Puzzled by LED using more power than expected

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  #1  
Old 01-04-14, 04:22 PM
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Puzzled by LED using more power than expected

This makes me curious -- where are the losses? Is the magnetic transformer performing well below its 80% efficiency rating?

I have 6 meters of LED strip with its total lengths for the circuit segments split roughly in half, on two 12VDC circuits. In other words, I have 8 LED strip sections and 4 of them on each circuit are almost half the total length. I have a 4x60W (240W total) magnetic transformer (so I can use a wall dimmer switch) and only using two of its four 60W terminals, one for each led strip segment.

When both transformer output circuit breakers are switched on, the Kill-A-Watt shows the LED strips are using 114W total.

However the strip specs are 11.3W/m so without any transformer losses, theoretically it should be approx. 68W, or 85W assuming 80% transformer efficiency and no wiring losses.

114 - 85 = 29W unaccounted for.

More clues: when only one strip is on, the Kill-A-Watt shows 58W. When the second strip is on, the Kill-A-Watt shows 68W. (58 + 68 = 126W which is greater than 114W when both are on). The Kill-A-Watt shows the transformer power draw with no output load at 6W.

I tested the Kill-A-Watt using light bulbs, it is pretty accurate within a couple watts. I am not accidentally confusing the W with VA readout.

Each of the two used terminals on the transformer 12VDC output goes to 5 ft lengths of 12AWG romex. Each of these two then split to four 14AWG romex circuits going to a total of eight LED strip sections. The 14AWG runs are about 15 feet on average (parallel/star topology coming from the 12AWG).

There is about 30 ft of extension cord on the input side just for testing so I could connect the Kill-A-Watt meter.

Even though 29W is relatively small, percentage wise it seems significant, and makes me really curious where the energy loss is, or whether I miscalculated something.
 
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  #2  
Old 01-05-14, 05:52 AM
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I think you're finding that the transformer is not 80% efficient as you assumed especially with the load your applying. Searching online I found some magnetic transformers efficiency only peaks at 80% under optimal conditions which is near it's rated output. When used at less than full capacity the efficiency drops and at low draw may be as bad as 40%.
 
  #3  
Old 01-05-14, 06:03 AM
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If your LED strips use a switching type regulator then the load current (power) is not a true sine wave. Is your power meter a true RMS reading unit?
 
  #4  
Old 01-05-14, 03:29 PM
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Pilot Dane's explanation seems likely, the transformer is probably only 60% efficient when it is lightly loaded. That's ok for my purpose, since the long term durability is most important (vs. more fragile electronic transformers).

I have another question - this transformer is 4x60W, 240W total. It has 4 output breaker switches. I guess that means it has the main core, plus 4 DC filtering circuits, or a main core + DC filtering circuit and 4 circuit breakers to limit each output to 60W for Class 2 purposes. Is there typically any difference in longevity and efficiency between using all 4 output circuits and spreading out the load, or using just 2 of these 60W output circuits with each using closer to 60W? (same total load, spread across 2 terminals vs. 4 terminals)
 
  #5  
Old 01-05-14, 04:20 PM
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I'm confused. A transformer inputs AC voltage and produces AC output voltage (s). Do you mean you have a DC power supply with 4 DC outputs? Transformers are very efficient.
 
  #6  
Old 01-05-14, 04:30 PM
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That's right, it's a DC power supply - magnetic transformer variety. The specs claim "80% efficiency typically" so I know it's not going to be a super efficient 120VAC to 12VDC supply. Something I didn't mention in my original Q - initially I was worried whether I have shorts against the aluminum channels when I soldered the wires onto the LED strips, using up some of the current, but now I'm thinking the output breakers on the transformer box would trip if that were the case, and it's probably not that unusual to get only 60% efficiency from the DC supply.
 
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