http://www.outdoorlanternsonsale.com/leds-super/
Super-capacitors and current available?
Can anyone advise how you calculate how long a capacitor takes to discharge for a nominated current draw from it?
With the advent of super-capacitors, I was wondering whether a 1 or 2 Farad cap would drive a string of 3-4 LEDs at 18m/A per device. Clearly they can, but will it be a flash from the LED as the cap discharges, or will it hold a 18mA on for a few seconds, minutes or hours?
I will use a string of LEDs so the 18mA goes further. But how do I work out the time a LED will stay lit?
Note this same request ws made in the Engineer section.
FYI, the cap I intend using is rated at 16V so I would probably charge to 13-14V. As regulators are wonderful things, I would use a low-dropout device. This would allow the LEDs to run at (say) 4.3-4.5V while the higher battery voltage drops. I am pretty sure it was a logarithmic function.
If the current draw is constant, it is easy to calculate:
change in cap voltage = -(1/C) x I x t
t = - C.V/I
So if a 2F cap has 5V across it and you draw 70mA from it then it will take -2 x -5 / 0.07 = 143 secs to discharge completely.
This gives you a rough formula for your circuit but it isn't accurate because your LEDs will not be drawing constant current unless you make a special circuit to do so.
The change in cap voltage is actually the integral of current wrt time / C. If current is assumed to be constant then this is just current x time / C. If you use series resistors to set the current then you'll get a sort of exponential voltage decay, the LEDS will start bright and fade quickly at first then fade more and more slowly.
Hope that helps.
 |
 Black 9 LED Super Bright Torch Lamp Flashlight Light $1.55 Time Remaining: 43m Buy It Now for only: $1.55 |
 Aluminium Compact Super Bright FlashLight 9 Bulbs LED $0.01 Time Remaining: 2h 36m |
 1pc New 3 watt Mini 3W Led Flashlight Light Super Bright Torch Lamp XT 7119 GIFT $3.69 Time Remaining: 3h Buy It Now for only: $3.69 |
 Outdoor Waterproof Super Bright LED 300LM Flashlight $16.19 Time Remaining: 3h Buy It Now for only: $16.19 |
 Outdoor Waterproof Super Bright LED 300LM Flashlight $16.29 Time Remaining: 3h 6m Buy It Now for only: $16.29 |
 1pc New 3 watt Mini 3W Led Flashlight Light Super Bright Torch Lamp XT 7119 GIFT $3.79 Time Remaining: 3h 10m Buy It Now for only: $3.79 |
 HOT Super Bright Light LED FlashLight KeyRing Keychain Lamp camping $1.88 Time Remaining: 4h |
 LOT 10 Super Bright Light LED FlashLight KeyRing Keychain Lamp camping $6.88 Time Remaining: 4h |
 LOT 30 Super Bright Light LED FlashLight KeyRing Keychain Lamp c $17.25 Time Remaining: 4h 6m |
 LOT 20 Super Bright Light LED FlashLight KeyRing Keychain Lamp c $11.99 Time Remaining: 4h 9m |
 SUPER BRIGHT 9 LED FLASHLIGHT WITH BATTERIES NEW $1.95 Time Remaining: 4h 38m Buy It Now for only: $1.95 |
 Super Bright CREE SST 50 LED 1600 Lumens Flashlight Torch + Holster $18.60 (4 Bids) Time Remaining: 4h 43m |
 2 Pk 9 LED Flashlight Set Lightweight Super Bright Blue Black Pocket Size $2.99 Time Remaining: 5h 4m |
 Super Bright CREE SST 50 LED 1600Lumens Bicycle Flashlight Torch+Mount+Holster $12.84 (4 Bids) Time Remaining: 5h 50m |
 3 X Super Bright 9 LED Flashlight Torch Aluminum Casing $1.00 Time Remaining: 6h 55m Buy It Now for only: $1.00 |
 Streamlight Nano Light LED Super Bight 10 lumens Key Chain $7.50 Time Remaining: 7h 54m Buy It Now for only: $7.50 |
 New 14 Led Table Lamp FL347 Super Bright FREE battery $6.69 Time Remaining: 8h 39m Buy It Now for only: $8.00 |
