led forward voltage ?

led

Went and picked up a LM317T. And I guess by what this says I need a 9 ohm resistor.for the 2 leds.Is that correct ? and to light 3 in series I would need to use a LM338T because that could take up to 5 amp. I didn't figure out the resistor for the LM338T yet.
Tks
Johnny
Thought that I would be able to find this SMALL cir. on the net some were.oh well

conntaxman said:

Went and picked up a LM317T. And I guess by what this says I need a 9 ohm resistor.for the 2 leds.Is that correct ? and to light 3 in series I would need to use a LM338T because that could take up to 5 amp. I didn't figure out the resistor for the LM338T yet.
Tks
Johnny
Thought that I would be able to find this SMALL cir. on the net some were.oh well

Click to expand...

We do not know what you are talking about.

You and we do not know if your LEDs are 2.7V or 3.7V or in between.
If they are 2.7V then three in series need at least 8.1V plus 3.2V for the current-regulator circuit. Use Ohm's Law to calculate their current or to calculate the value for their current-limiting resistor.

If your LEDs are actually 3.7V each then their total is 11.1V and they still need at least 3.2V for a current regulator circuit. Do you have at least 14.3V?

If your LEDs are in parallel then show your schematic.

5A is a lot of current. What are you doing with it? Burning out LEDs?

audio,I bought 3 of these LED and would like to run 2 in seires. off a 12dc car battary.here are the spec.
3W power consumption
Copper base
Voltage Input: 3.0V~3.27V
Current: 700mA
Brightness: 80 lumens (manufacturer rated)
Color Temperature: 6700K white
Size: 0.94 in x 0.47 in x 0.24 in
Tks
Johnny

conntaxman said:

audio,I bought 3 of these LED and would like to run 2 in seires. off a 12dc car battary.here are the spec.
3W power consumption
Copper base
Voltage Input: 3.0V~3.27V
Current: 700mA
Brightness: 80 lumens (manufacturer rated)
Color Temperature: 6700K white
Size: 0.94 in x 0.47 in x 0.24 in
Tks
Johnny

Click to expand...

The spec's for your cheap Chinese LEDs do not have enough detail for you to use them in a simple circuit.
You should use Name-Brand LEDs that have guaranteed detailed datasheet spec's.

Why do people buy cheap junk?

I see some 'smoked' LED's in his future.

The best way to drive these high power LED from a battery is with a switching power supply that produces constant current.

If you do it the simple way with dropping resistors then keep at least 25-30% overhead voltage drop across current limiting resistor.

This will be painful as the resistors will generate significant heat. Also don't forget the LED's need their own heat sink.

led

I didn't think that a small simple cir. would be hard for someone that knew electronics. At what I use to do, I Never would say ,,It cant be done,I never gave up,and I always figured it out.I was just looking on the net and they make flash lights with 3 watt leds,If the led drew 3 volts, and a car battary usually is 12 to 14 vdc, I cant see why I couldn't put 5 in seires that would be 15v.Sure the brightness would go up or down "alittle" but I dont care if these are just for when the power goes out.Or just run 3 volt battaries.Or get the older type lead acid battary and TAP into the battary just to get 3 volts.I still say that someone on this whole internet would have a cir. for these 1-2-3 watt leds that would run on a 12volt car battary.I only have 3 right now,but if I did have 5 I would try them in seires and see what happens.
Tks
Johnny
Still looking.

I know this is a little complicated but this is what is inside LED flashlight bulbs. It will take a voltage and boost it up and regulate the current. The LED voltage must be higher than the battery voltage. It will keep the current constant untill the battery drops down to 3 volts.
https://www.electro-tech-online.com/custompdfs/2009/10/tps61160.pdf
I have used many different types of LED drivers. Some work down to 0.6 volts.
There are some "buck down" parts but I usually use "boost up" type.

1 to 8 volts in.
https://www.electro-tech-online.com/custompdfs/2009/10/ZXSC310.pdf
https://www.electro-tech-online.com/custompdfs/2009/10/LM3410.pdf

hope this helps

led

ronsimpson-I know that I brought up a flash light,.but All im trying to do is run these leds off of a 12vdc car deep cycle battery when the power goes out.I dont think that it is a big deal./I fool around eletronic's just a bit,but electrical I can do pretty well.To build a cir. I don't do that.That isn't my trade.ALL I asked from those that do this stuff is a Small cir.[which I know is very simple].
oh well. I;ll keep looking and trying.
also I can just buy a car tail light with these leds in it, lol ..see how easy it is.
But thanks for the reply.
TKS
Johnny.

hi. m a newbie...... i wanted to light an led with 1W, 350mA....
I used a 20V derived from mains.... using 7805 I obtained a constant current source..
gave 20V i/p to 7805, betn o/p n common terminal of IC it gives 5V. across those two terminals i connected 13ohm to get 350mAs... the led is connected bet gnd n common of 7805....
i think this may help u......
bt u need to take care of ic as it gets heated much..... if u use this
hope this helps u

conntaxman said:

If the led drew 3 volts, and a car battary usually is 12 to 14 vdc, I cant see why I couldn't put 5 in seires that would be 15v.

Click to expand...

An LED does not draw 3V like a light bulb. Instead an LED draws as much current as it can until it burns up if nothing limits the current.

They cannot make LEDs with a precise voltage like a resistive light bulb so each LED has a different voltage (2.7V to 3.7V). Since an LED is a diode then its current changes a lot if a varying voltage is fed to it. The current will be way too high or will be nothing.
Therefore you must limit the current or regulate the current.

This is from National's data sheet.
LM317 can be used as a voltage or current regulator.
The LM317 tries to make 1.25 volts between ADJ and Vout.
In current mode it will sent current to the load (LED) untill there is 1.25 volts through the resistor.
1.2 ohms = 1 amp
12 ohms = 100mA
The regulator needs Vin to be > Vout by a couple of volts.
The LM317 will get hot so add a heat sink.

I hav tried above circuit n found that along with constant current this circuit also provides sufficient forward vltg for almost 4 LEDs......... this one is good

Need LED flash torch. pls...help me dear..................