{"id":923,"date":"2014-08-15T02:25:59","date_gmt":"2014-08-15T02:25:59","guid":{"rendered":"http:\/\/poojanwagh.opalstacked.com\/techblog\/?p=923"},"modified":"2014-08-13T02:47:23","modified_gmt":"2014-08-13T02:47:23","slug":"12v-dc-adapter-efficiency-measurement","status":"publish","type":"post","link":"https:\/\/tech.poojanblog.com\/blog\/unix-linux\/12v-dc-adapter-efficiency-measurement\/","title":{"rendered":"12V dc adapter efficiency measurement"},"content":{"rendered":"

So, you remember that pfSense build that I talked about before? Well, it was drawing around 25 W.<\/p>\n

I know, right? Unacceptable!<\/p>\n

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?<\/p>\n

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.<\/p>\n

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.)<\/p>\n

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:<\/p>\n

\"Multimeter<\/a>

Multimeter in series with dc supply chord<\/p><\/div>\n

And here are the readings on the Kill-A-Watt and multimeter:<\/p>\n

\"Kill-A-Watt<\/a>

Kill-A-Watt reads 20.6 W<\/p><\/div>\n

 <\/p>\n

\"Multi-Meter<\/a>

Multi-Meter reads 1.64A<\/p><\/div>\n

Which all means that my efficiency is 1.64A * 12V = 19.68 W \/ 20.6 W = 95.5%.<\/p>\n

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.)<\/p>\n

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.<\/p>\n

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.)<\/p>\n