy, it cannot successfully pass the barrier and is blocked on the input side.

The input impedance of the optocoupler itself is very low, and the internal resistance of the interference source is generally very large. From this perspective, the output signal of the interference source is based on the principle of voltage division, and the interference voltage applied to the LED is also very small.

Light is transmitted in a closed optical path, and the optical path will not be interfered by external light.

Why do we need to isolate?

The signal lines or power lines of electronic products may be affected by voltage surges caused by lightning, electrostatic discharge, radio frequency transmission, switching pulses, and power disturbances.

Lightning strikes can cause surge voltages of up to 10kV, which is a thousand times higher than the voltage limit of many electronic components. The product circuit itself may also contain high-voltage circuits. In this case, a safe and reliable way to connect the high-voltage circuits and digital low-voltage devices is required. Then isolation technology is a good technical means.

The main function of the optical isolator is to prevent such high voltages and voltage transients, so that surges in one part of the system will not interfere with or damage other parts.

What are the common optocoupler structures?

Common optocouplers have four forms, the difference lies in the photosensitive device used. Photoelectric transistors and photoelectric Darlington tubes are usually used in DC circuits, and photoelectric thyristors and photoelectric triacs are used to control AC circuits. In the phototransistor optocoupler, the transistor can be PNP or NPN. Darlington transistors are a pair of two transistors, one of which controls the base of the other transistor. Darlington transistors provide high gain capability.

Some points that I feel need to pay attention to when using optocouplers:

1 Selection of current-limiting resistance

2 The input and output side needs to be an isolated power supply

If the current-limiting resistor is selected too large, the forward current may be too small, which may cause the signal to be unable to be correctly transmitted; if the resistance is selected too small, the forward current may be too large, and the optocoupler may be easily damaged.

Power supply: The input side and the output side need to be powered separately. The two power supplies need no direct loop to achieve true optical isolation. Although the power supply voltages on both sides are different, if they share the ground, if you want to use an optocoupler for isolation, then The optocoupler is used for nothing, and the isolation effect is completely useless. Of course, logic level conversion can be done in this way, but it is a bit uneconomical to use optocouplers to only do logic level conversion, and a special logic level conversion chip can be used.

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