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I know this might be a little bit funny question. But how can you determine the current that will used up as well as the power rating, when you a designing a power supply unit for a device. Because usually, I will always know the voltage, say 5V, but how can I calculate the current the device will use up.

Thanks.

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3 Answers

up vote 4 down vote accepted

What current will be used is determined by the load, not the power supply. We've had a number of questions like this before, like "how do I limit the current of my 5A supply to 1A".

What your device will need is just the addition of the current for the different components on the board. You usually start with the power-hungry parts: motors, relays. For a microcontroller you'll have to look up in the datasheet what it needs at a given clock frequency.

If you have a lab supply which displays current you may use that as a rule-of-thumb, but you may want to give it some headroom. Used current is definitely useful if you want to design a power supply.

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@whoever downvoted: why? Downvoting without saying anything is cowardish. – stevenvh Apr 16 '12 at 11:29
+1 to cancel the downvote. There's nothing wrong with this answer. – Federico Russo Apr 16 '12 at 11:43
Please stop tagging me, I didn't downvote. ─ whoever 5 mins ago :) – clabacchio♦ Apr 16 '12 at 12:30
@stevenvh thanks for the answer. Does this include passive components as well – Paul A. Apr 16 '12 at 12:51
@Paul - Yes, but it's a much tougher job to calculate or measure this. Most often we make a rough estimation, like if the microcontroller needs 10mA, add another 10mA for the chicken feed. – stevenvh Apr 16 '12 at 12:55
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up vote 1 down vote

At a very simplistic level, if you know the voltage, and you know the resistance of your device, you will know the current needed: I=V/R

Alternatively, if you know what the power requirements are, again you can calculate: I=P/V

Your power supply needs to be able to deliver the required voltage at the greatest current your device may draw (usually plus a safety headroom factor)

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up vote 1 down vote

As Steven said, to design a proper power supply you have to know the maximum current consumption of your circuit and then add some headroom (depending also on how accurate the prediction is).

The first thing to do is to check the datasheets of the main components, depending on the kind of circuit: if you have motors, speakers, power LEDs, hungry microcontrollers...Sum up the power consumed by the hungriest components first, and then check what else remains.

You can also use a benchtop supply and measure the maximum consumption of the circuit, but take care of checking the maximum load and then don't trust it too much if it's lower than expected (looking at the datasheets).

Some margin is not only needed for safety, but also because if the supply works always at 100% capacity it will suffer from heat. Oversize it a bit will result in it being cooler and work better.

As always, you have a tradeoff between having enough headroom versus smaller price/size/weight.

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@PaulA. usually passive components are used in combination with active ones, so they will be already considered. For instance, if you have an LED, the limiting resistor will have the same current. But you have to understand the circuit to find the candidates to power consumption. – clabacchio♦ Apr 16 '12 at 13:09
Thanks! Things are now clearer. – Paul A. Apr 16 '12 at 13:30

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