How does one determine the power consumption of a device/load in a circuit?

Question

I have a question regarding circuit design, however i'm not sure whether i can express it in correct terms or if there's a fundamental flaw in my thinking.

• When using a device,how do we know exactly how much power it requires in order to start up/operate correctly?
• How do we calculate such requirements?

Let's say we have a circuit consisting of a power supply, resistor and some device like a chip or other microcontroller, or any other kind of load for that matter.

• How do we reason about the power supply needed for that device to work?
• Knowing that power is essentially voltage times current, how do I choose values for those components?

Forgive if my qeustion does make any sense Regards

Answer 1

Getting an estimate is fairly direct, although tedious.

You add up the power consumption of every device in the circuit. Usually with a giant spreadsheet.

You need to take care, particularly with modern digital logic, because the actual power consumption depends on a lot of factors. But basically you look up the data sheet values, then you add in any power consumed by loads you put on the chip.

I.e., this 74AHC00 chip specifies \$20\mu\mathrm A\$ of draw on the power supply -- but that's just for the chip. If you were using it to drive LEDs with \$10\mathrm{mA}\$ of current, you'd need to add in \$10\mathrm{mA}\$ for each LED.

So the chip, and each LED would be one line on your power consumption spreadsheet. Just multiply that by the number of parts on the board, and you have -- tedium.

It's a good idea to overestimate the power consumption on the first cut of a board -- you're never going to get away with just one rev of a board, things are always worse than they seem, and if you're way off on power consumption, it's easier to put in a smaller supply than a bigger one.