# Led Flashlight

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#### zsh8me2

##### New Member
I am going to try and make a flashlight using a Seoul P7 led Seoul Semiconductor, Inc, I was wondering what would the best way to build an led driver for this thing, I've been searching for generic led drivers and haven't had any luck. If someone could give me some insight or point me in a direction I would greatly appreciate it.

##### Banned
The forward current on those suckers is 2.8 amps, you're going to have to build your own driver for something that large =) You can use a current sense resistors as feedback into an opamp to control the gate of a FET. Use a unity gain stable opamp feed the center lead of a potentiometer into the - lead of the opamp and feed the voltage from the sense resistors in the + lead of the opamp, starting at it's lowest setting turn the POT up until a current meter attachd to the whole setup reads 2.5 amps or so, and then you know what values to use for your voltage divider (or you can just glue the POT in place)
Keep in mind with that much current going through it those things are going to need some serious heat sinking.

#### zsh8me2

##### New Member
Would you happen to have a circuit drawn for the above driver?
I would like to breadboard all of this before I start to make the flashlight body. I have a couple of LF351N 's and LM324N 's. Once I get a working circuit, I will probably try my hand at surface mount to keep the size small, hopefully. I'll have to do some checking on the current sense resistors. Thanks again

##### Banned
I've made this one before, works nicely. For a 1 ohm sense resistor at 2.5amps the voltage will be 2.5 volts (convenient) so instead of that DAC voltage you would change the 15.5 and 10k resistor values to give you 2.5 volts at the - of the opamp, or use a POT so you can adjust it. Keep in mind, at that current the sense resistor will be disipating almost 7 watts of power. You can use a smaller value sense resistor but you'll have to adjust the voltage at the - gate of the opamp to compensate. If you use a .1 ohm resistor it'll only dissipate 3/4's of a watt, but you'll need .25 volts at the inverting input. You follow all that?

#### zsh8me2

##### New Member
instead of that DAC voltage you would change the 15.5 and 10k resistor values to give you 2.5 volts at the - of the opamp

is that supposed to be "2.5 volts at the + of the opamp"?

"If you use a .1 ohm resistor it'll only dissipate 3/4's of a watt, but you'll need .25 volts at the inverting input."

Are you talking about a voltage divider from V+ to the inverting input, or a voltage divider from the point between the fet and top of the sensing resistor?

Is there a more efficient way to drive the led?

#### ronsimpson

##### Well-Known Member
op-amp and FET? This is not how flashlights are designed. Use a PWM or switching power supply.
What supply voltage?

#### zsh8me2

##### New Member
I'm gonna use either 3 AAA batteries or 2 123 batteries, so to answer the question 4.5 volts or 6 volts. This can be changed if needed though.

#### ronsimpson

##### Well-Known Member
NO.
You have a 10 watt LED. You can not get 10 watts out of AAA batteries.
1 watt=yes, 2 watt=maybe, 10 watt=no.

#### zsh8me2

##### New Member
I'm not married to any configuration, I can use AA C 123 or whatever, I can do 3 c's or 2+ 123's, I havent made anything yet, just trying to get my ducks in a row before I get to deep and find out something may not work.