How to Prevent a Spark at Relay Contacts?
The relay contacts’ jumping or switching on/off will cause the change of inductive load and further gas discharge. The current of relay contact on-off is small, there will be no electrical arc between contacts, but there will be a spark at the relay contacts. Due to the existence of inductance in the contact circuit, there will be overvoltage on the inductance when disconnecting the contact circuit. Together with the voltage supply in the contact gap, the contact gap that just separates a little will be broken down and discharged. Limited to energy, there will only be spark discharge. The conversion of capacitance between contacts and energy in inductance makes spark discharge disappear and reappear, becoming a high frequency signal, which will be crosstalk to related circuits by means of high frequency radiation, wire transmission and distributed capacitance, forming interference signals. Furthermore, spark discharges can also damage relay contacts and shorten the service life of contacts, so that sparks may ablate contact surfaces and make the surface uneven, resulting in failure of relay poor contact.
Therefore, the spark at the relay contacts should be avoided. And the practical preventing spark circuit is RC anti-spark circuit, as shown in the figure below. In the dotted frame of the figure, it is a semi-RC absorption circuit. The basic principle of RC anti-spark circuit is to connect the R and C phases in series, and then connect at both ends of the relay contacts in parallel, so that the energy in the inductance will not pass through the contacts but RC, and it absorbs only the self-induced potential generated when the contacts are disconnected. At the moment when the contacts of relay are connected, there is no absorption because the RC is short-circuited. Outside the dotted line frame, it is a fully-absorbed circuit. When the contacts are disconnected, the self-induced potential is exhausted on the load rL by the diode V.
In practical application, either one of the above circuits is feasible. However, it should be noted that the RC parameters should be appropriately selected. The parameters are mainly determined by experiments, and the capacitance C can be selected according to the load current 1A/1μF. When using a diode, the positive and negative polarities should be connected correctly, and the voltage withstand of the diode V should be sufficient. It should be pointed out that RC anti-spark circuit should be installed close to relay contacts, and the connection line should be shorter as possible to ensure the effect of anti-spark circuit.
Now take the automatic temperature control system of heat treatment furnace for example.
The system controls the contactor on/off with temperature controller (PID regulator with relay control output) and intermediate relay and then controls the electric heater. However, due to great fluctuation of temperature controller in use, especially the greater fluctuation with the more precise temperature control, the action of the intermediate relay will be more frequent. At the moment when the contacts of relay are connected, contacts will be impacted by several times more than the normal working current, while the electromagnetic energy stored in the circuit will be released between contacts in the form of sparks at the moment of disconnection. The frequent action of relay contacts causes the heating, ablation and even welding of electric contacts, which can not be used after several times of polishing, and the relay has to be replaced. Since several times of the working current flow, the contactor has been heated, the displacement of the iron core and the air gap of the magnetic circuit will increase because of frequent movement, resulting in a great deal of electromagnetic noise.
The above problems not only increase the maintenance workload, but also affect the production sometimes. Therefore, the factory has reformed the circuit and used RC anti-spark circuit. The parameters selected by RC circuit are as follows: C is 100pF/1000V and R is 1.2kΩ/1W. The RC element is directly connected to the electric contacts of the relay, and the working current of the contactor is reduced. After those measures, the service life of contacts of the relay can be greatly prolonged with significant effect, which can not only meet the requirements of production, but also reduce the workload of maintenance and save maintenance costs.