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# Estimating power supply AC load

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#### jasonc2

##### Full Member level 4
I have a 120VAC -> 24VDC power supply rated at 85% efficiency, and a 0.8W power dissipation with no load.

The device it is powering draws 110mA max = 2.64W.

I want to find a reasonable upper bound for the AC current draw and heat dissipation. Do the following calculations make sense:

3.91W total AC load / 120 VAC ≈ 33mA

So roughly 33mA load on the AC line max.

And also the power supply itself would dissipate 3.91W - 2.64W = 1.27W as heat max (assuming worst case that 0.8W no load dissipation is entirely heat).

Is that reasonable? If not, how can I estimate these values (and what other specs do I need on the power supply to do so)?

Thanks,
J

The calculations look good, but you did say "upper bound", not typical. You might need to take other factors into account. For example, suppose the line voltage assumes its maximum and minimum values. What does that do to your dissipation value?

jasonc2

Points: 2