Electric iron scheme design and development

2020-12-22 15:09:51 fandoukeji

An electric iron is an electrical appliance that uses the thermal effect of 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 with bimetallic 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 in use, 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. The touch switches are used 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 through the single-chip microcomputer, thereby achieving the purpose of safe temperature control, energy saving and consumption reduction.


1. The principle and problem 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 60~250℃. The commonly used thermostat structures are bimetallic structures. . The principle of temperature adjustment is that when the temperature of the soleplate rises to the temperature set by the temperature adjustment knob, the bimetallic strip turns from straight to bend, disconnects from the static contact, and cuts off the circuit; as the temperature of the electric iron gradually decreases, the bimetallic strip gradually changes from The bend becomes straight, and when the temperature drops to a certain level, the bimetallic strip contacts the static contact again; this is repeated continuously to control the temperature near the set temperature.

After adopting the thermostat, the safety of electric irons has been greatly improved, but there are still hidden safety hazards. The reasons include the following two aspects. Regarding the electric iron itself, the bimetallic sheet of the temperature-regulating electric iron loses its elasticity due to prolonged pressure or long-term high-power output leads to contact fusion and failure; on the user's side, it may cause fires due to improper operation, such as electric irons Placed incorrectly, placed horizontally or vertically for a long time; forgot to cut off the power.

At present, in view of the above-mentioned safety problems, the following two methods are mainly used for improvement: One is to use temperature sensors, attitude sensors and fuzzy control algorithms to develop intelligent electric iron controllers, but this method has complicated temperature control technology, high cost, and difficult to use. For household electric irons, they are mainly used for industrial irons or higher-end electric iron products; second, according to the use state or posture of the electric iron, make the state or posture last for a period of time before shutting down. This method is not It involves specific temperature control, but uses state, 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 improvement method still has the following problems:

(1) In terms of use status or posture detection, common posture sensors include mercury switches, metal ball rolling contact switches, etc. The former makes electric irons have environmental protection pressures in life links such as manufacturing, use and recycling, while the latter is prone to malfunction ;

(2) When the timer is used to control the power off, there are generally only three power output methods: during normal use, full power output; during a period of suspension of use (such as 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 poses a fire hazard, but also consumes serious energy waste.

2. Design of safety controller for electric iron based on single chip microcomputer

2.1 Basic design ideas of electric iron

Aiming at the problems of bimetallic temperature-regulating electric irons, this paper proposes a single-chip safety controller for electric irons. The basic design idea is shown in Figure 1.

图 1 基于单片机的电熨斗安全控制器示意图

The state of the human hand is detected by the touch switch installed at 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 use; when the human hand leaves the handle, there is no signal output, and it is considered to be in a pause state .

Due to the single-chip control, the electric iron can achieve "power attenuation output" when it is suspended. The control process is: the suspension is detected, and the output is half power immediately. In the subsequent ts time, the single-chip computer controls the conduction of the thyristor Angle, the output power is smoothly attenuated and decreased; if it is detected to be used within ts, it will immediately return to full power output; if within ts, the signal to be used is not detected, the single-chip microcomputer will control the thyristor to turn on The angle remains at zero and the thyristor is cut off, which is equivalent to power shutdown (this is an electronic shutdown), and the buzzer will sound to remind the user to unplug the power (this is a physical shutdown), and it will not be able to resume use until restarting.

Compared with the "half power output", the characteristics of the "power attenuation output" type controller are: (1) After detecting the suspension of use, the output power is immediately halved, and the power remains smoothly attenuated during the subsequent ts time. It is zero, but it can be easily and quickly heated to the set temperature when it is resumed, taking into account safety and efficiency; (2) The controller is more energy-efficient during the period of "suspended use" to "resumed use". The middle sector area is the energy saved in this time period.

图 2 触摸开关原理图

2.2 Use status detection

Figure 2 shows a self-designed touch switch. Its input contacts are led to the outside of the iron handle. When a person touches the touch switch contacts, the contacts receive clutter signals in the air through the human body. After the two-stage triode is amplified and filtered by the capacitor, it is finally input to the single-chip microcomputer as a signal of the use state of the electric iron. 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 suspended state, if the microcontroller wants to control the conduction angle of the thyristor, the AC zero-crossing detection must be performed first, as shown in Figure 3 for the AC zero-crossing detection circuit diagram. In the figure, the rectifier circuit composed of D1~D4 provides a pulsating DC waveform for the optocoupler PC817. When the waveform crosses from the highest point to the zero point, the optocoupler will be turned off, the transistor Q1 cannot be turned on, and the potential delivered to the RA2 port of the microcontroller is Zero, thus the single-chip microcomputer completes a zero-crossing detection, which provides a starting point for the single-chip microcomputer to control the thyristor chopped wave.

图 3 交流电过零检测原理图

2.4 SCR conduction angle controlled by single chip microcomputer

In Fig. 4, the triac is used as a controllable switch device in series with the electric heating element circuit in the soleplate of the electric iron, and the resistor R11 provides the pull-up current for the G electrode of the SCR controlled by the single-chip microcomputer. When the single-chip microcomputer triggers the G pole once, the thyristor will be turned on and will remain on until the voltage applied to the two extremes becomes zero. The power frequency alternating current half cycle is 10ms. If the single-chip microcomputer starts timing at the zero position of the alternating current, after 5ms the microcontroller RB3 port outputs a high level to trigger the thyristor G pole, then the thyristor will keep before the positive or negative half cycle of the alternating current reaches the zero point Turn on, at this time, the SCR 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 the average voltage of the heating element is changed by controlling the conduction angle of the thyristor during each period. With the extension of the pause time, conduction The angle and average voltage decrease step by step, and the output power of the electric iron also gradually decreases. If the ts time is up and the use is not resumed, an audible alarm signal will be issued.

图 4 单片机控制可控硅原理图

2.5 MCU control program flow

After the single-chip microcomputer is initialized, it continuously detects the use status of the electric iron, and determines the power output mode according to the signal feedback from the touch switch. After the half-power output, the state of use is still continuously detected, and if the use is restored, the full power is 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 achieve electronic shutdown. Improve safety and remind users to unplug the power supply through a buzzer to achieve physical shutdown.