WO2021031766A1 - 一种限压电路 - Google Patents

一种限压电路 Download PDF

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Publication number
WO2021031766A1
WO2021031766A1 PCT/CN2020/103038 CN2020103038W WO2021031766A1 WO 2021031766 A1 WO2021031766 A1 WO 2021031766A1 CN 2020103038 W CN2020103038 W CN 2020103038W WO 2021031766 A1 WO2021031766 A1 WO 2021031766A1
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voltage
resistor
varistor
circuit
thermistor
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PCT/CN2020/103038
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English (en)
French (fr)
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石小龙
张治成
章俊
黎科
张旭
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成都铁达电子股份有限公司
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Publication of WO2021031766A1 publication Critical patent/WO2021031766A1/zh

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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/04—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess voltage

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  • the invention relates to the technical field of regulated power supplies, and in particular to a voltage limiting circuit.
  • Switching regulated power supply is the power supply part of many electronic equipments. It has the advantages of high efficiency, good voltage regulation performance, and wide voltage adaptation range.
  • In the conventional 220 volt power supply line there are often abnormal rises in voltage due to faults such as poor neutral contact or open circuit, and the maximum value of the abnormal voltage can reach close to 380 volts.
  • the rectifier bridges, switching transistors, filter capacitors and other components that make up the switch-mode regulated power supply are more sensitive to abnormal overvoltages.
  • the input voltage fluctuations are too high, and these components are easily damaged. Therefore, a voltage limiting circuit is needed to reduce the grid voltage.
  • the fluctuation is limited to the safety value.
  • the smart electric energy meter for electric energy measurement uses a switching power supply. When it is connected to a 220-volt grid, its internal voltage limiting circuit is required to control the fluctuation of the grid voltage between 110-330 volts to ensure the safety and safety of components.
  • Figure 1 shows an existing voltage-limiting circuit widely used in old-fashioned electronic energy meters with weak communication functions. Because this type of energy meter does not require much power consumption during data transmission, only power is required. A power of less than 3 watts is enough, so it can achieve a good voltage limiting effect by using a voltage limiting circuit as shown in Figure 1.
  • the thermistor connected in series at the input of the power supply acts as a voltage divider and is connected in parallel at the output.
  • the varistor acts as a voltage limiter, and the thermal coupling between them increases the response speed and provides mutual protection, which has a good practical effect.
  • the communication function of the new smart electric energy meter has been greatly enhanced, and the corresponding power consumption has also increased.
  • This kind of intelligent electric energy meter has two working states, one is the simple metering state, and its power It consumes less than 3 watts, and the input current is within 10 mA. The other is the working state of metering plus communication (data transmission). At this time, its power consumption is about 6-10 watts, and the input current is 30- About 100 mA, the input current has a jump process when the two working states are switched.
  • the voltage limiting circuit in Figure 1 will cause the output voltage to drop below 100 volts after power switching (during the jump) due to too low output power and too narrow dynamic range, which cannot be stabilized between 110 and 330 volts. The switching power supply cannot work reliably.
  • One of the objectives of the present invention is at least to provide a voltage limiting circuit for how to overcome the above-mentioned problems in the prior art, which can adapt to the different working states of the new smart electric energy meter, and output to the switch when the grid voltage exceeds 330 volts.
  • the voltage at the power supply terminal is stably limited to between 110 and 330 volts.
  • the technical solution adopted by the present invention includes the following aspects.
  • a voltage limiting method After the thermistor is connected to the power grid in series as the input terminal and the varistor is connected to the power grid in parallel as the output terminal, the load circuit is connected in parallel at both ends of the varistor.
  • the load circuit When the connected grid voltage When the voltage is higher than the predetermined voltage and is at an abnormal voltage, the load circuit is protected by the thermal coupling of the thermistor and the varistor, and the voltage output to the load circuit is limited to its allowable input minimum voltage value and allowable Between the input maximum voltage values; characterized in that a voltage-controlled resistance branch is connected in parallel with the thermistor, and the conduction condition of the corresponding voltage-controlled resistance branch is set;
  • the voltage-controlled resistance branch includes: a voltage-controlled switch and a resistor; the voltage-controlled switch is connected in series with the resistor.
  • the conduction voltage value of the voltage control switch is greater than the difference between the maximum abnormal voltage peak value of the power grid and the maximum voltage value allowed by the load circuit, and is less than the maximum abnormal voltage peak value of the power grid and the allowable input value of the load circuit. The difference between the peak value of the minimum voltage value.
