2.10 Relays, solid-state relays

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2.10 Relays, solid-state relays [Copy link]

2.10.1 Relays

Relays use the magnetic force generated by the relay coil after power is applied to attract two electrodes together; the principle is known in junior high or high school physics. If you are interested, you can open one and have a look, it is very interesting.


Relays have:

1) Common terminal: COM

2) Normal open: normal open, NO

3) Normal close: normal close, NC


Main indicators of relays:

Working voltage: There are two types of working voltages, DC and AC;

on PCB, it is usually DC; AC relays are usually AC24V, of course, there are also AC220V. Domestic AC relays are much cheaper than imported ones, such as idec, OMRON, and Fujitsu, of course, the quality is also worse.

You can search for relays on Baidu, which has some introductions to some manufacturers' products.

The commonly used low-power relays are mostly national, and OMRON also has them, which are cheaper, 50 per box.


Commonly used voltages are:

DC5V/DC9V/DC12V/DC24V, etc.;

according to the number of contacts, it can be divided into:

single-pole single-throw/SPST

single-pole double-throw/SPDT

double-pole double-throw/DPDT

four-pole double-throw, etc.


It can also be divided according to the contact capacity:

generally DC30V/1A or DC30V/2A, AC110V/0.2A, and if it is larger, the relay cannot be used, because when the contact switches and the current passes through, arcing will occur, burning the contacts, making the switching unreliable; in this case, solid-state relays can be used.

In fact, the task of the relay on the PCB is to give a dry contact signal, which is used as a control signal, and transmits a signal, not energy. Or control a high-power switch, etc., so that the high-power switch controls the power transmission; however, the latter is not as convenient as a solid-state relay.

The package of the relay is usually DIP16 or above; there are also smaller relays, but the price is similar to that of DIP16.

Note: DIP16 relays have only 8 pins; so when designing the package, do not use DIP16 directly for the sake of convenience, as this will easily cause welding to be reversed during soldering, resulting in errors. The package of the relay should be specially designed, and the number of pins should be used.


2.10.2 Solid-state relays

Solid-state relays, to put it simply, are zero-crossing triggered thyristors triggered by light-emitting tubes.

It can be said that they are very similar to optocouplers, and also have 4 pins, but the difference is:

4) Directly use DC voltage control; it can be considered that there is already a current limiting resistor inside;

5) The end that receives the optical signal is a zero-crossing triggered thyristor;


it can be said to be an electronic switch, a contactless relay.

Because there are no contacts, large currents can pass without arcing, and the working life is independent of the number of switches.

The working life of a relay whose contacts are used as signals is 1 million to 3 million times, which can be checked in the technical manual;

solid-state relays can be considered unlimited, so solid-state relays are used in situations with large currents and frequent switching.

The signal passed through the contacts of solid-state relays is usually AC voltage, but there are also DC voltages.

Solid-state relays can pass AC power below AC220V or AC380V; its two indicators are:

1) working voltage, usually DC5V and DC12V, and there are also DC24V;

2) working current: AC220V/1A/2A/4A.


The quality of domestic solid-state relays is good, no different from imported ones, but the quality of the package is too poor. When you buy a batch, each one is different, and the soldering on the circuit board is crooked, which is not beautiful; as for low-power domestic relays, they are relatively rare in the market, and I dare not use them.

Imported solid-state relays are from the United States and Japan. Those above 4A are more expensive. The Japanese OMRON solid-state relay of 2A is only 8 yuan, and it is estimated that it can be cheaper if you bargain. It is small in size, and the package is absolutely accurate. It can be arranged neatly on the circuit board and does not take up space.


When will domestic products be as good as Japanese ones? When will

all Chinese people be as dedicated as the Japanese?

This forum is not a place to scold the Japanese, so I won’t say it anymore; I will share my meager experience with everyone. I think the Chinese can still drive foreign products out of the Chinese market in the mid-end and low-end markets of electronic products.