 White Guolin GL 037 42 1W LED Flashlight Super Mini Powerlight Aluminum Torch $2.00 (5 Bids) Time Remaining: 8h 44m |
 LED LENSER CREE COAST P14 LED FLASHLIGHT TORCH 210 LUMENS LIGHT SUPER FOCUS BEAM $47.39 Time Remaining: 9h 56m |
 SOLAR CHARGED FLASHLIGHT GAMA SONIC SUPER BRIGHT WHITE LEDS GS 72 $0.99 Time Remaining: 11h 30m Buy It Now for only: $2.99 |
 Cree Q5 3 Mode Zoom Super Bright LED Bulb Flashlight Torch $4.17 Time Remaining: 12h 57m Buy It Now for only: $4.17 |
 Cree Q5 Zoomable Super Bright LED Bulb Flashlight Torch $3.37 Time Remaining: 13h 17m Buy It Now for only: $3.37 |
 Lot of 4 Super Bright 51 LED Flashlight 2 Mode Blk $24.99 Time Remaining: 13h 50m Buy It Now for only: $24.99 |
 Super Bright CREE SST 50 LED 1600 Lumens Flashlight Torch + Holster $12.50 (5 Bids) Time Remaining: 14h 1m |
 Lot of 12 Super Bright 51 LED Flashlight 2 Mode Bk $84.99 Time Remaining: 14h 9m Buy It Now for only: $84.99 |
 super bicycle head light bright white led bike light $0.71 (2 Bids) Time Remaining: 14h 31m |
 Outdoor Waterproof Super Bright LED 300LM Flashlight $15.59 Time Remaining: 15h 15m Buy It Now for only: $15.59 |
 New SUPER Bright 5x XM L T6 CREE LED 5000 Lumens 5 Modes Flashlight Torch Set $92.99 Time Remaining: 15h 33m Buy It Now for only: $92.99 |
 LOT 8 Super Bright MINI 9 LED Flashlight Lamp Torch Light Camping $9.89 Time Remaining: 16h 12m Buy It Now for only: $9.89 |
 Super Bright 4000lm Lumen 3x CREE XML XM L T6 LED Flashlight Torch 5 Mode 18650 $46.58 Time Remaining: 16h 28m |
 MEGABRITE SELF POWERED SUPER BRIGHT LED Flashlights 2 Turbo XL NEW RED SILVER $29.99 Time Remaining: 16h 33m Buy It Now for only: $29.99 |
 5W Super Bright LED Mining Hunting Camping Spotlight Searchlight White light 5V $53.90 Time Remaining: 16h 39m Buy It Now for only: $53.90 |
 2000Lm 3X CREE XP G R5 Super LED Extended Flashlight 3 LEDs Torch 5 Mode DC102 $33.49 Time Remaining: 16h 40m |
 MINI 9 LED Super Bright Flashlight Lamp Torch Light Camping RED $1.99 Time Remaining: 16h 49m Buy It Now for only: $1.99 |
 MEGABRITE SELF POWERED SUPER BRIGHT LED Flashlights 2 Turbo XL NEW BLUE SILVER $29.99 Time Remaining: 16h 51m Buy It Now for only: $29.99 |
 SKY RAY 3x CREE XM L T6 LED High Super Power Flashlight Torch 3800Lm Waterproof $45.99 Time Remaining: 18h 24m Buy It Now for only: $45.99 |
 SKY RAY 3x CREE XM L T6 LED High Super Power Flashlight Torch 3800Lm Waterproof $43.19 Time Remaining: 18h 55m |
 Cree XM L T6 LED Flashlight Torch Light Lamp 800 Lumen Super Bright 18650 2 $33.29 Time Remaining: 20h 6m Buy It Now for only: $33.29 |
 Super Bright 350 Lumen G10 CREE XP G R5 1 Mode White LED Flashlight with Cord $16.99 Time Remaining: 20h 26m |
 Super Bright 1200 Lumen M3 T60 XM LT6 5 Mode White LED Flashlight Black $37.99 Time Remaining: 20h 26m |
 9 LED Super Bright Keychain Flashlight Lamp Light $2.72 Time Remaining: 20h 42m |
 3 Watt 1AA torch Super Bright LED Flashlight Lamp light $1.58 Time Remaining: 20h 57m Buy It Now for only: $1.58 |
 New 3 Modes Zoom Super Bright Police 3W LED Flashlight Torch + 18650 Charger $0.99 Time Remaining: 21h 9m |
 SUPER BRIGHT LED RED METAL BATTERY OPERATED FLASHLIGHT $10.22 Time Remaining: 21h 46m Buy It Now for only: $10.22 |
 3 PACK EVEREST MINI FLASHLIGHTS NIB 6 SUPER BRIGHT LEDS 3 COLORS $12.99 Time Remaining: 21h 47m |
 Black Super Bright C8Q2 Cree Q2 5 Mode White LED Flashlight Torch Lamp Light $19.99 Time Remaining: 22h 21m |
 Super Bright 900 Lumen AK P7 4 SSC P7 C SXO 2 Mode LED Flashlight Lamp Silver $35.99 Time Remaining: 22h 21m |
