How long does it take for the relay to open and close?
There are
so many factors that influence this question that there really is no clear answer that can be given. However, by building a test circuit, we can observe
the behavior
of
the relay
, including changes in inductance when the armature physically contacts the electromagnetic core, flight time, contact bounce and the impact of voltage, to explore the properties of the relay.
Hope this article can be helpful to you.
Let's take as an example the plug-in "
relays
"
often used in industrial applications
. That is, the relay is "inserted" between the two systems.
This article discusses relay operating speed. The hardware shown below is used to illustrate the principle.
Figure
1
: Picture showing an insert relay sandwiched between a Millenium Slim PLC and a three-phase contactor
Tech Tip
:
Turning off a large DC contactor or relay can be a stressful event. Recall that coils store energy in a magnetic field. Remember that the inductor "wants" to keep the current constant. The result is called an inductive kick, and the moment the coil is deenergized, the inductor does whatever is necessary to keep the current constant. Without relevant protection, the voltage will rise to hundreds of volts, causing arcing to maintain the current flow. This will destroy any semiconductor switches used to control the coil. Surge suppression diodes are often incorporated to provide an alternative path for current flow.
Next, let’s take a look at how long it takes to turn on and off the relay switch?
First, let's build a
test circuit for the relay opening and closing time.
Hardware parts:
Figure 2
: Test circuit for relay opening and closing time
3.
Relay closing time test
3.1
Analysis of relay shutdown test principle:
This relay driver may seem a bit complicated to design, but the high-side driver Q2 is actually necessary to connect the relay to ground. In this way, we can install a small value shunt resistor R5. Because this resistor is at ground, we can measure the relay current as easily as we would a small voltage drop across a known resistor.
The rest of the circuit includes the level-shifting transistor Q1, and a way to detect the state of the relay via the normally closed (normally closed) and normally open (NO) indicating LEDs.
Don't forget the D1 feedback diode on the relay coil. This diode protects transistor Q2 when the relay is closed. This diode has no effect on the relay opening, but it plays an important role in the relay closing.
Figure 3: Relay shutdown test schematic
Tech Tip: The difference between normally open (NO) and normally closed (NC) contacts of a relay
As shown below. The left side is normally closed (NC) type, and the right side is normally open (NO) type. Observe the difference between the two configurations of single-wire connections. We see the wire moving from NC (pin 12) to NO (pin 14).
Figure 4. Schematic diagram of normally open and normally closed relay experiments
Figure 5.
Finder (46 series) relay
More information:
Simple experiment: the difference between normally open (NO) and normally closed (NC) contacts of a relay
3.2
Test results:
The results are shown below. There are three panels:
-
Upper panel: The orange curve (CH 1) is the relay activation voltage measured at the collector of Q2. The blue curve (CH 2) is the relay current measured at the R5 shunt resistor.
-
Middle panel: The blue curve (CH 2) is the voltage measured at the normally closed contact of the relay.
-
Lower panel: The blue curve (CH 2) is the voltage measured at the normally open contact of the relay.
Tech Tip: Analog Discovery 3 operates as a dual-channel oscilloscope. When equipped with the 10X probe, it is capable of measuring signals up to +/- 250VDC. If using a 4-channel oscilloscope, the composite plot below can be constructed as a single screenshot.
Figure 6. Relay activation waveform, including coil current, normally closed and normally open contact voltages
Based on the data in the above figure, we observe:
-
电枢运动首次观察到在 4.7ms 时，常闭触点切换。
-
从4.7ms 到 7.6ms 有 2.9ms 的飞行时间。在这个“飞行时间”中，常闭和常开触点都没有连接到电路。
-
与常开触点的第一次接触发生在 7.6ms。
-
从7.6ms 开始一直延伸到8.8 ms，触点反弹 1.2ms 。
除了这些触点的变化，继电器电流中有一个微妙的下降。这发生在电枢运动时。推测，继电器电感变化，因为电枢的铁板与线圈的金属芯有物理接触。线圈电感的突然变化扰乱了继电器电流的缓慢斜坡。注意，如果电枢对着线圈保持在位置上，这种扰动就不会发生。
如果你想了解继电器关闭速度，请看下面内容：
继电器知识点 -
继电器的关闭需要多长时间?
