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LED Strip (SMD5050) Unexpected Current Measurements

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Justin Klein

New Member
Hey all,

I'm working on a small DIY project, and am a bit confused about the current draw I'm measuring from a roll of SMD5050 LEDs. The full roll (150LEDs, 16ft/5m) is supposed to be 21watt/12V (as stated by the seller, and from other references I've found online). As I understand it, this means it should draw 21W/12V=~1.75A. However, when I hook it up to a 12V battery pack (8xAAs) I'm only measuring ~500mA (it's on full brightness, white color aka all 3 RGB LEDs active). What could explain this large discrepancy?

The reason it matters is I plan to run them off of batteries, & am trying to calculate a reasonable balance of length vs runtime vs battery size vs cost (ultimately I'll use li-ion). All the resources I've found (i.e. https://www.reddit.com/r/DIY/comments/3abnek/trying_to_fix_my_electrofur_running_led_strips) say that I should estimate runtime as above: i.e. 21watts/12v=1.75A, so running this amount of LEDs off of a 2000mah battery pack (aka 8 AA's) should last for a bit over 1 hour. But the fact that I'm only measuring 500mA means it would run considerably longer. So then...if I need it to run for, say, 8 hours, does that mean I'd actually need a 1.75A*8=14000mah li-ion battery (as per calculations)? Or a 500mA*8=4000mah (as per measurements)? Or if I get a 8000mah battery, could I just double up the length of the strip?

Something tells me I'm missing something fundamental here, I just want to make sure that before I commit to a LED-length & battery size, I'm actually getting the numbers right... :)
 
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MikeMl

Well-Known Member
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1.75A for AA is bordering on the ridiculous... Normal discharge max discharge rate for those is <150mA

1.75A for 8h is 17Ah. Here is a link to 17Ah SLA, for size/weight comparison.
 

Justin Klein

New Member
1.75A for AA is bordering on the ridiculous... Normal discharge max discharge rate for those is <150mA
1) What I didn't understand is why the spec says it should draw 1.75A, but it's only drawing 500mA. I think from your reply it's because the AA's aren't capable of supplying that much current, correct?

2) If so, what problem might arise from trying to run these LEDs off of this AA battery pack? Is that a bad idea? I've only run it for short periods, but it *looks* to be nice & bright... :)

3) If it's safe/OK, would my calculation of 2000mah (for 8xAAs) at the 500ma it's actually drawing mean it should last approx. 4 hours? Or no, because of some side-effect of taxing the batteries more than I should?

4) If I were to buy a li-ion battery pack like (http://www.amazon.com/Talentcell-Re...27&sr=8-2&keywords=12volt+battery+lithium+ion), would I calculate its expected runtime based on the 500ma I'm seeing the LEDs draw right now? Or on the 1.75A they should theoretically be drawing? Or other?

The LEDs will be on a costume, so a heavy SLA battery would unfortunately not work...
 

ronsimpson

Well-Known Member
Most Helpful Member
aka 8 AA's
I looked up EverReady 1215 Super Heavy Duty AA battery.
At room temperature it has 0.4 ohm internal resistance. So a battery pack of 8AAs should have 3.2 ohms. At the current level you want that is volts of drop in you battery.
The graph shows that a 1A load will kill the battery in 6 minutes.
 

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Justin Klein

New Member
So, I'm still really at a loss as to how to figure this out. I'm obviously a beginner, this is just my first project. How can I choose the appropriate size Li-Ion battery?

I will say that I've definitely run these LEDs off the AA's for *much* longer than 6min already, and they're glowing bright. Been toying with them over several days, & haven't changed the batteries...
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
DO you have a link or manual to the meter your using to measure the current and the range you used? If your meter uses a 10 Ohm resistor in the current range your using, that would explain it.

If you have another meter, measure the voltage across the current meter while measuring current.
 

audioguru

Well-Known Member
Most Helpful Member
Does anyone use weak and old-fashioned Super Heavy Duty AA cells anymore? The AA alkaline cells from Energizer and Duracell can provide more than 1A for more than half an hour or 2Ah at low current for an hour.
The Amazon Lithium battery has reviews of many that did not work or were defective, other ones worked fine. Will you gamble that you get a good one? I have used thousands of Energizer AA alkaline cells and never had a bad one.
 