  • the value of the resistor should meet the following requirements: when the load circuit is in the minimum operating current state after the voltage-controlled switch is triggered to be turned on, the voltage drop generated by the resistor is greater than the grid The difference between the maximum abnormal voltage value and the maximum voltage value allowed by the load circuit;
  • the voltage drop generated by the resistor is less than the maximum voltage value allowed by the load circuit and the minimum voltage value allowed by the load circuit. Difference.
  • a voltage limiting circuit includes: a voltage-controlled resistor branch, a positive temperature coefficient thermistor and a varistor;
  • the voltage-controlled resistance branch includes: a voltage-controlled switch and a resistor, the voltage-controlled switch is connected in series with the resistor; the voltage-controlled resistance branch is connected in parallel with the thermistor to form a series-parallel branch One end of the series-parallel branch is the input electrode end of the voltage-limiting circuit, and a tap from the other end is the output electrode end of the voltage-limiting circuit; the other end of the series-parallel branch is also connected with the voltage
  • the varistor is connected in series; one end of the varistor is connected to the series-parallel branch, and the other end is the common electrode terminal of the voltage limiting circuit; the thermistor is thermally coupled with the varistor; the limiter
  • the voltage circuit adopts the above-mentioned voltage-limiting method.
  • the protected circuit When the connected grid voltage is higher than the predetermined voltage and is at an abnormal voltage, the protected circuit is voltage-limiting protection; and when the load power change exceeds the predetermined value, the temperature of the thermistor is higher than its The thermal equilibrium point above the Curie temperature point causes the voltage at both ends to rise to trigger the conduction of the voltage-controlled resistor branch and increase the power range output to the protected circuit.
  • the voltage-controlled switch is a discharge tube, and when the connected grid voltage is 220V, the DC breakdown voltage of the discharge tube is 70V-380V.
  • the resistance value of the resistor is 1.7 kohm to 2.4 kohm.
  • the nominal varistor voltage of the varistor is 390V
  • the maximum allowable AC working voltage is 250V.
  • the voltage-controlled switch, resistor, thermistor and varistor are packaged as a whole, and the corresponding electrode terminal is led out by the lead wire.
  • an intelligent electric energy meter includes a voltage limiting circuit using the above-mentioned voltage limiting method.
  • the present invention has at least the following beneficial effects:
  • a resistor divider circuit controlled by a dynamic switch is connected in parallel at both ends of the thermistor.
  • the voltage control switch can be adapted to the load circuit (Protected circuit) Different power states are dynamically turned on, so that when the resistor is connected to the voltage-limiting circuit when the voltage-controlled switch is turned on, the power output capability of the voltage-limiting circuit is improved, so that the voltage-limiting circuit can output dynamic range
  • the wider power value enables the output power value of the voltage limiting circuit to adapt to the power requirements of the load circuit in different working states under abnormal grid voltage; the resistor divider circuit can also make the output voltage to the switching power supply of the electric energy meter When the power is switched (during the jump), it can also be stabilized between 110 and 330 volts, thereby ensuring the normal operation of the power meter switching power supply.
  • Fig. 1 is a circuit diagram of a voltage limiting circuit used in an old-fashioned electronic watt-hour meter with weak communication function in the prior art.
  • Fig. 2 is a voltage limiting circuit diagram according to an exemplary embodiment of the present invention.
  • Fig. 2 shows a voltage limiting circuit according to an exemplary embodiment of the present invention.
  • the circuit of this embodiment mainly includes: a voltage-controlled switch 6, a resistor 7, a positive temperature coefficient thermistor 5, and a varistor 4;
  • the voltage-controlled switch 6 is connected in series with the resistor 7 and then connected in parallel with the positive temperature coefficient thermistor 5 to form a series-parallel branch; one end of the series-parallel branch is the input electrode terminal of the voltage limiting circuit 1. A tap from the other end is the output electrode terminal 2 of the voltage limiting circuit; the other end of the series-parallel branch is also connected in series with the varistor 4; one end of the varistor 4 is connected in parallel with the series The other end of the branch connection is the common electrode end of the voltage limiting circuit; the positive temperature coefficient thermistor 5 and the varistor 4 are thermally coupled.
  • the input electrode terminal of the voltage limiting circuit is connected to the 220 volt grid line, and the output electrode terminal is connected to the load circuit (using the electric energy meter switching power supply as an example).
