Let’s learn about the auxiliary contacts and external energy-saving boards in the high-voltage relay on the BMS
Latest update time：2021-11-22
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I have been busy doing testing recently. To be honest, there is still some pressure. It comes from the leadership and the project. In fact, it comes more from myself. If I want to deliver a high-quality product, on the one hand, it must be fully verified, and on the other hand, it must be fully verified. We also have to take into account the schedule. What should we do? We can only work overtime.
This time, let’s take a look at the energy-saving board and auxiliary contact on the high-voltage relay. The most representative one is the relay from TYCO, such as the model EV200HAANA, as shown below: It is an epoxy packaging type (the other common one is ceramic package type),
In the TE product specification book, you can see several features of EV200HAANA: it
has an external energy saver and a normally open (NO) auxiliary contact. We will focus on these two aspects this time. We will analyze it later. This relay is the basis.
From the appearance of the relay, you can see that there are two sets of external wiring, one is the auxiliary contact wiring, and the other is the coil power supply line.
Let’s first take a look at the characteristics of the auxiliary contact. Generally, the contact status when the relay is closed and opened is as shown in the figure below (from Panasonic’s official website): The contact of the relay has a bounce at the moment of closing or opening. ) process, this needs to be paid attention to during testing.
The picture below shows the timing between the main contact and the 12V power supply when the external power supply of this relay is 12V: the delay between the two is about 15ms, and it is found that the contact does have a transition period of closing and bouncing.
The figure below shows the closing timing diagram of the main contact and the auxiliary contact when powered on at 12V. It is found that both will bounce when closing, and the auxiliary contact closes before the main contact. The time difference between the two is about 500us. .
Similarly, the following figure shows the timing diagram of the main contact and the auxiliary contact when the coil is powered off. It is also found that the contact bounces when they are disconnected; from the timing above, the main contact is disconnected first, The delay between the two is about 500us.
Let’s turn our attention to the energy-saving board of this relay, as shown in the figure below: There is a rectangular plastic cover on the side of the relay, where the energy-saving board is placed.
Remove these two screws to expose the PCBA inside, and find that there are no components on the B side.
Look at the T side of this PCBA again. The devices are all arranged on this side. This small board has four external wiring harnesses: two of them are external power supply inputs (red and black wires), and the other two (black and black wires) are for relay coil power supply. output.
At this point, we need to answer a question first, that is, how does this energy-saving panel save energy? Regarding relay energy saving, it mainly refers to reducing the power consumption level of the coil. The specific implementation methods include dual coil scheme and PWM drive scheme. The working principle of the energy-saving board is still unclear. Let's take a look at the specific power consumption first.
As shown in the figure below, the nominal resistance of this relay coil is 3.14Ω, so when powered by 12V, the long-term working current should be 3.82A.
In addition, the specification book indicates that the maximum inrush current of the coil is 3.8A, which is consistent with the above;
the holding current of the coil is 0.13A@12V
, which is equivalent to the equivalent impedance of the coil being 92Ω; so the operating current is from 3.82A Reduced to 0.13A, this is the role of the energy-saving board.
Next, use an oscilloscope to actually measure the voltage and current waveform of the energy-saving board. The test bench is as follows: use a DC power supply to provide 12V power to the power supply terminal (red and black wires) of the relay, and use an oscilloscope to observe its power supply terminal (red and black wires) and the coil. The voltage and current at the terminal (black black wire).
The picture below is the power-on current waveform of the relay power supply end: indeed, the maximum current will reach about 4A at startup, and then after 100ms, the current waveform reaches a stable state.
Expand the current waveform in the steady state, as shown below: It can be seen that the energy-saving board uses PWM driving method.
Look at the voltage and current waveforms at the relay coil end: similarly, it reaches a stable state after about 100ms.
The waveform after the coil end is stabilized is expanded as follows: it is found that the current in the coil is floating up and down, but will not drop to 0A; and the voltage at both ends of the coil is switched periodically, which is very similar to a switching power supply.
From the above waveforms, we can draw a basic conclusion: when the energy-saving board of this relay works, it uses PWM mode to drive the coil, thereby reducing power consumption.
I haven’t had time to analyze the circuit principle of this energy-saving board and the structure of the internal contacts of the relay. I will learn more about it later when I have the opportunity.
Summarize:
In the past few years, I saw that people mostly chose relays from Panasonic and TYCO, but now they mostly choose relays made in China, such as Hongfa, SCII, BYD, etc.; all the above are for reference only.
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