 NEW 90 Lumen Super Bright LED Flashlight Torch AAA $1.04 (2 Bids) Time Remaining: 22h 43m |
 Micro Super Bright White LED Head Lamp Flash Light I $2.48 Time Remaining: 23h 19m Buy It Now for only: $2.48 |
 Life Gear Super Bright 360 Lumens LED Flashlight Black $66.48 Time Remaining: 23h 20m Buy It Now for only: $66.48 |
 Olight i6 Paladin World Class Super Bright Cree XM L LED Flashlight NEW $54.59 Time Remaining: 23h 56m Buy It Now for only: $54.59 |
 New 9 Bulb LED Torch Super Bright LED Flashlight Lamp $1.95 Time Remaining: 23h 59m Buy It Now for only: $1.95 |
 Brand New 9 Led Soft Super Bright Touch Torch Light $7.10 Time Remaining: 1d 54m Buy It Now for only: $7.10 |
 1pc New 3 watt Mini 3W Led Flashlight Light Super Bright Torch Lamp XT 7119 GIFT $3.79 Time Remaining: 1d 2h 26m Buy It Now for only: $3.79 |
 1pc New 3 watt Mini 3W Led Flashlight Light Super Bright Torch Lamp XT 7119 GIFT $3.03 Time Remaining: 1d 2h 32m Buy It Now for only: $3.03 |
 Super Bright 5W 60 Lumens LED Flashlight aluminum Torch $9.98 Time Remaining: 1d 2h 35m Buy It Now for only: $9.98 |
 Super Bright CREE SST 50 LED 1600 Lumens Flashlight Torch + Holster $12.84 (4 Bids) Time Remaining: 1d 3h 28m |
 Outdoor Camping Super Bright 17 LED Lantern w Decorative Compass $21.41 Time Remaining: 1d 3h 40m |
 Head Lamp Hands Free Super Bright 7 Bulb LED Light NEW $12.99 Time Remaining: 1d 3h 57m Buy It Now for only: $12.99 |
 17 LED Camping Outdoor Super Bright Compass Lantern $23.24 Time Remaining: 1d 4h 6m |
 Silver Super Bright Portable 9 LED Torch Flashlight $1.77 Time Remaining: 1d 4h 26m Buy It Now for only: $1.77 |
 Silver Super Bright Portable 9 LED Torch Flashlight + Gift LED Keychain Flashlig $2.17 Time Remaining: 1d 4h 26m Buy It Now for only: $2.17 |
 1pc New 3 watt Mini 3W Led Flashlight Light Super Bright Torch Lamp XT 7119 GIFT $3.70 Time Remaining: 1d 4h 42m Buy It Now for only: $3.70 |
 1pc New 3 watt Mini 3W Led Flashlight Light Super Bright Torch Lamp XT 7119 GIFT $3.40 Time Remaining: 1d 4h 49m Buy It Now for only: $3.40 |
 Super White Bright 9 LED Heavy Duty Compact Aluminum Flashlight Mini Pocket $2.99 Time Remaining: 1d 4h 52m Buy It Now for only: $3.99 |
 17 LED Camping Outdoor Super Bright Compass Lantern $24.54 Time Remaining: 1d 5h 53m |
 Super Bright CREE SST 50 LED 1600 Lumens Flashlight Torch + Holster $10.52 (7 Bids) Time Remaining: 1d 5h 56m |
 Lot of 6 Super Bright 32 LED Flashlight FREE SHIPPING $27.99 Time Remaining: 1d 6h 16m Buy It Now for only: $27.99 |
 NEW NEBO SUPER UV CSI LED FLASHLIGHT W COMPASS 5063 $23.99 Time Remaining: 1d 6h 18m Buy It Now for only: $23.99 |
 Lot of 24 Super Bright 9 LED Flashlight FREE SHIPPING $44.99 Time Remaining: 1d 6h 20m Buy It Now for only: $44.99 |
 Flashlight Super Bright P5 LED Weather Shock Resistant NEW $8.00 Time Remaining: 1d 6h 22m Buy It Now for only: $9.90 |
 Lot of 6 Super Bright 21 LED Flashlight FREE SHIPPING $31.99 Time Remaining: 1d 6h 23m Buy It Now for only: $31.99 |
 NEW 14 LED SUPER BRIGHT TORCH FLASHLIGHT GOLDEN FROM NY $6.96 Time Remaining: 1d 6h 56m Buy It Now for only: $6.96 |
 INNOVAGE OUTDOOR super bright new in the box 12 LED POWER camping LANTERN $14.95 Time Remaining: 1d 7h 2m Buy It Now for only: $32.95 |
 Mini 9 Bulb LED BRIGHT Flashlight Torch Super bright $3.10 Time Remaining: 1d 7h 2m Buy It Now for only: $3.10 |
 Super White Bright 9 LED Heavy Duty Compact Aluminum Flashlight Mini Pocket $2.99 Time Remaining: 1d 7h 42m Buy It Now for only: $3.99 |