  • the normal working voltage range of the switching power supply is Between 110 and 330 volts, the varistor voltage is 390 volts, and the maximum allowable AC working voltage is 250 volts.
  • the voltage limiting circuit of the present invention has three different working modes, which are as follows:
  • the first working mode When the grid voltage connected to the input electrode end is lower than 250 volts, the varistor will be in a state of high resistance without heating, the thermistor is also in a normal temperature state, and the thermistor value is zero power resistance. , Is a low resistance state. At this time, no matter whether the electric energy meter is in the measurement state or the communication state, the current flowing through the thermistor is several milliamperes or tens of milliamperes, which is lower than the minimum operating current of the thermistor at room temperature and cannot cause the thermistor to heat up. , The resistance value rises.
  • the thermistor is in a low resistance state, the voltage drop on the thermistor is very small (the partial voltage is very small), and it can be approximated that the voltage limiting circuit does not work, and the grid voltage is almost directly applied to the switching power supply of the energy meter .
  • the second working mode when the grid voltage connected to the input electrode terminal exceeds 250 volts (at an abnormal voltage value), the varistor begins to have current flowing and starts to generate heat, and the heat generated by the varistor is thermally coupled to the thermistor The resistance of the thermistor is increased due to heat, and the partial pressure on the thermistor is increased, thereby slowing down the current flowing into the varistor.
  • the grid voltage increases, when the current value flowing into the varistor When it reaches 10-30 mA, the thermal equilibrium can be reached above the Curie temperature point, and the thermistor shares part of the abnormal overvoltage, so that the voltage output to the switching power supply terminal is limited to 330V.
  • the current flowing through the thermistor only needs to be shunted a few milliamps into the switching power supply.
  • the heat generated by this incremental current cannot make the temperature of the thermistor continue to rise from the thermal equilibrium point.
  • the resistance of the thermistor will not continue to increase, and the partial voltage across the thermistor will not continue to increase.
  • the on-voltage value of the voltage-controlled switch in the voltage limiting circuit is set to be higher than At this voltage value, the voltage-controlled switch will not be turned on, and accordingly, no current will flow through the branch connected in series with the voltage-controlled switch.
  • the output power of the voltage limiting circuit can ensure the normal operation of the electric energy meter in the metering state.
  • the voltage across the varistor is less than 250 volts, and the varistor returns to a high-resistance non-conducting state. There is no heat generation, and the thermistor is not coupled with heat, and the resistance starts to drop, and its partial pressure Then it decreases, and finally returns to the first working mode.
  • the third working mode If the electric energy meter is in the abnormal state of the grid voltage and working in the communication state, the current output from the voltage limiting circuit to the switching power supply needs to increase to several tens of milliamperes, which will cause the thermistor to increase due to the current passing. The additional power consumption of more than 1 watt will cause the temperature of the thermistor to deviate from the thermal equilibrium point and continue to rise. Above the Curie temperature of the PTC thermistor, a slight increase in temperature will cause the resistance value The sharp increase of, the partial pressure across the thermistor will also rise rapidly, and the voltage output to the switching power supply terminal of the electric energy meter will also drop.
  • the switch will be turned on.
  • the resistor and the thermistor are connected in parallel to improve the power output capability of the voltage limiting circuit and ensure the output to the switching power supply
  • the voltage at the terminal is maintained between 110 and 330 volts.
  • the conduction voltage of the voltage-controlled switch should be selected to satisfy the It is greater than the difference between the maximum abnormal voltage peak of the power grid and the highest voltage peak allowed by the voltage limiting circuit, and is smaller than the difference between the maximum abnormal voltage peak of the power grid and the lowest voltage peak allowed by the voltage limiting circuit.
  • a discharge tube can be used as the voltage control switch. When used in a voltage limiting circuit of an electric energy meter connected to a 220 volt power grid, the DC breakdown voltage of the discharge tube can be selected at 70 Between ⁇ 380 volts.
  • the lower limit of the resistance value of the resistor connected in series with the voltage control switch should meet the minimum operating current after the electric energy meter is switched to the communication state, the voltage drop generated by this resistance value is equal to the maximum abnormal voltage of the power grid The difference between the value and the maximum allowable voltage value output to the switching power supply terminal.
  • the upper limit of the resistance value of the resistor should meet the maximum working current after the electric energy meter is switched to the communication state, the voltage drop generated by this resistance is equal to the maximum allowable voltage value output by the voltage limiting circuit to the switching power supply terminal The difference from the minimum allowable voltage value. If the electric energy meter working in the 220V grid is set, it needs to consume 10 watts of power in the communication state.
