LED flashlight voltage... how would you do it?

Thread Starter

trader007

Joined Feb 27, 2010
249
Building my own PCB for a flashlight. My constraints are-

LED's- 6v modules, qty 6, with 650ma of current each (4a total).

Supply voltage- 12v DC. When the cord is unplugged, the internal lithiums will give 6.5v-8.4v DC. There is a battery charger IC that will take the 12v to charge the batteries. This device needs to run without the batteries (in case they go bad or I don't have them), so I need to make the circuit work from 6v-12v.

Because of this, I was planning on running the LED's in series/parallel to need 18v. I think this is ideal because I would use this step up converter to turn anything from 6v-12v into 18v. https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP3426/document_id/1125

Would you guys do something different? I've been designing the schematics in kiCad but until im kinda stuck on what the LED voltage should be in an ideal world. Battery life is a concern but if I get >80% efficiency I would be more than happy. If I configure the LED's for 12v I would have to isolate and switch the voltage booster off when it's running on a 12v supply... which I could do but if I run on 18v then I can simply isolate with a diode I believe.
 

Picbuster

Joined Dec 2, 2013
1,007
Building my own PCB for a flashlight. My constraints are-

LED's- 6v modules, qty 6, with 650ma of current each (4a total).

Supply voltage- 12v DC. When the cord is unplugged, the internal lithiums will give 6.5v-8.4v DC. There is a battery charger IC that will take the 12v to charge the batteries. This device needs to run without the batteries (in case they go bad or I don't have them), so I need to make the circuit work from 6v-12v.

Because of this, I was planning on running the LED's in series/parallel to need 18v. I think this is ideal because I would use this step up converter to turn anything from 6v-12v into 18v. https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP3426/document_id/1125

Would you guys do something different? I've been designing the schematics in kiCad but until im kinda stuck on what the LED voltage should be in an ideal world. Battery life is a concern but if I get >80% efficiency I would be more than happy. If I configure the LED's for 12v I would have to isolate and switch the voltage booster off when it's running on a 12v supply... which I could do but if I run on 18v then I can simply isolate with a diode I believe.
A Led is a current driven device.
In the led specs currents are given.
Ohms laws will produce a serial resistance. ( Vbat - (N x Vled))/( max led current as specified)
Vbat = connected max voltage.
N = number of identical leds in serial.
N=1 when used in parallel. ( each led carry a serial resistor)

P_Resistor = I^2 x Resistor
To dim the led use PWM signal.

Some led do carry an internal resistor and they do have a specified working voltage.

Picbuster
 

DickCappels

Joined Aug 21, 2008
6,449
Aasuming the boost circuit is 100% efficient (it is not), the current from your batter would be 12 amps. How heavy of a flashlight can you carry, and how long would you carry it (how long would the flashlight be on before replacing or recharging the batteries?
 

Thread Starter

trader007

Joined Feb 27, 2010
249
What I would do differently right from the start would be not to constrain the device to 6 volt LED modules.

Well...you asked.
What would the ideal voltage be to you? I could run them at 6v and use a buck driver. And if i really should, i could go with different LED's. Just a bit more work.
 

Thread Starter

trader007

Joined Feb 27, 2010
249
Aasuming the boost circuit is 100% efficient (it is not), the current from your batter would be 12 amps. How heavy of a flashlight can you carry, and how long would you carry it (how long would the flashlight be on before replacing or recharging the batteries?
it will be run by two 18650 batteries. I don't plan on running the LED's at full power... they will be on a PWM driver and i will adjust the max rate where it needs to be to not over amp the batteries or overheat the heatsink. I want more power than I need because i'll be using the same PCB for other projects.
 

djsfantasi

Joined Apr 11, 2010
6,493
it will be run by two 18650 batteries. I don't plan on running the LED's at full power... they will be on a PWM driver and i will adjust the max rate where it needs to be to not over amp the batteries or overheat the heatsink. I want more power than I need because i'll be using the same PCB for other projects.
18650 batteries? Have you investigated their capacities?

Taking the following scenario as your first option, let’s run the numbers

Here is the capacity of your chosen batteries:
The highest capacity for an 18650 battery is the NCR18650G which clocks in at 3600mAh. A high capacity cell that is readily available is the 3500mAh LG MJ1. But the most commonly used 18650 batteries on the market have capacities of 2000mAh, 2200mAh, 2500mAh and 2600mAh.

Depending on how you wire your led modules, the current requirements will change. Two 18650 batteries will supply 7.4v. Your only configuration option is placing all six modules in parallel. That will draw 4A. Using the lower capacity of an 18650, you’ll get only 30 minutes of runtime at most. Using a high capacity battery, you might get 50 minutes. Likely, it’ll be significantly less.
 
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