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Iron solution
An electric iron is an electrical appliance that uses the thermal effect of an electric current to heat the soleplate to iron various fabrics. At present, household electric irons are mainly divided into ordinary electric irons without a thermostat and relying on people to control the power-on time, and temperature-regulating electric irons using bimetallic strips for temperature control. The former is very dangerous when people forget to turn off the power switch; the latter still maintains a large power output when the thermostat fails or people are not using it, and there is also a fire hazard.
In order to improve the safety of household electric irons and reduce its energy consumption, this paper proposes and designs a safety control device for electric irons based on touch switches, single-chip microcomputers and thyristors, using touch switches to detect the use status of electric irons. The output power of the electric iron is controlled by controlling the conduction angle of the thyristor by the single-chip computer, so as to achieve the goals of safe temperature control, energy saving and consumption reduction.
1. The principle and problems of electric iron temperature adjustment
In order to meet the ironing temperature requirements of different fabrics, a thermostat is added on the basis of ordinary electric irons, which can control the temperature of the soleplate to be continuously adjustable between 60 and 250 ° C. The commonly used thermostat structure is a bimetal structure. . The principle of temperature adjustment is that when the temperature of the bottom plate rises to the temperature set by the temperature adjustment knob, the bimetal sheet changes from straight to curved, disconnects from the static contact, and cuts off the circuit; as the temperature of the electric iron gradually decreases, the bimetal sheet gradually changes from When the temperature drops to a certain level, the bimetal sheet will be in contact with the static contact again; so repeated, the temperature will be controlled near the set temperature.
After using the thermostat, the safety of the electric iron has been greatly improved, but there are still potential safety hazards. The reasons include the following two aspects. In terms of the electric iron itself, the temperature-regulating electric iron bimetallic piece fails due to long-term pressure loss of elasticity or long-term high-power output leads to contact fusion; on the user's side, it causes fire due to improper operation, such as: electric iron Misplaced, placed flat or vertically for a long time; forgetting to cut off the power.
At present, the following two methods are mainly used to improve the above security problems: one is to use temperature sensors, attitude sensors and fuzzy control algorithms to develop intelligent electric iron controllers, but this method is complicated in temperature control technology, high in cost, and difficult to use. It is used for household electric irons, mainly for industrial electric irons or higher-end electric iron products; the second is to make the state or posture of the electric iron last for a period of time before shutting down according to the use state or posture of the electric iron. It involves specific temperature control, but uses status, attitude sensors and timers to control the shutdown of the power supply to ensure safety. It has the advantages of low cost and safer use, so it is widely used.
However, the second improved method still has the following problems:
(1) In terms of use status or attitude detection, common attitude sensors include mercury switches, metal ball rolling contact switches, etc. The former causes environmental protection pressure in the life links of electric irons such as manufacturing, use, and recycling, while the latter is prone to misoperation ;
(2) When the timer is used to control the power off, there are generally only three power output modes: during normal use, full power output; during a period of suspension of use (such as set to ts), half power output; after ts time, zero power output. Since the half-power output is still maintained during the suspension period, the temperature of the soleplate of the electric iron is relatively high, which not only presents a fire hazard, but also causes serious waste of energy consumption.
2. Design of electric iron safety controller based on single chip microcomputer
2.1 Basic design idea of electric iron
Aiming at the problems existing in the bimetal temperature-regulating electric iron, this paper proposes a single-chip electric iron safety controller, and its basic design idea is shown in Figure 1.
The state of the human hand is detected by the touch switch installed on the handle of the electric iron, that is, when the human hand holds the handle, the touch switch outputs a signal, and it is considered to be in the use state; when the human hand leaves the handle, the touch switch has no signal output, and it is considered to be in the paused state .
Due to the use of single-chip microcomputer control, the electric iron can realize "power attenuation output" when it is suspended in use. The control process is: when the suspension is detected, the half-power output will be output immediately. During the subsequent ts time, the single-chip microcomputer will control the conduction of the thyristor. Angle, so that the output power decays smoothly; if within the ts time, it is detected that the resumption of use is detected, it will immediately restore the full power output; if within the ts time, no signal for resumption of use is detected, the microcontroller will control the SCR Angle is kept at zero, and the thyristor is cut off, which is equivalent to power off (this is electronic shutdown), and the buzzer sounds to remind the user to unplug the power supply (this is physical shutdown), and it cannot be used until it is turned on again.
