Path Design of Hot Negative Hydrolysis Vacuum Gauge
From the operating principle of the hot negative electrode hydrolysis vacuum gauge, it can be seen that the measurement of Ii is not difficult.It is nothing more than the use of a high output impedance calculation amplifier and the use of electrical dispersion offset method to amplify the DC signal. The vacuum relay switches the feedback resistor to grasp the added value. The measured vacuum is not very low. It is a good step to adopt logarithmic amplification. It is necessary to pay attention to the heat drift. The focus of the design is on the stable grasp of the two voltages and electron currents. How the channel design works is the key. Carefully synthesize the principle of Figure 1 and synthesize it using the equivalent model of Figure 2. Among them UC = 200V, UF = 50V, Ie = 0.5mA (when 10-1Pa ～ 10-4Pa) or 5mA (when 10-4Pa ～ 10-8Pa). It can be seen from Fig. 2 that the ion current Ii does not have to do with Ie, UC, and UF, so it is only necessary to consider the path design that the other three master each other.
Figure 2 Equivalent mold for hydrolysis regulation
It is not difficult to find from Figure 2 that UC is an absolute golden chicken independent quantity, so the channel adopts a voltage regulator to cash in. Obviously, the crux of the problem lies in the need to ensure that both Ie and UF are stable at the desired value. Ie is composed of the electrons emitted by the heating of the silk (hot negative electrode) by the higher potential electrode (anode), so the absolute potential of the two electrodes is vivid, so Ie's grasp is realized by the direct current of mastering the silk. There are many steps to achieve these two parameters, and the step of adopting series resistance is the easiest and most reliable. As shown in Figure 3, then the resistance R = UF / Ie inserted in series, the problem is greatly simplified, so only using UF as the mastering goal can achieve the design goal. Then the opposite electron flow Ie is needed, and only the resistance value of R can be changed, and concrete realization can adopt the steps such as a substitute to switch the opposite R to lose the required Ie.
Figure 3 Sampling points
Therefore, it is necessary to share the power supply with the rest of the channels, and consider the characteristics of small silk resistance and large DC power.The steps of pulse-width modulation (PWM) to grasp the push-pull conversion have been adopted, and the PWM chip has adopted the widely used UGN3525PWM controller.The specific principle The block diagram is shown in Figure 4.
Figure 4 Principle of vacuum gauge grasp
Transformer T1 has two sets of secondary, of which: one set of input 160V exchange electricity, 200V DC power is easily input to provide the electrode potential UC; the other group provides the 24V DC power required by the entire set of filter voltage stabilization. R in FIG. 3 is composed of (R1 // R3) + R4 + R8 + R6. Therefore, the emitted electrons of silk have nothing to do with the nominal heat of silk, and because of the thermoelasticity of silk, the electron flow does not change with high-frequency silk direct current, but is rather smooth. In view of this, the input of the conversion transformer does not have a whole set of filtering paths, but instead provides high-frequency exchange electricity to silk indirectly. In that way, the use of large DC power devices is prevented, which undoubtedly increases the design difficulty of the transformer and the entire set of voltage stabilization paths, and the simultaneous sound greatly reduces the size of the whole machine. UF is sent to the inverting output terminal of UGN3525 in the PWM control path through the divided voltage at the sliding end of the potentiometer R8, and is compared with the external reference of UGN3525. If it is lower than the reference voltage, the PWM control path will increase the pulse amplitude. The universal input DC power of the T2 secondary is popularized by the power conversion. The increase of silk direct current will increase the electron flow, and the increase of the electron flow will cause the partial pressure on R8 to pick up, and vice versa.
Because of the thermoelasticity of silk, easy feedback can easily produce the scene of silk sparkle. The electron flow Ie will be greatly stabilised by the silk sparkle, and the ion flow Ii will also be stable, which will bring great hardship to the amplification of the search path. Therefore, C3 is added in the path to provide an exchange bypass for the ripple of the electron flow Ie, to prevent the ripple from being attenuated by the partial pressure of the resistor, and to effectively grasp the ripple of the electron flow.
The use of R5 and C4 can not effectively restrain the stability of the power supply. Moreover, it can weaken the change of the input of the PWM controller, thereby weakening the stability caused by overshoot. A proper amount of integration links is unnecessary in the differential amplification link of the PWM mastering channel. The time constants of the integrated integration links have an indirect relationship with PWM efficiency and silk characteristics, so they must be carefully selected during theoretical use.
In order to protect the silk from being burnt out of control, measures such as infinite flow are also required in the path. R7 in Figure 4 can play a role of current limiting. In addition, when the measurement vacuum reaches above 10-6Pa, pay attention to increase the performance of the degassing path and degas the electrode.
The vacuum gauge channel designed using the above steps has been used in large quantities as a whole of our production, and its performance is stable and reliable. Because the PWM control is adopted and the DC conversion of the transformer is adopted, there is no indirect grasp of the large DC current of the hot negative electrode, which increases the power consumption, greatly reduces the unit of the large DC heating element, and makes the temperature rise of the whole machine lose invalid restraint. Then you need to design a commercial vacuum gauge, and only add amplification, search and foresight.