Reasons for contact ablation in pressure controllers and improvements

Thread Content

Due to the advantages of pressure controllers, such as unrestricted installation orientation, easy adjustment, and high control precision, pressure switches are widely used in various fields. However, many pressure controllers have experienced ablation of the microswitch contacts since they were installed and put into use, resulting in erroneous operations that affect safe operation. This article mainly discusses the causes of contact ablation in pressure controllers and the methods for improvement. 　　Based on the experience of technical personnel, there are mainly two reasons for the ablation of the contacts in pressure controllers: one is the load DC relay coil of the microswitch; since this coil is inductive, the magnetic energy stored in it is released, generating a strong arc that causes the electrical contacts to ablate. Another type is the microswitch contacts, which generate an open arc when energized, especially when they are disconnected. Since the pressure changes continuously and gradually, the pressure-sensing element (bimetallic strip) causes the microswitch contacts to move in small increments at times. The opening of the contacts is not sudden; continuous discharge sparks are generated, resulting in contact ablation (pits appear on the positive pole, while small protrusions form on the negative pole). To address these two issues, we made the circuit modifications as shown in the figure below. The guiding principle for improving this circuit is to reduce the current value when the microswitch is turned on and off. Use transistors (bipolars) to replace microswitches as the load control element. The improved working principle is as follows: when a signal current indicating a pressure drop is transmitted in the circuit. Upon contact, YK acts as a trigger signal, enabling BG to conduct current; the coil of relay JZ11 is energized and thus closes, allowing current to flow in each circuit and enabling the load power switch to operate. When the pressure rises to a certain level, YK breaks the connection, BG stops functioning due to the lack of a triggering signal, and the coil of relay JZ11 loses its energy and opens. A pressure-sensitive element (bellow) is used to control the on and off state of the microswitch YK, thereby achieving control over the connection and disconnection of the load. 　　As can be seen from Figure 2, the current flowing through the microswitch YK when it is closed is:
Ib = Ee/Rb + (1+β)Ra
= 220/910 + (1+45)×4600
≈ 1.03 mA
Compared to the current that flows when the original microswitch is used (with the operating current of the intermediate relay being 50 mA), this value is about 50 times lower. Therefore, the microswitch will no longer experience ablation. 　　The operating voltage of the relay in the middle of JZ11 is: Va=1+β)Ib, Rc≈217.9V; therefore, the intermediate relay can operate reliably. 　　Through experiments, the improved pressure controller was connected to the DC 220V JZ11 relay, and after being powered on continuously for 100 times over a period of half an hour, the JZ11 relay functioned reliably each time. The experimental parameters were as follows: Ib=1.02mA, Va=221V, Vrb=0.95V, Vbr=0.71V, Vcr=1.6V.