4.1
继电器打开测试原理解析：
和继电器开闭测试原理图稍微有一点不一样。对高侧驱动程序和R4 的位置有轻微的修改。将原来的 MPSA56 换成了电压更高的
2N5401
。这是必要的，因为当继电器停用时，我们将遇到更高的电压。R4 电阻移动，使其与反激二极管 D1 串联。
图7. 继电器打开测试原理图
技术小贴士
:你可能会反对在这种高压情况下使用 1N4001 二极管。毕竟继电器 K1 的感应反打会发展到近 100V。然而，在这种情况下，1N4001 二极管并没有受到压力，因为当继电器断电时，它会传导约 0.7VDC 的二极管降。在正向方向上，它将遇到一个 24VDC 。预期的电压和电流都在 1N4001 二极管的设计最大值之内。
电感中的磁场会储存能量。当我们关闭晶体管Q2时，磁场会崩溃，从而在K1线圈上产生电压尖峰。如果我们把继电器（或更准确地说，是继电器内部的电感）想象成一个人，我们可以说电感在晶体管Q2关闭前后都试图保持电流恒定。
电感与“恒定电流”作用相关的特性会产生电压。如果不加以控制，这个电压会上升到几百伏甚至上千伏，以维持电流。过高的电压会破坏晶体管Q2，除非它被钳制住。
请注意，我们正在使用高边驱动器（Q2）来控制继电器。请花点时间观察这个电压尖峰的极性。许多读者可能会基于之前用低边NPN晶体管驱动的继电器实验，而假设这个尖峰是正的。但在这个例子中并非如此。相反，当Q2关闭时，在Q2集电极测得的电压会立即从24伏直流电压变为负电压。这个尖峰的幅度只受到电阻R4和二极管D4正向传导的限制。请参考上面电路图，确保你明白当Q2集电极变为负电时，二极管D4是正向偏置的。
大多数系统都不会额外添加电阻R4，而是直接将续流二极管跨接在继电器线圈上。这种配置非常常见，比如这次实验用的工业继电器，就包含一个可选的二极管模块4，如下图所示。
图8. 实验中所用Finder继电器的续流二极管和LED指示模块
这个并联二极管使用起来既有效又简单。但遗憾的是，它会导致继电器打开速度变慢。这是因为继电器的打开速度与其电感时间常数有关，而电感时间常数由下面的公式决定：
其中，L是继电器线圈的电感，R是继电器内部电阻。与原始驱动电压（本例为24伏直流电压）相比，二极管实际上相当于短路。
回想一下你在学校学到的电容放电电路，当时遇到的初始充电问题，能量就是通过电阻耗散的。这里的情况与之类似。能量储存在电感器的磁场中，当与电源断开时，端子就会短路，所有能量都在电感器内部电阻和少量二极管中消耗掉。
结果如下图所示，包含三个面板：
-
上面板：橙色轨迹（CH 1）是Q2集电极测得的继电器激活电压。蓝色轨迹（CH 2）是跨接在R5分流电阻上测得的继电器电流。
-
中面板：蓝色轨迹（CH 2）是继电器常闭触点测得的电压。这个常闭触点正在由断开状态转为关闭状态。
-
下面板：蓝色轨迹（CH 2）是继电器常开触点测得的电压。
图9.
继电器的断电波形，包括线圈电流、常闭和常开触点
根据上图的数据，我们观察到：
前面文章中提到的电流中的摆动仍然存在。这种飞行时间内的变化是由于衔铁的金属板离开电感器的中央铁芯时电感发生变化造成的。
与前面的文章相比，带有并联续流二极管的继电器打开速度较慢。在这个L/R系统中，继电器从激活到最终弹跳需要8.8毫秒，我们定义这个时间为t0。它关闭需要14毫秒。
如果你想提高继电器提高打开速度，请看下面内容：
我们是不是走进了死胡同？
工程师们的集体智慧告诉我们，继电器更快地断电是一个值得追求的目标。工作原理是线圈能量耗散会导致触点更快移动。这应该能延长继电器的使用寿命，因为更快移动的触点能更好地熄灭负载下触点打开时自然形成的电弧。
不幸的是，这个有限的实验并不支持这个理论。相反，它表明继电器的飞行时间与L/nR时间常数关系不大。回想一下，我们定义飞行时间为双极触点的时间，即衔铁在运动但没有与常闭或常开触点连接的时间。
这种矛盾的想法也得到了接触弹跳时间和弹跳特征的支持。就像一个篮球，移动更快的触点不是有更多的动能，导致它跳得更高，持续弹跳更长时间吗？但看起来并不是这样。
对此，你有何看法？
更多有关
继电器
技术文章，请点击以下链接，也欢迎大家在文末留言讨论。
The working speed of a relay is one of its important performance characteristics. Understanding this characteristic can help us select the appropriate relay, optimize circuit design, and achieve more precise equipment control. However, the switching time that affects its working speed is not easy to obtain. Friends, you may wish to obtain it through the methods provided in this article. Do you have any other methods, experiences or questions about obtaining the relay switching time?
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