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rmn_tech

Member
2A at low current? 2A is 2A how can you reduce a current of 2A and still have 2A? 1A for more than half an hour 2A for longer? It doesn't make sense to me could you explain please. Thankyou.
 

siliconsurfer

New Member
I'm not sure that this is of any help, but a while back I bought a few low cost led lamps that shipped from China. They were specified as 20w each, but I measured 12 W, that's a pretty large discrepancy from what the manufacturer specified. Although they are pretty bright anyway.
 

audioguru

Well-Known Member
Most Helpful Member
2A at low current? 2A is 2A how can you reduce a current of 2A and still have 2A? 1A for more than half an hour 2A for longer? It doesn't make sense to me could you explain please. Thankyou.
My typo error, it should read 2Ah at low current. The Energizer AA alkaline cell datasheet has a graph that shows its current and duration when its voltage has dropped to 1.0V in 20 hours at 100mA (low current, 2000mAh). The graph shows the voltage has dropped to 1.0V in 1 hour at 800mA and looks like it could last half an hour at 1A. A Super Heavy Duty cell might drop to 1.0V in 1 hour with 100mA.
 

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Justin Klein

New Member
This is all a bit over my head. In super-layman's terms, I'm just trying to figure out how feasible it would be to power these LEDs off of batteries, in terms of determining how large a battery I'd need to power them for a given period of time.

To add to confusion, I just found a seller claiming to have 500cm of 5050's that run off of only 3x AA's (so 4.5V): http://www.ebay.com/itm/5050-SMD-RG...trip-Flexible-With-battery-box-/221771987157?

I messaged them and they said they've tested it as running for a full 4-5hrs off of just 3 batteries. When I hooked my strip up to 4.5V, it was only able to illuminate Red, and barely at all...
 

audioguru

Well-Known Member
Most Helpful Member
Simple. His LED strips are designed to be powered from 4.5V. Your LED strips are designed to be powered from 12V (maybe a car battery). Do you trust this new guy? He connected 500cm to 3x AA's but the spec's say you can connect only "several" 50cm strips together.
 

Justin Klein

New Member
1) So then, I guess that means there actually are different types of 5050's? My understanding just from reading forums etc was that they're "all" 12V. This makes me think that's not actually true...?

2) As a follow-up, since I plan to run it from "small" (aka not car-) batteries, 4.5V would be much preferable, as it requires 3x AA's at a time rather than 8x. Is there any reason I'd want to get 12V and use more than twice the number of batteries? Similarly, if I were to use a li-ion battery, 5V are much easier to come by (& cheaper) than 12V...so what benefit might it have to choosing a 12V strip vs a 5V (again, assuming the ansewr to #1).

Thanks much again! :)
 

siliconsurfer

New Member
How did you get 12 watts? Measure Voltage X Current? How did you set the current or voltage?
I used an adjustable power supply and set the voltage across the led to 35 V, that was a bit higher than the "recommended voltage" of 33 V. Then I measured how much current that actually ran through the circuit, about 340 mA. So, P = U I, gives 11.9 W, or http://www.sengpielaudio.com/calculator-ohm.htm

I think the key thing to realize is that the "recommended voltage of 33 V" is not what is going to deliver the quoted 20 W, but rather, the voltage that will give a reasonable long lifetime for the leds. I never tried to push it any further.
 

tunedwolf

Well-Known Member
The LEDs will be on a costume, so a heavy SLA battery would unfortunately not work...
How feasible would it be to have one really decent LED light source and do the points with some cheap plastic fiber optic?
It won't likley be as bright and the viewing angle probably not anywhere near what you would get from a bare LED chip, but circumstances dependant, you may be able to make it work in your application, especially on a darkened stage etc. As a bonus, you shouldn't have anywhere near as much maintenance to deal with due to the costume movement damaging connections etc :)
 

audioguru

Well-Known Member
Most Helpful Member
A 5050 LED measures 5mm x 5mm. There are 3 LED chips inside with the same color or RGB colors.
A strip with 5050 LEDs can have the LEDs in series for 9V to 12V or in parallel for 3V to 4.5V.

For the same brightness and duration the battery for series LEDs is the same size as the battery for parallel LEDs because the power is the same.
 

Justin Klein

New Member
Alright, that makes sense :) So if I'm understanding correctly, 3 AA's would just run a 5V strip for (3/8) as long as 8 AAs would run a 12V strip.

In which case, just for convenience, since portable cell phone chargers are so much more plentiful than 12V battery packs, it seems 5V would be a better choice. Much easier & more cost-effective to find 5V battery packs than 12V, all else equal. Again, if I'm understanding correctly ;)
 
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