Chapter 845 Photoelectric Coupling
The benefits of primary-secondary isolation of control are self-evident. Problems on one side will not affect the other side, especially when the secondary is high voltage, high current, high power, and the working environment is complex, which can greatly improve reliability and maintainability.

To give a simple example, if the secondary is struck by lightning, the impact of the lightning stroke can theoretically only reach the isolating switch, and the upper parts are protected. These days, the controllers above are all expensive items.

The question is, how to isolate?

"Chief Engineer Gao, this is going to be used in a DC circuit, right? How to isolate it? An isolation transformer? That thing can only be used in AC circuits." Mr. Cui's words had nothing to do with making things difficult for Gao Zhendong, but he really didn't understand.

He did not mention the electromagnetic relay which also has isolation function. The purpose of developing this electronic switch is to solve the defects of the relay. It is useless to mention it, so he went back.

In fact, what he said was not entirely correct. There are isolation transformers used in DC circuits, but they are more troublesome. Gao Zhendong did not dwell on the problem of isolation transformers. He solved it in other ways. The isolation transformer is not small and has special functions, but there is nothing else suitable here.

"Instead of an isolation transformer, we use an optocoupler!"

Photocoupler? This time, Mr. Cui and Mr. Pan were completely confused. What the hell is this? I have never heard of it! Chief Engineer Gao has all kinds of new things here, but he is a little unfriendly to those who communicate with him.

It's right that they haven't heard of it. The principle of photocoupler was only proposed in 1963, and now no one in the world knows about it.

If you don’t understand, just ask: “Mr. Gao, what is a photocoupler?”

"Ah, this is a new isolating switch device I designed. It uses the photoelectric principle to couple the primary and secondary, so I named it a photoelectric coupler."

So it was newly designed by Chief Engineer Gao? That's fine.

This situation made the comrades in 1218 a little excited. Is this also something new? Hey, how can this be so embarrassing?
They knew that this meant one thing: another first prize of the Lightning Technology Award was won!

It's a new component, and it's brand new in principle. The department won't agree to it unless we win first prize.

If the situation of power electronic components is still unclear, at least this electronic switch is enough to make them happy.

"Mr. Gao, you talk and we remember." When it comes to new things, you can only learn them at the beginning, and there is not much room for discussion. It is not too late to discuss them after you have learned them well.

Gao Zhendong stood up and picked up the chalk.

"Actually, the principle is very simple. Everyone knows the photoelectric effect, right?"

Without waiting for anyone to answer, he continued on his own, saying what the hell is going on if he doesn't even know the answer.

"This device actually has a simple structure. The primary controls a light-emitting element, which is powered to emit light when it is turned on. The secondary is a photosensitive element, which turns on when it senses light. Of course, this is a normally open element. The normally closed element is the opposite. It turns on when the primary is not emitting light, and turns off when the primary is emitting light."

The normally open and normally closed contacts in the electronics industry are terms that can easily confuse ordinary people. The "open" here means opening the circuit or disconnecting, and the normally open type means "disconnected" in the normal state, not "open" in the normal state, which means exactly the opposite.

Correspondingly, the normally closed type refers to the normal state of "closed", which means the switch is turned on, rather than the normal state of "closed", which is completely twisted for ordinary people.

This has the exact opposite meaning to the normally open and normally closed fire doors in fire protection, and the normally open and normally closed safety doors in security.

Fortunately, the people here understand this. They are all amazed at Gao Zhendong's ingenious idea. Hey, using a photoelectric switch, can't the primary and secondary be isolated?

high! It’s really high!

It makes sense that Chief Engineer Gao’s last name is Gao.

"Hey, that's a good idea, it's safe!"

"Not only is it safe, but if the photosensitive element is chosen properly, the current in the control electrode will not be merged into the output electrode. For some sampling circuits, this further improves the sampling performance." This is Chief Engineer Cui, who is indeed an experienced senior technician who thinks more and further than ordinary people.

"This is a bit like a relay, haha." Well, I went back.

"Then what light-emitting device is better? Incandescent lamps have poor integration. It's not that they can't be used here, but the effect doesn't seem to be very good." asked a comrade.

What he thought made sense. Incandescent lamps and even neon tubes all have a volume problem.

Before Gao Zhendong could answer, President Cui started laughing: "Are you stupid? The best light-emitting devices used here are already produced in our factory, hahahaha."

Everyone suddenly realized, hey, how could they have forgotten about this!
The LED produced by our factory is unique. Perhaps this is the reason why Chief Engineer Gao chose our 1218.

Gao Zhendong nodded: "Yes, use LED, and don't use visible light LED, the efficiency of white light is too poor."