2.11 Linear power supply, transformer, rectifier bridge and three-terminal voltage regulator


PCB needs power supply, so it needs power supply. There are two kinds of power supplies, one is linear power supply, such as 7805, which is a linear power supply;

the other is switching power supply, which directly rectifies and filters AC220V into high voltage, drives the power tube to work above 100Khz, and then rectifies and filters into low voltage.

The working principle of switching power supply can be found on Baidu.

Linear power supply requires industrial frequency transformer to convert AC220V into low voltage, which becomes pulsating DC after full bridge rectification and large capacitor filtering, and then passes through a three-terminal voltage regulator to output DC voltage.

The output end also needs large electrolytic capacitor filtering. Note that you should choose electrolytic capacitors with large enough capacitance and withstand voltage. If the withstand voltage is not enough, you will hear the sound. If the capacitance is not enough, the voltage difference between the rectifier output voltage and the output voltage is not enough, the output voltage will have ripples, and the single-chip computer system will not work properly.

Usually, DC5V is needed on the circuit board. Generally speaking, a 7805 circuit board plus a heat sink is basically enough. The greater the power, the larger the heat sink.


Transformers have various specifications:

1. Ordinary E-type silicon steel sheet wound transformers. The largest number of transformers are now produced by Xiongying, Puning, Guangxi. They have passed the Great Wall certification and have good quality. The biggest advantage is that they are cheap and have a large spot volume; the usual specifications are single 9V, 12V, double 9V, 12V, etc.; maybe this place produces silicon steel sheets and has such minerals.

2. Ring transformers, like an ellipse;

3. Recently, it seems that there are more circular transformers. The latter two types of transformers should have small leakage, less heat, and high efficiency. They are used for high-power rectification. Of course, the unit wattage is also expensive. According to the 1 yuan/W calculation budget, it should be about the same.

Both types of transformers need to be customized, and various specifications can be customized.

4. The transformer soldered on the circuit board is more famous for the Bingzi brand. It has been advertised in "Electronic Technology Application" and "Electronic Products World" for many years.


How to calculate the power required for the transformer?

First, calculate the power consumption. For example: Dc5V/0.25A, which is 1.25W power consumption.

The efficiency of 7805 is 50%, and the efficiency of the transformer is 50%, which means that a 6W transformer is needed.

The input voltage difference of DC5V/0.25A is about 3V, and the voltage rectified by the 6V transformer is about 6×1.4＝8.4V, which is about right. Therefore, use an AC6V/6W transformer. Then find a similar transformer on the market.

Note: The larger the current, the larger the voltage difference; when the circuit is working, you can use an oscilloscope to see if the waveform of the power supply is stable.


The 78 series chips are more common and the best are the LM780X of National Semiconductor and the MC780X of Motorola.

The 78 series is a high-voltage difference voltage regulator chip. Now there are low-voltage difference voltage regulator chips, including LM2930, LM2937, LM2940C, LM2990 and other 4 series.

If you need a larger power supply, either use a switching power supply or buy a finished linear power supply, such as the power supply of Liaoning Chaoyang 4NIC, which is of good quality and good price.



Appendix: An article found on the Internet:


The difference between switching power supply and linear power supply .


The adjustment tube of the linear power supply works in the amplification state, so it generates a lot of heat and has low efficiency (about 35%). It needs to add a large heat sink, and also needs a large power frequency transformer. When multiple
voltage the transformer will be even larger. The adjustment tube of the switching power supply works in the saturation and cut-off state, so it generates less heat, has high efficiency (more than 75%), and saves a large transformer. However, the DC output of the switching power supply
will be superimposed with a large ripple (50mV at 5V output
typical), which can be improved by connecting a voltage regulator diode in parallel at the output end. In addition, since the operation of the switching tube will generate a large spike pulse interference, it is also necessary to connect a magnetic bead in series in the circuit to improve it. Relatively speaking, the linear power supply
does not have the above defects, and its ripple can be made very small (less than 5mV). For places where power efficiency and installation volume are required, it is better to use a switching power supply. For places where electromagnetic interference and power purity are required (such as
capacitor leakage detection), linear power supplies are often used. In addition, when isolation is required in the circuit, most of the time, DC-DC is used to power the isolated part (DC-DC is a switching power supply in terms of its working principle).
In addition , the high-frequency transformer used in the switching power supply may be difficult to wind.



