A simple temperature control circuit to help you understand the circuit diagram

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A simple temperature control circuit to help you understand the circuit diagram [Copy link]

In fact, to understand circuit diagrams, the basics are very important. The so-called basics here are circuit component graphics and text symbols. If you have a basic understanding of these, reading the diagram will be much easier.

An automatic temperature control circuit

Understanding components: In the figure, S is a switch, which is used to connect the circuit; FU is a fuse, which protects the circuit; T is a transformer, which converts 220V electricity into the working voltage required by the circuit; VC is a rectifier bridge, which is a full-wave rectifier circuit composed of four rectifier diodes in the form of a rectifier bridge structure. (Related reading: Summary of calculations for rectifier circuits) KM and KA are both relays. KM is directly used to control the actuator, and KA controls KM. We call KA an intermediate relay. HL is an indicator light, which is used to indicate the current working status of the circuit. The WJ2 electric contact pressure thermometer marked in the figure is the core intelligent component of this circuit. We found a similar thermometer with the same function. The appearance is as follows:

This thermometer can be used to measure the temperature of gas, liquid, and steam, and control the upper and lower limits of the temperature. By designing a reasonable circuit, the temperature can be maintained within a certain range. Its wiring terminals are as follows

Terminal annotation of an electric contact pressure indicating thermometer

Working principle: Combined with the wiring annotation picture of the electric contact pressure thermometer, let's take a look at the circuit diagram interpreted in this article. We can see that line number 403 belongs to the moving contact, and line numbers 401 and 402 are ignored for the time being.

When we need the circuit to work, we need to press the switch S. At this time, the 220V AC power supply is connected, transformed by the transformer T, and rectified by the rectifier circuit. At this time, the coil of the intermediate relay KA is connected, and the two contacts of KA are actuated. One of the KA normally open contacts connected to the KM coil is closed, so that the relay KM coil is energized, and then the normally open contact (made contact) of KM is closed, and the circuit is connected; the other contact of KA is actuated, connecting HL1, and the HL1 indicator light becomes bright (from this point we can see that the intermediate relay KA is a Z-type relay, that is, the contacts are like a transfer switch, separated in one way, and connected to another way). When the temperature reaches the operating temperature of the electric contact pressure thermometer, 403 is disconnected from 401 and then connected to 402, which causes the intermediate relay KA to lose power, and then the KM coil to lose power. The KM coil loses power, and the KM normally open contact returns to the normally open state, thus disconnecting the electric furnace heating element; at the same time, the other contact of KA will also act, disconnecting the connection with HL1 and restoring the connection with HL2. At this time, the switch S is not disconnected, the HL2 indicator light is on, and the heating is completed.

Through analysis, we can see that line number 401 should be the upper limit contact of the electric contact pressure thermometer, and 402 is the lower limit contact.

The analysis of this circuit diagram is basically complete. Many of the manuals of the electrical appliances we buy contain simple circuit instructions. If we understand the circuit diagram, when the appliance fails, we can simply judge from the diagram where the failure is. Then, when we take it for repair, we can also estimate the price and avoid being cheated out of a lot of money.

shuangchafeng

The article is well illustrated and very detailed.