So, you remember that pfSense build that I talked about before? Well, it was drawing around 25 W.
I know, right? Unacceptable!
So, one of the things I tried to figure out is what the main power draw is. The PicoPSU accepts a 12V input. Could the ac/dc adapter be hurting my efficiency?
The adapter I have is a 12V / 5A (60W) adapter that I got for around $10 off Amazon. I figure it wasn’t anything special, and was probably not terribly high efficiency.
I decided to measure the efficiency of the ac/dc adapter. I did this by measuring the power into the adapter with a Kill-A-Watt. And to measure the output power, I used an ammeter (the ammeter function of a multimeter). (I assumed the PSU was delivering 12V; if it was a little low, my efficiency assumptions would be wrong.)
To do this, I had to cut one of the chords on the adapter and place the multimeter in series. (I could alternatively cut the input chord on the PicoPSU, but I figure an adapter is easier to come by than a PicoPSU.) Here’s a picture of how that looks:
Multimeter in series with dc supply chord
And here are the readings on the Kill-A-Watt and multimeter:
Kill-A-Watt reads 20.6 W
Multi-Meter reads 1.64A
Which all means that my efficiency is 1.64A * 12V = 19.68 W / 20.6 W = 95.5%.
I gotta say, I did not expect it to be that high. This is pretty cool. (And a bit surprising, since it seemed like the ac/dc adapter gets pretty warm.)
I should also note that the power draw is less than I remember. I thought it was more like 25A, but I never wrote it down, so I can’t be sure. I’ll take 20A, and be quite happy.
The current on the multimeter bounced around a bit, from 1.6 A to around 1.8 A, but it was well under 2A. Originally, I was interested in getting a (smaller) 2A supply, but I don’t see any need to, with this being so high efficiency. (And who knows, the smaller supply may be lower in efficiency, since they tend to be more compact.)
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