Compared with the "half power output", the characteristics of the "power decay output" type controller are: (1) After the suspension of use is detected, the output power will be halved immediately, and the power will maintain a smooth decay in the subsequent ts time until It is zero, but when it is resumed to use, it can be heated up to the set temperature conveniently and quickly, taking into account both safety and efficiency; (2) During the period from "suspended use" to "resumed use", the controller is more energy-efficient, as shown in Figure 2 The middle fan-shaped area is the energy consumption saved in this period of time.
2.2 Use status detection
Figure 2 shows a self-designed touch switch. Its input contacts are led to the outside of the handle of the electric iron. When a person touches the touch switch contacts, the contacts receive clutter signals in the air through the human body, and the signals pass through The two-stage triode is amplified, and after being filtered by a capacitor, it is finally used as a status signal of the electric iron to be input to the single-chip microcomputer. Compared with mercury switches and metal ball rolling contact switches, this touch switch has the characteristics of reliable performance, environmental protection and safety, and low cost.
2.3 AC zero-crossing detection
In the state of suspending use, the single-chip microcomputer must first perform AC zero-crossing detection to control the conduction angle of the thyristor. Figure 3 shows the AC zero-crossing detection circuit diagram. In the figure, the rectifier circuit composed of D1~D4 provides the pulsating DC waveform for the optocoupler PC817. When the waveform crosses from the highest point to the zero point, the optocoupler will be closed, and the transistor Q1 cannot be turned on. The potential sent to the RA2 port of the microcontroller is Zero, so the single-chip microcomputer completes a zero-crossing detection, which provides a starting point for the next step of the single-chip microcomputer to control the thyristor chopping wave.
2.4 Single-chip Microcomputer Controls SCR Conduction Angle
In Figure 4, the bidirectional thyristor is used as a controllable switching device in series on the electric heating element circuit in the soleplate of the electric iron, and the resistor R11 provides a pull-up current for the single-chip microcomputer to control the G pole of the thyristor. When the single-chip microcomputer triggers the G pole once, the thyristor will be turned on, and it will remain on until the voltage applied to the two terminals becomes zero. The half cycle of power frequency alternating current is 10ms. If the microcontroller starts timing at the zero point of the alternating current, and the RB3 port of the single chip microcomputer outputs a high level after 5ms to trigger the G pole of the thyristor, the thyristor will keep on before the positive or negative half cycle of the alternating current reaches the zero point. Conduction, at this time, the thyristor controls the half-power output of the electric iron. The essence of the power attenuation output of the electric iron is to divide the pause time ts into several segments (such as 50 segments), and change the average voltage of the heating element by controlling the conduction angle of the thyristor within each segment. As the pause time prolongs, the conduction The angle and average voltage decrease step by step, and the output power of the electric iron also gradually decays. If the ts time expires and the use has not been resumed, an audible alarm signal will be issued.
2.5 MCU control program flow
After the microcontroller is initialized, it continuously detects the use status of the electric iron, and determines the way of power output according to the signal fed back by the touch switch. After the half-power output, the use status is still continuously detected, and if the use is resumed, the full-power output will be output immediately; otherwise, the output power will be gradually attenuated within the specified time. After the ts time is up, the thyristor will be cut off to realize electronic shutdown. Improve safety, and remind users to unplug the power through the buzzer to achieve physical shutdown.
Summarize
The safety controller uses a single-chip microcomputer as the control core, uses a touch switch to detect the use status of the electric iron, and controls the conduction angle through the single-chip microcomputer to realize the gradual attenuation of the output power of the electric iron during the suspension of use. Experiments show that the electric iron safety controller has the characteristics of safety and reliability, energy saving and consumption reduction, convenient use and low cost, and has a good market promotion prospect.
The above are the details of the electric iron solution introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have the function development needs of electric irons, you can rest assured to entrust them to us. We have rich experience in custom development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad: Songhan, Yingguang, Jieli, Ankai, Quanzhi, realtek, with MCU, voice IC, Bluetooth IC and module, wifi module. Our development capabilities cover PCB design, single-chip microcomputer development, Bluetooth technology development, software customization development, APP customization development, WeChat official account development and other hardware and software design. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the development of electronic education products.
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