  • the switching power supply terminal voltage is 330V
  • the maximum allowable value the input working current is a minimum of 30 mA
  • the switching power supply terminal voltage is At the lowest allowable value of 110 volts
  • the input working current is the maximum value of 90 mA
  • the maximum abnormal voltage of the power grid is 380 volts.
  • this setting ignores the partial pressure of the discharge tube's lowest sustaining on-voltage value and the shunt effect of the thermistor under different resistance states. In application, some engineering corrections can be made. As long as there is experimental verification support, large errors in the upper and lower limits of the resistance are acceptable.
  • the voltage control switch discharge tube
  • the voltage output to the switching power supply terminal will drop to 110 ⁇ 330 volts, thus ensuring the normal operation of the power meter switching power supply.
  • the varistor will reduce or stop heating, causing the thermistor resistance to decrease, and the current flowing through the thermistor increases, and the corresponding shunt flowing through the resistor begins to decrease.
  • the current flowing into the watt-hour meter's switching power supply plummets from tens of milliamps to a few milliamps, and the shunt flowing through the resistor will be less than the minimum sustaining discharge current of the discharge tube (the maintenance The current needs to be greater than about 20 mA to keep the discharge tube on), the discharge tube will be disconnected, and the circuit will return to the state of the second working mode.
  • the varistor will stop heating at this time, the resistance of the thermistor will decrease, and the shunt will increase, resulting in a decrease in the current flowing through the resistor.
  • the shunt flowing through the resistor is reduced to less than the minimum sustain current of the discharge tube or the voltage across the thermistor is reduced to less than the minimum sustain voltage of the discharge tube (the sustain voltage needs to be greater than about 30 volts to keep the discharge tube on) , The discharge tube will be disconnected, and the circuit will directly return to the state of the first working mode.
