# Power calculation for a microcontroller and a peripheral device

Discussion in 'Power Electronics' started by Abhinavrajan, Aug 8, 2016.

1. ### Abhinavrajan Thread Starter Member

Aug 7, 2016
83
1
I have heard people talking about the power calculation of a device and mostly micro controller.

I haven't obtained much clarity on that.

Please give me an Explain to understand a bit more clearly.

Suppose, I have a microcontroller and few peripherals.
Input to the microcontroller is +5V and there are 3 peripherals which require a voltage and a load current of 3V 5A,5V 3A and 3V 2A respectively.

And by the way, can a device have a high requirement of load current than its input voltage ?
Ex. Can a device have a rating like 2V and 3A load current ( Where the load current is greater than the input supply voltage ? )

2. ### #12 Expert

Nov 30, 2010
18,094
9,678
P=IE
Power = Current times Voltage
List all the loads, their voltage supply, and their current needs.
Calculate the power for each.

Yes, a device with a 2 volt supply can use 3 amps.
Volts and amps are completely different from each other. Can a plastic tube with 1 PSI have a flow of two ounces per minute?
Obviously it can.

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3. ### avayan Member

Oct 30, 2015
32
6
This is a very tricky question because the power is most likely not going to be constant. For example, you specify one of your loads is 3V @ 5A. If what you mean is that you have a power supply that can supply 5A for an output of 3V, this does not mean you will have 5A flowing into your 3V load. It means that the power supply can output 3V and the load can sink as much as 5A if needed and that if the load needs more than 5A, then the voltage will droop to something (hard to know what).

You may want to check the terms for Instant Power and Average Power. Instant Power is kind of useless when it comes to knowing how much power an application is using. You would only use it to determine whether your components can handle such amount. Average Power, on the other hand, is the one you will have to worry about the most. This is the one which dictates how quick your battery will drain or simply how much energy you are using across time.

The equation given by #12 is still correct. Power = Current * Voltage. The problem is that whereas voltage is kind of constant for the great majority of applications, the current hardly ever is. Most of the time you will use an average current (or RMS current) as that is more meaningful. For example, a microcontroller may be sinking current at different rates as it is executing code. Sounds nit picky, but if you want to fully understand the inner workings of power utilization on microcontroller based applications, you may need to go there.

And as stated by #12, there is no limit as to what is the relationship between voltage and current. A car uses a 12V battery which must be able to supply hundreds of amps for the car to start. There is a gazillion of cases out there where the current is insane and the voltage is not.

However, for the most case you will always prefer to have lower currents than voltages because of the other notorious power equation P = I^2*R. This is why in power distribution voltages are gargantuan (many thousands of volts) and the currents are tiny. You will have to play with both of these equations depending on the application. P=I^2*R only plays a role when you are designing the microcontroller (if at all), though, so I doubt you will see it too many times when dealing with microcontrollers alone.

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