The spectrum of white light is complete and uniform, which means that under the same power, its power is basically distributed to each band, but the photosensitive element is wavelength selective and generally works best for wavelengths within a certain range. In this way, the energy that is not within its wavelength range is wasted.

"What for?"

"Let's use infrared rays. Gallium arsenide is a direct bandgap material that can directly emit infrared rays." Gao Zhendong answered without hesitation. This was unheard of for the comrades in 1218, but for Chief Engineer Gao, it was as easy as eating.

Although they were not clear about what "direct bandgap materials" were, since Chief Engineer Gao said that it could emit infrared rays, then it must be able to do so.

But there is a problem.

"Where does gallium arsenide come from?" There's no need to ask what it is. They know lead sulfide, and they can think of it by analogy with gallium arsenide, but...

But they don’t even know where to get this thing!

Gao Zhendong was stunned. He was so careless.

"Haha, don't worry about that. I'll take care of it."

Gallium arsenide can be pulled directly using the Cz method, which is a bit more troublesome, but it can still be pulled out. The quality of the pulled crystals is not very high, but it is still fine to use in light-emitting components, and the amount used is extremely small, so it should still be supported.

The Cz direct pulling method is the method used by Gao Zhendong and his team to develop the single crystal furnace.

As a senior military fan with a technical bent, Gao Zhendong had collected a lot of materials related to gallium arsenide in his previous life, but he just needed someone to be responsible for its implementation.

Tang Shaohui: Huh? Why do I feel like sneezing?

In Gao Zhendong's previous life, gallium arsenide came into practical use two years later. In fact, it was not as rare as most people imagined.

After hearing Gao Zhendong's words, the comrades from 1218 suddenly remembered where they were sitting.

The Third Rolling Mill and the Third Factory of the Seventeenth Ministry of Machinery! Materials are their main business! Think about where high-purity single crystal silicon comes from?

That's okay.

"Then we will use ordinary LEDs as samples first, and then replace them when gallium arsenide materials come out." He has experience in scheduling the project. It can be done without gallium arsenide, but the efficiency will be lower. However, having and not having are two different things. Poor efficiency can be compensated by technical means. If there is no best, the second best can be used.

The wavelength range of LEDs of various colors is actually limited to a certain range, which is much better than white light.

Don’t be fooled by the fact that many LEDs look red, yellow, or green, as if the light is dyed by the plastic package to appear in those colors. In fact, the colors of these plastic packages are just for the convenience of users to distinguish. Even if a completely transparent plastic package is used, the light will still be red or yellow, and it will be that natural color.

"Well, your arrangement is fine. Let's use visible monochromatic light first. It's at least stronger than white light." Gallium arsenide light-emitting tubes were basically standard equipment for all optocouplers later, but before that, there was still a period of time when visible light was used. It's not that it can't be used. It's always stronger than incandescent lamps and neon tubes.

"Mr. Gao, I have thought of many possibilities regarding photosensitivity, but it's a bit confusing. Can you help me sort it out?" Mr. Cui said with a smile. He has already thought of several ways, but he hasn't figured it out completely.

"Well, you talk and I listen. If there's anything you don't understand, I'll explain it to you."

Although he didn't know what Mr. Cui was thinking, Gao Zhendong was confident that in the current environment, he could explain everything he could think of.

Gao Zhendong was very happy that his comrades could think of solutions on their own and take the initiative.

President Cui also thinks so.

"The first light-sensing method I considered was to use a photoresistor, but I always felt like something was wrong with it."

Anyone who works in optoelectronics will definitely think of this first. The advantage of photoresistors is that they have high linearity, simple structure and low cost.

But photoresistors have a fatal flaw.

"Well, photoresistors can realize this principle, but they have a fatal problem. The switching speed is too slow. Not to mention high-speed circuits, even low-speed circuits are difficult to use."

The switching speed of a photoresistor is extremely slow, the worst being hundreds of milliseconds, which is roughly the same as that of an electromagnetic relay.

After Gao Zhendong's reminder, Mr. Cui remembered that this thing really had this problem.

"That won't work. What we want to develop is a high-speed electronic switch." President Cui knew the purpose of this project very well.

Gao Zhendong did not completely deny it: "Not necessarily. You can try it out. After all, this thing can really play an isolation role, and its size is much better than that of electromagnetic relays. At the same time, its structure is extremely simple and the cost is low. It still has a certain role at this stage. And it has high linearity, so it can also be used in special circumstances."

This type of photoresistor optocoupler is really simple to the extreme. In theory, you can just package a light-emitting tube and a photoresistor face to face. The reason why the outer shell is needed is mainly to prevent external light from interfering with the photoresistor.

(End of this chapter)