Low power consumption is an extremely important concept in the Internet of Things. Most IoT nodes need to be powered by batteries. Only by correctly measuring the power consumption of the wireless module can we accurately estimate how much battery is needed for the 5-year battery life. This article will explain the detailed measurement methods for you.
In many applications of the Internet of Things, terminal devices are typically battery powered and have limited available power. Due to the self-discharge of the battery, the actual use of electricity in the worst case is only about 70% of the nominal power. For example, the commonly used CR2032 button battery, the nominal capacity of one battery is 200mAh, and actually only 140mAh can be used.
Since the battery's power is so limited, it is important to reduce the power consumption of the product! Let's take a look at the commonly used methods of measuring power consumption. Only when these methods of measuring power consumption are clear can the product power consumption be optimized.
Step 1: First, Power Consumption Measurement
The power consumption test of the wireless module is mainly to measure the current, and here is divided into two different tests of quiescent current and dynamic current. When the module is in sleep or standby state, because the current does not change, keep a static value, we call it quiescent current. At this time, we can use a traditional multimeter to measure, just need to connect a multimeter in series with the power supply pin to get the required measurement value, as shown in Figure 1.
When measuring the emission current of the module's normal operating mode, the total current is in a state of change due to the short time required for signal transmission. We call it dynamic current. The response time of the multimeter is slow, it is difficult to capture the changing current, so you can't use the multimeter to measure. For changing the current, you need to use the oscilloscope and current probe to measure. The measurement result is shown in Figure 2.
Step 3: Second, the Battery Life Calculation
Wireless modules often have two modes of operation, operating mode and sleep mode, as shown in Figure 3 below.
The above data comes from our LM400TU product. According to the above figure, the transmission interval between two transmission packets is 1000ms, and the average current is calculated:
In other words, the average current is about 2.4mA in 1 second. If you use a CR2032 power supply, you can ideally use about 83 hours, about 3.5 days. What if we extend our working hours to one hour? Similarly, it can be calculated by the above formula that the average current per hour is only 1.67uA. The same section of the CR2032 battery can support the equipment to work 119,760 hours, about 13 years! From the comparison of the above two examples, increasing the time interval between sending packets and extending the sleep time can reduce the power consumption of the whole machine, so that the device can work longer. This is why the products in the wireless meter reading industry are generally used for a long time because they only send data once a day.
Step 5: Third, Common Power Problems and Causes
In order to ensure the low power consumption of the product, in addition to increasing the packet interval time, there is also a reduction in the current consumption of the product itself, that is, Iwork and ISleep mentioned above. Under normal circumstances, these two values should be consistent with the chip data sheet, but if the user is not used properly, there may be problems. When we tested the module's emission current, we found that installing the antenna had a great impact on the test results. When measuring with an antenna, the current of a product is 120mA, but if the antenna is screwed off, the test current is soared to nearly 150mA. The power consumption anomaly in this case is mainly caused by the mismatch of the RF end of the module, causing the internal PA to work abnormally. Therefore, we recommend that customers take the test when evaluating the wireless module.
In the previous calculations, when the transmission interval is getting longer and longer, the working current duty cycle is getting smaller and smaller, and the biggest factor affecting the power consumption of the whole machine is ISleep. The smaller the ISleep, the longer the product life will be. This value is generally close to the chip data sheet, but we often encounter a large amount of sleep current in the customer feedback test, why?
This problem is often caused by the configuration of the MCU. The average MCU power consumption of a single MCU can reach the mA level. In other words, if you accidentally miss or mismatch the state of an IO port, it is likely to destroy the previous low-power design. Let's take a small experiment as an example to see how much the problem affects.
In the test process of Figure 4 and Figure 5, the test object is the same product, and the same configuration is the module sleep mode, which can obviously see the difference of test results. In Figure 4, all IOs are configured for input pull-down or pull-up, and the tested current is only 4.9uA. In Figure 5, only two of the IOs are configured as floating inputs, and the test result is 86.1uA.
If the operating current and duration of Figure 3 are kept constant, the transmission interval is 1 hour, which brings in different sleep current calculations. According to the results of Fig. 4, the average current per hour is 5.57 uA, and according to Fig. 5, it is 86.77 uA, which is about 16 times. Also using a 200mAh CR2032 battery power supply, the product according to the configuration of Figure 4, can work normally for about 4 years, and according to Figure 5 configuration, this result is only about 3 months! As can be seen from the above examples, the following design principles should be followed to maximize the duration of use of the wireless module:
1. Under the condition of satisfying the application requirements of customers, extend the interval of sending packets as much as possible, and reduce the working current during the working period;
2. The IO status of the MCU must be correctly configured. The MCUs of different manufacturers may have different configurations. Refer to the official data for details.
LM400TU is a low-power LoRa core module developed by ZLG Zhiyuan Electronics. The module is designed with LoRa modulation technology derived from military communication system. It combines unique spectrum widening processing technology to perfectly solve small data volume in complex environment. The problem of ultra long distance communication. The LoRa network transparent transmission module embeds the self-organizing network transparent transmission protocol, supports the user's one-button self-organizing network, and provides a dedicated meter reading protocol, CLAA protocol and LoRaWAN protocol. Users can directly develop applications without spending a lot of time on the protocol.