  • the voltage-limiting circuit in this example connects a resistor divider circuit (voltage-controlled switch) controlled by a dynamic switch in parallel to the thermistor, which dynamically improves the power output capability of the voltage-limiting circuit and makes the voltage-limiting circuit more effective.
  • the dynamic output power range is wide, which can not only adapt to the power requirements of the electric energy meter when the grid voltage is abnormal but also work in the communication state, but also ensure that the voltage output to the electric energy meter switching power supply can be stabilized at 110 during the jump process. ⁇ 330 volts, thus ensuring the normal operation of the power meter switching power supply.
  • the voltage limiting circuit of the present invention can also be applied to other electrical appliances with low-power switching power supplies, such as LED lighting appliances.
  • the switching power supply does not have to have both large and small powers.
  • the corresponding voltage limiting circuit has only two working states: working mode 1 and working mode 3.

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Abstract

一种限压电路，其包括：电压控制型开关（6）、电阻器（7）、正温度系数热敏电阻（5）和压敏电阻（4）；其中，电压控制型开关（6）与电阻器（7）串联后再与正温度系数热敏电阻（5）并联形成串并联支路；串并联支路一端为限压电路的输入电极端（1），其另一端为限压电路输出电极端（2）；串并联支路的另一端还与压敏电阻（4）串联；压敏电阻（4）一端与所述串并联支路连接，其另一端为限压电路的公共电极端（3）；正温度系数热敏电阻（5）与压敏电阻（4）热耦合。通过该电阻器（7）分压电路的动态导通提高限压电路的功率输出能力，适应于电能表不同工作状态的功率要求；还能使输出至电能表开关电源的电压在功率切换时(跳变过程中)也能稳定在110～330伏之间，从而保证了电能表开关电源的正常工作。

Description

一种限压电路 技术领域
本发明涉及稳压电源技术领域，尤其涉及一种限压电路。
背景技术
开关型稳压电源是许多电子设备的电源部分，它具有效率高、稳压性能好、电压适应范围宽等优点。在常规的220伏供电线路中，经常有由于零线接触不良或断路等故障造成的电压异常升高，异常电压最高值可达到接近380伏。而组成开关型稳压电源的整流桥、开关三极管、滤波电容器等元器件对异常过电压比较敏感，输入电压波动太高，容易损坏这些元器件，因此，需要一款限压电路，将电网电压的波动限制在安全值以下。电能计量用的智能电能表就采用了开关电源，当它接入220伏电网时，需要其内部的限压电路将电网电压的波动控制在110～330伏之间，以确保元器件的安全和开关电源的正常工作。
图一示出了现有的一种广泛应用在通信功能不强的老式电子式电能表中的限压电路，由于这种电能表在数据传输时不需要太大的功耗，只需要电源提供3瓦以下的功率就够了，因此其采用如图一的限压电路就能实现很好的限压效果，该电路中串联在电源输入端的热敏电阻起分压的作用，并联在输出端的压敏电阻起限压作用，它们之间的热耦合即提高了响应速度，又相互提供了保护，具有不错的实际使用效果。然而随着物联网技术的发展，新型智能电能表的通信功能大幅增强，相应的功耗也要增加，这种智能电能表有两种工作状态，一种是单纯计量的状态，此时它的功耗在3瓦以下，输入的电流在10毫安以内，另一种是计量加通信(数据传输)的工作状态，此时，它的功耗在6～10瓦左右，输入的电流在30～100毫安左右，两种工作状态切换时输入电流有一个跳变过程。图一的限压电路就会因输出功率太低，动态范围太窄而使输出电压在功率 切换后(跳变过程中)降低至100伏以下，不能稳定在110～330伏之间，电能表开关电源就不能可靠地正常工作。
