How to calculate the power consumption of a designed device?

Question

Arduino

A weather station was created on the Arduino Nano, to which the temperature and humidity sensor DHT11 is connected, as well as the Nokia LCD 5110 display, which will display all the information.
According to the datasheet:
1) DHT11 current is 2.5 mA, and the supply voltage is 5 V, can we assume that the power consumption of the sensor is 12.5 mW?
2) The backlit display current is 20mA, and the supply voltage is 3.3V, can we assume that the power consumption is 66mW?
3) the Arduino current (information from their website) is 19 mA and we supply 5 V to it. Accordingly, the power consumption is 95 mW?
4) on one site it was said that one output consumes 40 mA, and since we have 6 involved (1 from the sensor and 5 from the display), can we assume that Arduino additionally consumes 240 mA, which means that an additional 1.2 W?
5) Does the sensor polling frequency depend on the current consumed, if so, how?

Answer 1

Measure the current consumed by the weather station, multiply by the supply voltage - and get answers to all questions.

Answer 2

Wrong at the core.
1. 5V voltage came from somewhere. Those. there is a certain stabilizer which has efficiency. If you have a switching regulator like LM2576T, then it has an efficiency of 95%, and if 7805 or similar (as most likely it is), then it all depends on the input voltage, the same applies to 3.3V. As a result, we have (2.5+20+19)= 42.5mA. If the initial voltage is 9V, then the power consumption is 382.5mA
2. Arduino pins, do not consume anything (nothing at all). Consumes the load that is connected to these outputs. 40mA is the maximum allowable current, i.e. if you connect a small LED with a current consumption of 10-20mA, then everything will be fine, but motors, LED strips, etc. must be connected to the outputs through transistors. In your case, the information outputs of the sensor and display consume microamps, i.e. this current can be neglected.
3. The polling frequency does not depend on the current in any way. The current consumption depends on the clock frequency of the controller, according to the datasheet on the Atmega 328 at a frequency of 16 MHz (as in arduino), the current consumption is 19 mA, and at a frequency of 1 MHz from the internal generator 5 mA. at a very low frequency (for example, from watch quartz), the current is less than 1mA. Those. if you have nowhere to hurry, but you need to save a couple of mA, you can programmatically change the frequency to an internal generator (it will be difficult to replace the quartz). True, in such a situation, you will not be able to flash the board via USB, you will need a programmer for direct programming of the MK via SPI

Answer 3

4. 40 mA is the maximum allowable current that the output can deliver. In real interfaces, of course, there will be no such current.
5. Conversely, the current consumption depends on the sampling rate. How exactly it depends - it is impossible to say in advance. If the datasheet does not have a graph or formula for calculating this case, the only way to find out is to assemble the circuit and measure it.

Answer 4

The easiest way to measure current consumption is to power the circuit from a laboratory power supply and see what the indicator shows.