# Very basic power problem

#### sabotatorepg

Joined Oct 24, 2016
1
Hello all!

I'm not mother tongue speaker (and hence writer) and I'm facing a quite complicated problem which requires me basic electronics knowledge that unfortunately I don't currently have (hope to fill the gap soon ).

I need to electrically connect a small module (a closed box which contains some tools that I will use for a scientific experiment) to a bigger structure via a 9 pin sub D connector. The bigger structure will provide (via the first pin) a non regulate voltage line of nominal 28 V DC (min 20 V max 36 V). The power budget (the developer of the bigger structure wrote down exactly this) is 3W.

I think that the budget is 3W for every hour. Does it make sense? Or is the budget the total amount of energy, not referred to a temporal unit?

As I wrote above, I know that inside the small module there will be some tools that I will alternatively activate (for example a CO2 sensor that will take only one recording every single day). I've found, in the description of the CO2 sensor, the following: Current Consumption : Normal mode : 20mA, Peak : 200mA, Sleep mode : < 0.5mA.
Let's suppose that the experiment will last ten days and in the small module there will only be a CO2 sensor; that the CO2 sensor will reach the consumption peak (200mA) for a second during each reading and it will be in sleep mode during the rest of the 10 days (<0.5 mA).

Is it possible to asses how many W I will consume? Is it possible to asses if the power budget (3W) will be sufficient to carry out all the experiment? It would also be very nice for me to understand how to calculate everything and the relation between Watt and Ampere in this experiment

Ciao e grazie!!!!

#### wayneh

Joined Sep 9, 2010
16,399
Watts are an instantaneous rate, like miles per hour. Power is in units of energy per unit time. Watts are power. Watts = volts x amps.

Total energy is then power (watts) multiplied by an elapsed time, just as miles = mph x hours.

Personally, I would clarify the 3W figure. Is that the maximum allowable at any time, or is it some sort of average? Maybe your box could use 5W as long in a brief pulse as long as the average stays below 3W.

To determine your power needs, it's essential to know the profile (versus time) of power usage by each component.

#### ErnieM

Joined Apr 24, 2011
8,055
Well first off power is a measure of energy over time, so one watt is defined as one joule of energy per second.

A power budget of 3 watts implies you are allowed to draw a maximum of 3 watts / 28 volts = .1 amp at all times. However your CO2 sensor needs twice that once a day to take its reading.

I see two choices: go back to the engineer who specified 3 watts and ask if you can have a brief peak of twice that once a day, or design some sort of hold up circuit (large cap or even a super cap) that can provide the over current once a day for the short period.

Ask first, your system may well not even notice that brief current demand.

And welcome to the forums!

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