发明内容
本发明的目的之一至少在于，针对如何克服上述现有技术存在的问题，提供一种限压电路，能够适应新型智能电能表的不同工作状态，在电网电压超过330伏时，将输出至开关电源端的电压稳定地限制在110～330伏之间。
为了实现上述目的，本发明采用的技术方案包括以下各方面。
一种限压方法，将热敏电阻作为输入端串联接入电网和压敏电阻作为输出端并联接入电网后，在所述压敏电阻两端并联接入负载电路，当接入的电网电压高于预定电压处于异常电压时，通过热敏电阻和压敏电阻的热耦合作用，对负载电路进行限压保护，将输出至所述负载电路的电压限制在其允许输入的最小电压值与允许输入的最大电压值之间；其特征在于，在所述热敏电阻上并联一条电压控制型电阻支路，并设置相应的电压控制型电阻支路的导通条件；
当接入的电网电压高于预定电压处于异常电压，所述负载电路功率变化超过预定值时，热敏电阻的温度高于其居里温度点上方的热平衡点时，使其两端电压上升从而触发所述电压控制型电阻支路导通，增大输出至被保护电路的功率范围。
优选的，上述限压方法中，电压控制型电阻支路包括：电压控制型开关与电阻器；所述电压控制型开关与电阻器串联。
优选的，上述限压方法中，电压控制型开关的导通电压值大于电网最大异常电压峰值与负载电路允许输入的最大电压值峰值的差值，小于电网最大异常电压峰值与负载电路允许输入的最小电压值峰值的差值。
优选的，上述限压方法中，电阻器的取值应该满足：当触发所述电压控制 型开关导通后、所述负载电路处于最小工作电流状态时，所述电阻器产生的压降大于电网最大异常电压值与负载电路允许输入的最大电压值的差值；
当触发所述电压控制型开关导通后、所述负载电路处于最大工作电流状态时，所述电阻器产生的压降小于负载电路允许输入的最大电压值与负载电路允许输入的最小电压值的差值。
进一步的，一种限压电路，包括：电压控制型电阻支路、正温度系数热敏电阻和压敏电阻；
其中，所述电压控制型电阻支路包括：电压控制型开关与电阻器，所述电压控制型开关与电阻器串联；所述电压控制型电阻支路与所述热敏电阻并联形成串并联支路；所述串并联支路一端为所述限压电路的输入电极端，其另一端引出一个抽头为所述限压电路输出电极端；所述串并联支路的另一端还与所述压敏电阻串联；所述压敏电阻一端与所述串并联支路连接，其另一端为所述限压电路的公共电极端；所述热敏电阻与所述压敏电阻热耦合；所述限压电路采用上述限压方法，在接入的电网电压高于预定电压处于异常电压时，对被保护电路进行限压保护；并当负载功率变化超过预定值时，热敏电阻的温度高于其居里温度点上方的热平衡点，使其两端电压上升从而触发所述电压控制型电阻支路导通，增大输出至被保护电路的功率范围。
优选的，该限压电路中，电压控制型开关为放电管，当接入的电网电压为220V时，所述放电管的直流击穿电压值为70V～380V。
优选的，该限压电路中，电阻器的阻值取值为1.7千欧姆～2.4千欧姆。
优选的，该限压电路中，当接入的电网电压为220V时，所述压敏电阻的标称压敏电压为390V，最大允许交流工作电压为250V。
优选的，该限压电路中，电压控制型开关、电阻器、热敏电阻和压敏电阻 被封装成一体，相应的电极端由引线引出。
进一步的，一种智能电能表，该智能电能表中包含有采用上述限压方法的限压电路。
综上所述，由于采用了上述技术方案，本发明至少具有以下有益效果：
本发明的限压电路通过在热敏电阻两端并联一条由动态开关(电压控制型开关)控制的电阻器分压电路，通过相应的元件参数的设置，该电压控制型开关能够适应于负载电路(被保护电路)不同功率状态动态导通，从而在电压控制型开关导通时将电阻器接入至限压电路时，提高该限压电路的功率输出能力，使限压电路能够输出动态范围较宽的功率值，使限压电路输出的功率值能够适应于电网电压异常状态下的负载电路不同工作状态的功率要求；通过该电阻器分压电路还能使输出至电能表开关电源的电压在功率切换时(跳变过程中)也能稳定在110～330伏之间，从而保证了电能表开关电源的正常工作。
附图说明
图1是现有技术中应用在通信功能不强的老式电子式电能表中的一种限压电路图。
图2是根据本发明示例性实施例的限压电路图。
附图标记：101-热敏电阻；102-压敏电阻；1-输入电极端；2-输出电极端；3-输入电极端；4-压敏电阻；5-热敏电阻(正温度系数热敏电阻)；6-电压控制型开关；7-电阻器7。
具体实施方式
下面结合附图及实施例，对本发明进行进一步详细说明，以使本发明的目的、技术方案及优点更加清楚明白。应当理解，此处所描述的具体实施例仅用以解释本发明，并不用于限定本发明。
图2示出了根据本发明示例性实施例的限压电路。该实施例的电路主要包括：电压控制型开关6、电阻器7、正温度系数热敏电阻5和压敏电阻4；
