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Power usage of LED light using AC adapter

by daddybruce56
Tags: ac-dc adapter, led light
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Feb5-10, 06:24 PM
P: 1
I have a question about how much power Im using to run a LED light that I modified to
run off an AC-DC adapter instead of 3 AAA alkaline batteries. The light is a Sylvania Lighting Dot It Black Led Swivel, which uses about 1/4 of a watt (4.5 V X 54.7mA) on batteries. The light would dim unacceptably after about 20 hours of use. So as to not burn through lots of batteries I did this modification (I know I could have purchased rechargeable batteries but since I hang on to old AC-DC adapters for this very reason, it was a no cost project. It works great always max brightness.

Here are the specs on the adapter:

INPUT: AC 120V 60HZ 8W
OUTPUT: DC 4.5V 600mA
Class 2 Power Supply for use with audio products without antenna connections. Masugarbagea Electric CO. LTD. Made in China. It has a polarized two-prong plug. I have no idea how old it is, what it was originally used for or, what the exactly is the internal circuitry. It gets moderately warm after running a while with nothing connected.

Measured 6.1VDC unloaded output of the adapter (Measured same with adapter connected to light LED OFF, at the battery terminals where adapter wires are soldered. Measured 5.25V DC at terminals with light ON. I also measured the current -- about 55mA DC approximately the same as the battery amount above, which comes from www.ledmuseum.orgs review of the puck, style Dot-It.

Is the 8W AC standby or max? (Is that the power it uses just plugged in with no load or what it uses with the DC load drawing the full 600mA?) Can I take the actual DC load (55mA) divided by the max DC load (600mA) and multiply that by 8W to get .73W that gives me ~=1W used total? Am I assuming anything that is wrong? Also, is the 5.25V DC a problem?

Any way it comes out it is still less power than the 25-watt R14 miniflood light I was using. The LED does not put out as much light as the miniflood but its enough to illuminate a computer keyboard in semi-dark room, which is what I wanted.

Thanks, daddybruce56
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Bob S
Feb5-10, 08:06 PM
P: 4,663
The 8W is the total input power when it is putting out the rated load (2.7W). The VA (volt-amps) will be more.

Bob S

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