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This post was last edited by sdscj0122 on 2020-3-14 at 17:24. Simple additional circuits for relays are often used to modify certain characteristics of the relay, protect its electrical contacts, or safeguard other electronic components. This article provides a detailed introduction to the four types of additional circuits for relays: the acceleration engagement circuit, the delay operation circuit, the spark suppression circuit, and the protection transistor circuit. Relay auxiliary circuit: yunrun.com.cn/tech/2856.html. 1. Acceleration circuit for relay activation: For relays used in DC circuits, let R0 be the resistance of the coil itself. A resistor R is connected in series with the coil, and a capacitor C is connected in parallel with this resistor, as shown in Figure 1. When the switch K is closed, since the charging current of the capacitor also flows through the coil, the current passing through the coil for a short period of time is greater than the steady-state current I=U/(R0+R), which results in an accelerated response. If the series resistance R is still calculated based on the coil’s rated current, the actual current will exceed the rated value for a short period of time; however, the heating effect is not significant. http://yunrun.com.cn/upload/201912/30/201912301328071397.png Figure 1: Circuit for accelerating the relay’s engagement. The supply voltage in Figure 1 should be higher than that when no acceleration circuit is used, and the heat dissipation power of the resistors should be calculated based on the steady-state current. The capacitance value can be chosen as needed, as long as the voltage rating is higher than the supply voltage. The induced voltage when the circuit is interrupted cannot be applied to the capacitor. If the supply voltage is already fixed and the coil resistance is high, adding a series resistor may result in a steady-state current that is slightly lower than the holding current; at first glance, it seems that the above method cannot be used in such a situation. However, when the switch is first closed, the capacitor acts as a short circuit, and as long as the current during this period is greater than the holding current, the relay can still be activated. As for the steady-state current, although it is smaller than the holding current, as long as it remains greater than the releasing current, the holding state can be maintained. Therefore, the resistance value of the series resistor does not necessarily need to be calculated based on the pull-in current. Changhui Instruments reminds everyone to note that the accelerated pickup circuit should not be used with AC relays. 2. Delay action circuit: If capacitor C is connected in parallel across the coil, the circuit shown in Figure 2 is obtained. When the switch is closed, the charging current creates a voltage drop across R, which results in a slower increase in the voltage across the coil and thus an extended holding time. Similarly, when the switch is turned off, the discharge of capacitor C and its recharging due to the induced voltage further prolong the release time. http://yunrun.com.cn/upload/201912/30/201912301336028282.png Figure 2: Relay delay action circuit. If it is desired to merely prolong the release time, the circuit shown in Figure 3 can be used. When the power is turned on, the diode D is in the off state and does not function. But when the switch K is opened, the induced electromotive force in the coil will generate a current through the diode, causing the magnetic flux in the core to decay slowly and delaying the release action. http://yunrun.com.cn/upload/201912/30/201912301341122257.png Figure 3: Relay delay circuit (diode). The circuit in Figure 3 occupies less space than that in Figure 2; however, it only delays the release time, with no effect on the pickup time. Some relays have two coils on their core. Telephone relays are an example of this. The main coil is used to generate magnetic flux; if the ends of the auxiliary coil are short-circuited with diodes, the operating time can be extended. Depending on the connection direction of the diodes, the process can be made to proceed slowly either during absorption or during release. Properly applying the above methods can slow down the action time by 5 to 10 times. If transistor-based delay circuits are used, the delay can naturally be extended even further; however, that falls under the application of time relays. Note: The delay action circuit is limited to use with DC relays. 3. Spark suppression circuit: Eliminating sparks at the relay contacts is not only important for protecting the relay; it is also highly beneficial for explosion safety and for preventing electromagnetic interference. The two delay circuit configurations shown in Figures 2 and 3 also have a spark-suppression function. If switch K is considered to be the contact of another relay, when it disconnects the circuit, the induced voltage generated in the coil is short-circuited by capacitor C or diode D; once the energy is absorbed, no sparks will be generated at K. In addition, the circuits shown in Figures 4, 5, and 6 can be used. http://yunrun.com.cn/upload/201912/30/201912301344270380.png Figure 4: Spark-suppression circuit. Figure 4 shows a circuit in which the normally open contact KH of the relay is connected to an inductive load L. To prevent sparks from being generated at contact KH due to the induced voltage on L in the event of a power outage, an RC circuit is connected in parallel with contact KH. Since the voltage across C is zero when the circuit is connected, and there is no sudden change in the voltage across C when it is disconnected, the charging current creates a voltage drop across the internal resistance of L, thereby preventing sparks from forming at contact KH. By the time the voltage across the capacitor increases, the air gap at the contact point has grown to such an extent that arcing becomes impossible. Resistor R is used to prevent excessive discharge current of the capacitor when the contacts close. Generally, choose 2μF for C and 50Ω for R. The L in Figure 4 can also be the coil of an intermediate relay or contactor. http://yunrun.com.cn/upload/201912/30/201912301346273441.png Figure 5: Spark-suppression circuit (suitable for relay coils with or without inductive resistance). Some telephone relay coils have inductive resistance built into them; they are marked as “double-wound” and the resistance value is indicated, allowing for easy construction of a spark-suppression circuit. Figure 5 shows the use of resistors and coils in parallel, which also plays a role in protecting KH. Although some energy is consumed in the resistor, since its resistance value is several times higher than that of the coil, the economic implications resulting from energy consumption can be ignored. http://yunrun.com.cn/upload/201912/30/201912301345333155.png Figure 6: Spark-suppression circuit (suitable for relay coils with or without inductive resistance, where the inductive resistance is connected in series with the coil). In Figure 6, the inductive resistance is connected in series with the coil; then, a set of normally closed contacts KD on the relay is used to short-circuit this resistance. When the relay is powered and activated, KD opens, allowing the resistance R to be included in the coil circuit. As long as the current at this point is greater than the release current, the relay will remain in the activated state. However, due to the lower current level, no significant sparks are produced when KH opens. Relays with zero-resistance features are still a minority; those without such features can be constructed using discrete components, and it’s not difficult to do so, with very noticeable results. 4. Circuit for protecting transistors: If transistors are used as the switches to control the engagement and disengagement of a relay, then when the transistors are turned off, the induced voltage in the relay coil could damage the transistors. To protect the transistors, it is necessary to dissipate the energy of the induced voltage. Both of the circuits shown in Figure 2 and Figure 3 can accomplish this task, which is why they are often found in the electronic circuits that drive relays. In particular, using diodes connected in reverse parallel across the coil requires little space and is simple to implement. The four additional relay circuits shared in this article all fall under basic electrical knowledge, which is commonly encountered by instrument technicians and electricians; mastering their use can help solve various on-site problems. Recommended reading: Case studies on relay selection, How to correctly choose the alarm operation mode of relays, How to improve the breaking capacity of relay and contactor contacts