Selection of the pressure sensitive resistance
1, Zinc Oxide application principle of pressure sensitive resistors
Pressure sensitive resistance is a kind of voltage limiting protection device. By using the non-linear characteristic of the pressure sensitive resistance, when the over-voltage occurs in the voltage between the two electrodes, the voltage clamp can be clamped to a relatively fixed voltage value, so as to realize the protection of the circuit. The main parameters of the pressure sensitive resistance are: voltage, current capacity, junction capacitance, response time and so on.
The response time of the varistor is ns grade, faster than the air tube, tube slightly slower than TVS, electronic circuit for overvoltage protection of the response speed can meet the requirements of the general situation. Junction capacitance varistors in hundreds to thousands of pF magnitude range, not many cases direct application in the protection of high frequency signal line, used in the protection circuit in communication, because of its larger capacitance increase in leakage current, give full consideration to the needs in the design of protection circuit. Varistor flow capacity is larger, but the ratio of the gas discharge tube is small.
Voltage sensitive resistor in parallel with protected electrical equipment or components. When the circuit is in lightning overvoltage and transient overvoltage Vs, equipments and components of varistors and protected at the same time under Vs, the varistor response speed is fast, with nanosecond time quickly showed excellent conductive properties of nonlinear (shown in Figure 3, the varistor breakdown region) voltage decreases rapidly, far less than Vs, voltage equipment and components that protected the actual bearing is far lower than the voltage of Vs, so that the equipment and components from overvoltage impact.
2, Zinc Oxide pressure sensitive voltage of choice
According to the voltage of the protected supply voltage, the voltage of the specified current is V1mA. General selection principle is:
For the DC loop: V1mA = 2.0VDC
For AC circuit: V1mA = 2.2V effective value
Note that the selection criteria of varistor varistor voltage is higher than the supply voltage, to meet the protection needs while protecting the device can, as far as possible to choose the varistor varistor voltage is high, which can not only protect the device, also can improve the service life of the varistor. For example, to protect the breakdown voltage is Vdc=550Vdc, the working voltage of the V=300Vdc device, so we should choose the varistor varistor voltage for 470V varistor, the varistor voltage range (423-517), the power supply voltage the breakdown voltage of 300Vac 470-47=423Vdc device is greater than the maximum negative error, maximum error is less than 470+47=517Vdc 550Vdc positive pressure device.
Must also pay attention to the selection:
(1) it is necessary to ensure that the continuous operating voltage will not exceed the maximum allowable value in the voltage fluctuation, otherwise it will shorten the service life of the pressure sensitive resistance;
(2) in the power supply line and the earth between the use of pressure sensitive resistance, and sometimes due to the bad grounding between the line and the ground voltage rise, so the use of more than the line between the line and the use of higher voltage of the voltage sensitive resistor.
3, through the selection of flow
Usually the product is given through the flow is given by the product standard waveform, shock times and the gap time pulse test can withstand the maximum current value of the product. The impact of number of products can bear is a function of waveform, amplitude and time, when the current waveform amplitude decreased 50% times of shock can be doubled, so in practical application, the maximum flow rate surge current absorption pressure sensitive resistance should be less than the product.
Figure 1 shows the circuit connection diagram of the circuit surge and transient protection using the voltage sensitive voltage regulator. For the application of the pressure sensitive resistance, can be divided into four types:
The first type is the connection between the power supply line or the power line and the earth, as shown in Figure 1 (a). As a voltage sensitive resistor, the most representative of the use of the power line and the long distance transmission of the signal line to meet the lightning strike and so that there is a surge in the case of electronic products, such as the protection of the protection of electronic products. General online access between the pressure sensitive resistor can be effective between the lines, and online and to access the pressure sensitive resistance between the transmission line and the ground between the induction pulse effective. If the connection between the wire and the wire connection is combined in two ways, it can absorb the surge pulse better.
Second types of connections in the load, see Figure 1 (B). It is mainly used to absorb the inductive pulse caused by the sudden opening and closing of the inductive load in order to prevent the component from being damaged. Generally speaking, as long as in parallel with the inductive load on it, but according to the current type and energy of different size, and can be considered RC series absorption circuit combination.
The third type is the connection between the contacts, see Figure 1 (C). This connection is mainly to prevent the induced charge switch contact arc is burned, and the general contact varistor can be connected in parallel.
Fourth types are mainly used for the protection of semiconductor devices connected, see Figure 1 (D). This connection is mainly used for thyristor, power transistor and other semiconductor devices, commonly used methods and the protection devices in parallel, to limit the voltage protection device is lower than the rated voltage, which is an effective protection of semiconductor devices.
5, selection principle
If the voltage level of electrical equipment Vo is low, while the surge energy is relatively large, you can choose the varistor voltage V1mA is low and the chip size larger varistors; if Vo is high, you can choose the varistor varistor voltage V1mA high, which can protect the electrical equipment, and can prolong the service life of the varistor.
