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When plugged into wall warts, the flexible LED strips I want to make into a wearable display draw 3A @ 12V per 5m.

To do a 30 x 15 pixel display that's 15m, or, 9A @ 12V running around my body. Is this prima-facie unsafe? If it's not, am I going to need to carry the batteries around on a trolley?

I believe the LEDs will light at 9V and just not be as bright (which may be what I want, anyhow) and I'll be running PWM'd patterns so it won't be a 100% duty cycle but I'd at least like to establish a lower bound.

Robert Atkins
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    You left out one important constraint: How long you plan on wearing your project between recharge cycles / new batteries – W5VO Mar 15 '11 at 18:09
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    the ebay page cites 2.2W consumption at 12V, which corresponds to ~200mA per 5m. – crasic Mar 15 '11 at 18:34
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    even if you want a constant color you don't need to drive the LED "full on" just choose your PWM frequency > 60hz (flicker as this speed isn't detectable to the human eye) and control the duty cycle to determine brightness. What you'll find that the brightness difference is small between "full on" and PWM with maybe a 50% duty cycle but your current usage goes way down. To put it another way, the light output vs current relationship for most LEDs is logarithmic. – Mark Mar 15 '11 at 19:20
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    It it was 12V/9A, that's half again the wattage of a fairly powerful heated jacket, so unless you were planning on wearing it in a blizzard you would get quite hot fast. But as crasic states, it's only a tenth that. – Pete Kirkham Mar 15 '11 at 23:42
  • @W5VO, that's the lower bound I'm trying to get a handle on. Even though the eBay page does say 2.2W @ 12V, the seller said in an email I'd need one 3A power supply per 5m length. Similar strips whose published specs I trust more (http://bliptronics.com/item.aspx?ItemID=84) say 1-3W/m which sounds more sensible to me. – Robert Atkins Mar 18 '11 at 05:10
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    I suspect the eBay seller knows their "3A" supplies are not really up to providing a full 3A, and that selling people one wall-wart per 5m strip is therefore more reliable and good for business. Dropping the voltage to 9v will not help, for the same brightness you'll consume the same power so just draw more amps, depending on the way you're powering it all. – John U Mar 18 '13 at 09:11

2 Answers2

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Expanding on my comment, The ebay page cites 2.2W consumption, with a 12V supply voltage this corresponds to 200mA per 5 meters. Assuming this is correct, 15 meters of lighting will draw approximately 600mA. A high end rechargable NiMh AA battery is rated at around 3000mAh. With this in mind, a battery pack of 9 AA batteries (12 V) will power the display for at most 5 hours (theoretical limit), but likely closer to 2.5.

However this is based on several assumptions, primarily that the rated power consumption is true and represents an "always on state", because if the power supply is rated with half or less of the LED's on at the same time it could skew your performance (and drive the supply current way up)

Keep in mind that 600mA at 12V corresponds to 7.2W, so be sure that you use fat enough gauge wire for the return current, and that any resistors are beefy enough to handle the power on individual lines.

P.S. $80 dollars for 5 meters is excessive, you might be better off rolling your own

Community
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crasic
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  • Assumptions are bad. These strips have 5050 rgb leds. 3 per segment, 10 segments per meter. 20mA per color, per segment (leds in series). 3 colors, so that's 60mA per segment. Times 10 is 0.6A per meter. Times 5 is 3 Amps per 5 meters, at full on white at 12V. That's 36 Watts. Since it's a dream led strip, which tends to have some controller (auction does not state which, nor does it the power supply), the numbers can be assumed to be lower due to pwm, to slightly higher due to the IC's current draw. But at 15m, we are looking at 108 Watts of power needed, if at full draw. – Passerby Mar 18 '13 at 07:21
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As far as I know, 12 V is totally safe. Shock can't even be felt below dozens of volts. The current draw of the circuit is irrelevant, unless your wires are too thin and they get hot and burn you.

Or are you asking how heavy the batteries will be?

endolith
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    The potential to be shocked does not entirely depend on the voltage of the supply. Its true that the conductivity of *dry* skin is low enough to act as an insulator at low voltages, however given wet skin or a wet wound and the chance for shock goes way up even at 12 volts. You can in fact be *killed* by a 9v battery if you wire it up properly (like sticking a probe directly into the blood stream). – crasic Mar 15 '11 at 18:52
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    I'd suggest that if there are people trying to stab DMM probes into your blood vessels to either side of your heart, you have other problems. :) – Warren Young Mar 15 '11 at 20:22
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    Doesn't anyone lick 9Vs to see if they're fresh anymore? – overslacked Mar 15 '11 at 21:02