其中，所述电压控制型开关6与电阻器7串联后再与所述正温度系数热敏电阻5并联形成串并联支路；所述串并联支路一端为所述限压电路的输入电极端1，其另一端引出一个抽头为所述限压电路输出电极端2；所述串并联支路的另一端还与所述压敏电阻4串联；所述压敏电阻4一端与所述串并联支路连接，其另一端为所述限压电路的公共电极端；所述正温度系数热敏电阻5与所述压敏电阻4热耦合。
具体的，在实际使用中，将限压电路的输入电极端与220伏电网线路连接，将其输出电极端与负载电路(以电能表开关电源为例)连接，开关电源正常工作的电压范围在110～330伏之间，此时选择压敏电阻的压敏电压为390伏，其最大允许交流工作电压为250伏。根据电网电压的稳定状态和电能表的工作状态，本发明的限压电路会有三种不同的工作方式，分别如下：
第一种工作方式：当输入电极端接入的电网电压低于250伏时，压敏电阻会处于高阻不发热的状态，热敏电阻也处于常温状态，热敏阻值为零功率阻值，为低阻状态。此时不管电能表是在计量状态还是通信状态，流过热敏电阻的电流在几毫安还是几十毫安，都低于热敏电阻在常温时的最小动作电流，不能引起热敏电阻发热、电阻值上升。此时，由于热敏电阻处于低阻状态，热敏电阻上的压降非常小(分压极小)，可以近似认为限压电路不起作用，电网电压几乎直接加在电能表的开关电源上。
第二种工作方式：当输入电极端接入的电网电压超过250伏(处于异常电压值)时，压敏电阻开始有电流流过并开始发热，压敏电阻产生的热量热耦合到热敏电阻使热敏电阻的阻值因受热而增大，热敏电阻上的分压加大，进而减 缓了流进压敏电阻的电流，随着电网电压的升高，当流入压敏电阻的电流值达到10～30毫安时，就可在居里温度点上方达到热平衡，热敏电阻分担了一部分异常过电压，使输出到开关电源端的电压限制在330伏。这时如果电能表工作在计量状态，流过热敏电阻的电流只需要分流几个毫安流入开关电源，这点增量电流产生的热量并不能使热敏电阻的温度脱离热平衡点继续上升，热敏电阻的阻值也就不会继续增大，热敏电阻两端的分压也就不会继续增加，此时如果将限压电路中的电压控制型开关的导通电压值设置为高于这个分压值的电压值，电压控制型开关就不会打开导通，相应地，则不会有电流从串联有电压控制型开关的支路流过。此时，限压电路的输出功率能够保证电能表在计量状态下的正常工作。
当电网电压恢复正常后，压敏电阻两端的电压小于250伏，压敏电阻又回到高阻不导通状态，没有热量产生，热敏电阻耦合不到热量，阻值开始下降，其分压随之降低，最终回到第一工作方式。
第三种工作方式：如果电能表又处于电网电压异常状态，又工作在通信状态，限压电路输出到开关电源的电流需要增加到几十毫安，就会使热敏电阻因通过电流的增加而额外产生1瓦以上的功耗，产生的额外热量就会使热敏电阻的温度脱离热平衡点继续上升，在正温度系数热敏电阻的居里温度点上方，温度的微小上升会导致阻值的急剧增加，热敏电阻两端的分压也会迅速上升，输出到电能表开关电源端的电压也就会下降。一旦热敏电阻两端的分压超过了电压控制型开关的导通电压，开关就会导通，将电阻器与热敏电阻并联，提高了限压电路的功率输出能力，保证了输出到开关电源端的电压维持在110～330伏之间。
为使本发明的限压电路能够适应电网电压的稳定状态和电能表的工作状态 实现第二种与第三种工作方式的切换，该电压控制型开关的导通电压的选值应满足使其大于电网最大异常电压峰值与所述限压电路允许输出的最高电压峰值的差值，小于电网最大异常电压峰值与所述限压电路允许输出的最低电压峰值的差值。在本发明进一步的实施例中，电压控制型开关可选用放电管来充当，当用在接入220伏电网的电能表限压电路中时，该放电管的直流击穿电压值可选在70～380伏之间。
进一步的，与电压控制型开关串联的电阻器的阻值的选值下限应满足使其在通过电能表切换到通信状态后的最小工作电流时，此阻值产生的压降等于电网最大异常电压值与输出到开关电源端的最大允许电压值的差值。电阻器的阻值的选值上限应满足使其在通过电能表切换到通信状态后的最大工作电流时，此阻值产生的压降等于所述限压电路输出到开关电源端的最大允许电压值和最小允许电压值的差值。如果设定工作在220伏电网中的电能表，通信状态下需消耗10瓦功率，开关电源端电压为330伏最高允许值时，输入的工作电流为最小值30毫安，开关电源端电压为110伏最低允许值时，输入的工作电流为最大值90毫安；电网最大异常电压为380伏。那么，电阻器阻值的下限就是RL＝(380伏—330伏)/0.03安培＝1.7千欧，电阻器阻值的上限R H＝(330伏—110伏)/0.09安培＝2.4千欧。在这里，为了方便说明工作原理并省去繁杂的非线性计算，本设定忽略了放电管的最低维持导通电压值的分压和热敏电阻在不同阻值状态下的分流影响，在实际应用中，可以进行一些工程修正，只要有实验验证支持，阻值的上下限有很大的误差都是可以接受的。
按照上述思路优选元件参数后，在电网异常电压达到330伏以上高位时，一旦电能表进入通信状态，电压控制型开关(放电管)就会导通，输出到开关电源端的电压就会降到110～330伏之间，从而保证了电能表开关电源的正常工 作。而这时压敏电阻将减少或停止发热，致使热敏电阻阻值下降，流过热敏电阻的电流增加，相应的流过电阻器的分流开始减小，当热敏电阻阻值降低到一定程度后，压敏电阻两端的电压又开始上升，压敏电阻又开始重新发热，阻止了热敏电阻阻值的进一步下降，输出到电能表开关电源的电压又重新平衡在330伏。