Pressure sensitive resistor is mainly used in various electronic products over voltage protection circuit, it has a variety of models and specifications. The main parameters of the selected voltage sensitive resistors (including the nominal voltage, maximum continuous working voltage, maximum limit voltage, current capacity, etc.) must meet the requirements of the application circuit, especially the nominal voltage to be accurate. The nominal voltage is too high, the pressure sensitive resistor is not over voltage protection, the nominal voltage is too low, the pressure sensitive resistor is easy to malfunction or breakdown.
6, Zinc Oxide‘s use of pressure sensitive resistors
Varistor is a kind of non polarity overvoltage protection element, either AC or DC circuit, only the varistor and the protected electrical equipment or components in parallel to achieve the purpose of protection equipment (Fig. 4)
When the voltage is higher than the prescribed voltage current, peak current current amplitude is less than the varistor (if the varistor is enough to make equipment components damage), varistors in breakdown region, can be instantaneous over voltage limit in low frequency, the amplitude of inrush current through little varistors (<100A/cm2), is not sufficient to produce a deterioration of the varistors; voltage amplitude is high when the varistor will, over voltage limit at a relatively low level (less than the voltage level of equipment, at the same time) through the impact current pressure-sensitive resistor is large, the performance of varistor deterioration failure coming, when the current through the fuse great, the fuse opens, the electrical equipment and components from overvoltage impact can be avoided due to deterioration caused by line breakdown voltage sensitive resistor Short circuit between L-N, L-PE (recommended fuse specification see Table 1).
In the overvoltage protection of the circuit, the voltage sensitive resistor can not be damaged in theory if it is working in the pre breakdown and breakdown area of Figure 3. But due to the power supply voltage varistors under long-term circuit transient overvoltage and overvoltage impact super energy absorption and random circuit of energy storage element release energy, therefore, also can damage the varistor, its life varies according to the voltage and energy is subjected to different circuit.
In the electronic ballast and energy saving lamps over-voltage protection of the pressure sensitive resistance, generally less than 20W selection of MYG07K series, 30W-40W generally use MYG10 series of pressure sensitive resistance to do over-voltage protection
First, the problem of the connection line of the pressure sensitive resistance
The connecting wire varistor circuit access to rough enough, the size of the line connecting note recommended: ground for more than 5.5 mm2 connecting line should be as short as possible, and walk the line, because the impact of current will produce additional voltage on connection inductance, the protected equipment at both ends of the voltage limit.
|Pressure sensitive resistance through flow
||≥ 0.3 mm2
||≥ 0.5 mm2
||≥ 0.8 mm2
||≥ 2 mm2
For example: if the MY varistor at both ends of each of the 3 cm long wiring inductance L it roughly 18 nH, if the 8/20 impact current of 10 KA into the varistor, the current rise speed of 8 m / 10KA as s, while the additional inductance voltage UL1, UL2 generally
UL2=L UL1= (di/dt) 10-9 * =18 (10 * 103 * 10-6) V =22.5
This makes the limit voltage increased by 45V.
Two, the series and matching of the pressure sensitive resistance
The pressure sensitive resistance can be easily used in series. In series, the two resistors are connected in the same diameter (with the same flow rate), and the voltage of the paint is constant, and the constant working voltage and the limiting voltage are added together. For example, in the high-voltage power surge arrester, requiring continuous operating voltage of up to several thousand volts, tens of thousands of volts, is the multiple ZnO pressure sensitive resistance valve and up (in series) and get the.
The pressure sensitive resistor can be connected in parallel, which aims to obtain larger flux, or reduces the current density in the resistance body under the condition that the impulse current peak value is certain.
When asked to get through traffic, great such as 8/20 (50 ~ 200) KA, and the breakdown voltage is low (e.g., less than 200V), the resistance body diameter / thickness ratio is too large, there are difficulties in manufacturing technology, and with the increase of the diameter of the resistor, the resistor micro homogeneity therefore, through flow may not with resistor area in proportion to increase. At this time, it may be a more reasonable method to use the small diameter of the resistance chip in parallel.
Due to the high non linearity, the parallel connection of the pressure sensitive resistance sheet need to be especially careful, only after careful matching, the parameters of the same resistance chip in parallel, to ensure that the current in the resistance between the uniform distribution. According to this demand, the company dedicated to provide users with a pair of resistance.
In addition, the longitudinal connection of several voltage sensitive resistors, using a pair of matched after the parameters of the pressure sensitive resistor, when the impact of intrusion, appear in the transverse voltage difference can be very small. In this case, pairing is also meaningful.
Three, the series and parallel connection of the pressure sensitive resistance and gas discharge device
Varistor can and gas discharge tube, air gap, micro discharge gap gas discharge device connected in series (Figure 10.5a), the series combination of the normal work to meet the two basic conditions: first, the system voltage limit DC breakdown voltage should be lower than the gas discharge device G; the fire in the system, G the upper limit voltage, arc current varistor in MY should be less than G to maintain the current, in order to ensure the arc G.
This combination has a series of small capacitance, high working frequency; small leakage current and good safety; and does not exist in MY system varistor voltage ageing, and high reliability, but also the corresponding slow gas discharge device caused by "voltage".
The varistor is also available with the gas discharge tube in parallel, in order to reduce the impact of the ignition voltage of the gas discharge tube.