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Circuit for high power LEDS - power question

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hoghunter

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I am putting together a project that will light up an outdoor area. I am using (3) high power, 3w leds (1.5V/700ma). The leds are in series and are preceded by a 'buck puck' led driver. I want to power them using (3) 18650 rechargeable batteries 3.7V & 3000mah each for a total of 11.1 volts. I'm trying to figure out how long this system would provide light before needing a recharge. I have tried a few online calculators, but am not sure of my entries or the results. Thanks for any help.

I assumed a total forward voltage = 6.5V (including led driver)

My other option is to power the unit with a 12v AGM or lead acid 7 ah battery via a lead plugged into a jack on the unit.
 

Externet

Active Member
Those cells are considered discharged at a certain value. The 'buck puck' should not drain them beyond. That means the battery voltage range available to run the LEDs is to be accounted for.

I do not know of 1.5Vf leds. So calculation is crippled before starting. 700mA drain can in theory provide 4.28 hours of operation, but such 3000mAH capability is likely not continuous use. There is a duty-cycle pattern associated with discharging such batteries that usually only the manufacturer knows, IF.

Another point is the typical senseless plan to use LEDs to its maximum rated current. De-rate them for longevity. Your "total forward voltage" does not make sense to me. What "unit" jack/lead are you talking about ?
 

hoghunter

Member
The 'jack' would be a typical power in type with the pos pin down the center and ground on the sides. The 'lead' from the jack would connect to a 12v battery approx 5' away.....
Also, these are 3w 'high power' star leds. Not your typical low power 20ma diode.
 
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Externet

Active Member
Yes; There is no 20mA mentioned. Instead, 700mA and a questionable 1.5Vf as 1.5 x 0.7 ≠ 3W
 

hoghunter

Member
Good point! The specs for that particular LED, based on the supplier website are:

Model: IR-3W-060-01 - IR Infrared
Color Frequency: 940nm (IR Infrared)
Luminous Intensity: 250-300mW
Viewing Angle: 60°
Radiation Pattern: Lambertian
Forward Voltage: 1.4v - 1.6v
Typical Voltage: 1.5v
Typical Forward Current: 700mA
Maximum Pulse Current: 900mA
Style: 3watt STAR Configuration


In my particular case, the Vf would be 4.5v as I am using three of these in series. So, 4.5v*.7amps=3.15w. However, I suppose one of his spec numbers is incorrect? I am obviously new to all of this.
 

audioguru

Well-Known Member
Most Helpful Member
You should buy Name brand LEDs from a Western LED manufacturer, not cheap Chinese no-name-brand LEDs made in somebody's basement.
A real LED manufacturer will have a correct datasheet.

It looks like the LEDs are 1W and are mounted on a metal star that is usually used for 3W LEDs.
 

hoghunter

Member
You should buy Name brand LEDs from a Western LED manufacturer, not cheap Chinese no-name-brand LEDs made in somebody's basement.
A real LED manufacturer will have a correct datasheet.

It looks like the LEDs are 1W and are mounted on a metal star that is usually used for 3W LEDs.
That maybe true. The leds were purchased from www.pcboard.ca out of Canada. His site is very informative. Some were bought through www.ledsupply.com out of Vermont.
Any suggestions for a supplier are welcome.
 

ci139

Active Member
typical senseless plan to use LEDs to its maximum rated current
if you are not about to incubate varanus eggs then the optical sensors are best sensitive in the iR wave range
+ if you are not "exposing" somehow continuously you can perhaps sync. the illuminator sensor duty cycles . . .
 

Externet

Active Member
OK, got it. --->Infrared LEDs. Vf=1.5 x 3 series = 4.5V
Fed by 3 series lithium for 11 V at full initial 700mA; 11V - 4.5V divided by 0.7 A = use 10Ω in series. Current will diminish while cells discharge to their rated minimum recommended.
For 12V lead-acid; 12V - 4.5V divided by 0.7A = use 11Ω to 12Ω
Or, your 'puck driver' set for 4.5Vf / 700mA instead of resistors.

Check their time on until Li cells reach their minimum voltage (2.5V ¿?) with or without your 'puck driver'
 

hoghunter

Member
OK, got it. --->Infrared LEDs. Vf=1.5 x 3 series = 4.5V
Fed by 3 series lithium for 11 V at full initial 700mA; 11V - 4.5V divided by 0.7 A = use 10Ω in series. Current will diminish while cells discharge to their rated minimum recommended.
For 12V lead-acid; 12V - 4.5V divided by 0.7A = use 11Ω to 12Ω
Or, your 'puck driver' set for 4.5Vf / 700mA instead of resistors.

Check their time on until Li cells reach their minimum voltage (2.5V ¿?) with or without your 'puck driver'
Thank you. Great info. I will definitely be using a (PWM capable) LED driver board with this setup. However, after a preliminary test last evening, I will only need (2) 3w leds (plenty of light). So, allowing 1.5v for each LED & 2 volts for the driver board..... I would have 5Vf.
If I have (3) 18650 batteries at 3000mah each, and 5Vf / 700mah, how long should the lights run? If I use a separate 12v lead acid 7ah, should it run approx 2.3 times as long? Is there a way to calculate the time the lights should run?
 

audioguru

Well-Known Member
Most Helpful Member
The Canadian LED distributor does not know simple arithmetic or maybe he is fooling you. The website says the 3W LEDs are from 2.0V to 3.5V at 700mA which is a power of 1.4W to 2.45W (not 3W).
A Lithium-ion 18650 cell is 4.2V when fully charged and slowly drops to 3.0V when it should be disconnected. Your 3-cells battery will be 12.6V when fully charged and be 9.0V when almost dead. Simply calculate the current and guess the brightness after some discharge time.

All LEDs have a range of forward voltage. Your IR LEDs "might be" 1.5V or they might be 1.2V to 1.8V. You can measure them and sort them or you can calculate a battery voltage and resistor value so that low voltage LEDs do not burn out and high voltage LEDs will be bright enough.
 
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