当电能表结束通信功能回到计量状态后，流入电能表开关电源的电流从几十毫安骤降为几毫安，流过电阻器的分流就会小于放电管的最低维持放电电流(该维持电流需大于约20毫安才能保持放电管导通)，放电管就会断开，电路回到第二工作方式的状态。
如果电能表仍然还工作在通信状态时，电网电压就已经恢复到正常电压，这时压敏电阻也会停止发热，热敏电阻阻值下降，分流增加，导致流过电阻器的电流减小，直到流过电阻器的分流减小到小于放电管的最低维持电流或者热敏电阻两端的电压降低到到小于放电管的最低维持电压(该维持电压需大于约30伏才能保持放电管导通)，放电管就将断开，电路直接回到第一工作方式的状态。综上，本实例中的限压电路通过给热敏电阻并联一条由动态开关控制的电阻器分压电路(电压控制型开关)，动态提高了限压电路的功率输出能力，使限压电路的动态输出功率范围较宽，既能够适应于电能表既处于电网电压异常状态、又工作在通信状态时的功率要求，也保证了在跳变过程中输出至电能表开关电源的电压能够稳定在110～330伏之间，从而保证了电能表开关电源的正常工作。
除了在电能表的应用外，本发明的限压电路还可以应用在具有小功率开关电源的其它电器中，例如LED照明电器，在这些应用中，开关电源不一定要有大、小两种功率状态，对应的限压电路也就只有工作方式1和工作方式3两种 工作状态。
以上所述，仅为本发明具体实施方式的详细说明，而非对本发明的限制。相关技术领域的技术人员在不脱离本发明的原则和范围的情况下，做出的各种替换、变型以及改进均应包含在本发明的保护范围之内。

Claims (10)

  1. 一种限压方法，将热敏电阻作为输入端串联接入电网和压敏电阻作为输出端并联接入电网后，在所述压敏电阻两端并联接入负载电路，当接入的电网电压高于预定电压处于异常电压时，通过热敏电阻和压敏电阻的热耦合作用，对负载电路进行限压保护，将输出至所述负载电路的电压限制在其允许输入的最小电压值与允许输入的最大电压值之间；其特征在于，在所述热敏电阻上并联一条电压控制型电阻支路，并设置相应的电压控制型电阻支路的导通条件；
    当所述负载电路功率变化超过预定值时，热敏电阻的温度高于其居里温度点上方的热平衡点时，使其两端电压上升从而触发所述电压控制型电阻支路导通，增大输出至被保护电路的功率范围。
  2. 如权利要求1所述的限压方法，其特征在于，所述电压控制型电阻支路包括：电压控制型开关与电阻器；所述电压控制型开关与电阻器串联。
  3. 如权利要求2所述的限压方法，其特征在于，所述电压控制型开关的导通电压值大于电网最大异常电压峰值与负载电路允许输入的最大电压值峰值的差值，小于电网最大异常电压峰值与负载电路允许输入的最小电压值峰值的差值。
  4. 如权利要求2所述的限压方法，其特征在于，所述电阻器的取值应该满足：当触发所述电压控制型开关导通后、所述负载电路处于最小工作电流状态时，所述电阻器产生的压降大于电网最大异常电压值与负载电路允许输入的最大电压值的差值；
    当触发所述电压控制型开关导通后、所述负载电路处于最大工作电流状态时，所述电阻器产生的压降小于负载电路允许输入的最大电压值与负载电路允许输入的最小电压值的差值。
  5. 一种限压电路，其特征在于，所述限压电路包括：电压控制型电阻支路、 正温度系数热敏电阻和压敏电阻；
    其中，所述电压控制型电阻支路包括：电压控制型开关与电阻器，所述电压控制型开关与电阻器串联；所述电压控制型电阻支路与所述热敏电阻并联形成串并联支路；所述串并联支路一端为所述限压电路的输入电极端，其另一端引出一个抽头为所述限压电路输出电极端；所述串并联支路的另一端还与所述压敏电阻串联；所述压敏电阻一端与所述串并联支路连接，其另一端为所述限压电路的公共电极端；所述热敏电阻与所述压敏电阻热耦合；所述限压电路采用如权利要求1-4任一所述方法，在接入的电网电压高于预定电压处于异常电压时，对被保护电路进行限压保护；并当负载功率变化超过预定值时，热敏电阻的温度高于其居里温度点上方的热平衡点，使其两端电压上升从而触发所述电压控制型电阻支路导通，增大输出至被保护电路的功率范围。
  6. 如权利要求5所述的限压电路，其特征在于，所述电压控制型开关为放电管，当接入的电网电压为220V时，所述放电管的直流击穿电压值为70V～380V。
  7. 根据权利要求5所述的限压电路，其特征在于，所述电阻器的阻值取值为1.7千欧姆～2.4千欧姆。
  8. 根据权利要求5所述的限压电路，其特征在于，当接入的电网电压为220V时，所述压敏电阻的标称压敏电压为390V，最大允许交流工作电压为250V。
  9. 根据权利要求5-8任一所述的限压电路，其特征在于，所述电压控制型开关、电阻器、热敏电阻和压敏电阻被封装成一体，相应的电极端由引线引出。
  10. 智能电能表中包含有采用权利要求1-4任一所述限压方法的限压电路。
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