2.12 Switching power supply chip


Compared with linear regulators, switching power supplies play a role in voltage conversion on computer motherboards, industrial computer motherboards and various circuit boards. For example: converting low voltage, such as batteries, into stable 3.3V
or 5V, or converting high voltage into DC5V, DC3.3V, or converting DC5V into 3.3V and 1.8V. For example, ARM circuit boards require such chips, 3.3V to
power ARM, and 1.8V to power the core of ARM.

Because of the use of switching circuits, the power supply chip has a high operating frequency, low heat generation and high efficiency.

Similarly, chip giants, such as MAXIM, LINEAR and TI, are the most outstanding in power conversion chips, both in terms of product types and quality;

netizens who often watch the world of electronic products and electronic technology applications must have a deep impression of maxim's power chips.

We often use tchmode DC-DC Power Supplies, which are divided into n types. I am more familiar with step-down power supply chips, which are so-called chips that drop from high voltage to low voltage; from low to high, of course, it is step-up power supply chips.

Like Philips electric shavers, there must be power management chips in them. When the battery voltage drops, but the battery has power, it can output a constant voltage, squeeze out the last bit of power in the battery, and can be used comfortably for a long time.

For example, your circuit board only has DC24V voltage, but you also need a DC5V/2A power supply. With such a high voltage drop, it is obviously not appropriate to use a linear regulator. If you use a DC/DC module, the cost is too high and the volume is relatively large, so you have to choose a chip to complete this function.

According to the complex choices given by maxim, according to your requirements, select more items, maxim will give a list, and you can choose the chip that best suits your needs in the list.


If it is generated as an isolated voltage, for example: 5V is converted into isolated 5V, used in optocouplers, communications and other circuits, it is not worthwhile to make the isolation circuit yourself. It is better to buy a ready-made DC/DC module. Domestic modules do a good job in this regard, about 30 yuan, and the quality is still good.

The DC/DC module can completely connect the input and output power grounds together and be used as an ordinary non-isolated power converter.


2.11 Device selection, chip batch number, packaging knowledge, etc.


When buying chips in the market, be careful not to buy old chips or disassembled chips; some old chips and disassembled chips can be used, while some simply cannot; even if they can be used now, their lifespan is definitely limited;

old is old, and it will definitely not be hidden from those who are interested; mainly from the newness of the chip pins and the quality of chip printing.

Old chips often have uneven pins, dark tin coating, and the packaging tube is not new enough. The surface of the chip is often white, because it has been polished and then the manufacturer's name and batch number are re-screened.

Therefore, it is best to buy laser-marked components. Laser-marked products must be new or unused. And when buying, you need to pay attention to the batch number of the component, which should be the product of the last year or the last few weeks. In short, don't waste money and affect the quality of the product.

The same product has different quality from different manufacturers. I prefer products from American brands such as TI, Motorola MC, MAXIM, and LINEAR; there are fewer fakes;

for example, the max7219 chip has the following logo:

MAXIM

MAX7219CNG


MAXIM is the manufacturer of the chip;

MAX7219CNG is the manufacturer's model, and CNG is related to the subtle differences in the package and the 7219 series of products;

03 refers to the year 2003; 23 refers to the 23rd week of the year; there are 52 weeks in a year, so this number will not exceed 52;

laser marking is to form a dark mark on the chip that has a clear contrast with the chip surface; Zhongguancun's Daheng Company has a laser marking machine business; but it does not necessarily mark on the chip;

sometimes, laser marking will produce a dark yellow mark, which is related to the laser power and the chip packaging material.