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led forward voltage ?

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conntaxman

New Member
I bought some 3 watt leds,and they don't have the forward voltage,I want to find the 3 different resistors for running these on 12vdc, by putting all 3 in seires ,just 2 in seires and just one.Here is all the spec. they gave.
------------------------------------------------------
3W LED 80 Lumens Emitter 700mA LXHL-PW09
.
Note each color channel is able to take a maximum of 700mAh of current, 3W rating
A single LED package providing the world's highest brightness of 80 lumens
Ultra slim size, super high luminous efficacy, and low electricity consumption to replace the conventional bulbs
High quality head dissipating base connection board
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
--------------------------------------------
Thank you very much.
John
ps I bought these on ebay. from china.
 

Sceadwian

Banned
You shouldn't use a resistor for high wattage LEDs their forward voltage can change a lot because they run at a pretty high temperatures, you'll want an honest to goodness constant current source to supply them, and even with that you'll need proper heatsinking to drive it at it's rated wattage.

Also you should probably try to google the datasheet for the LED...
https://pdf1.alldatasheet.com/datasheet-pdf/view/228326/LUMILEDS/LXHL-PW09.html
3.0 to 4.4

In the data you posted it also says 3.0 3.27 so I'm not sure why you said it doesn't have a forward voltage.
 
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conntaxman

New Member
led

Sceadwian/thank you.I didn't realize that the voltage that it uses to run on is called the forward voltage,so now I know that the F/V is Voltage Input: 3.0V~3.27V. Tks alot,....Guess I should have bought 4 and run then in seires so that I could run them on a 12vdc battary,Right.Tks again, now I just have to find or make a heat sink.
Tks
Johnny
 

audioguru

Well-Known Member
Most Helpful Member
The datasheet is of a Philips Name-Brand Lumileds Luxeon LED, not for the cheap Chinese copy. The forward voltage of the cheap copy could be anything.

The forward voltage range of the Philips one depends on its colour as shown on its datasheet.
 

ronsimpson

Well-Known Member
Most Helpful Member
"Guess I should have bought 4 and run then in seires so that I could run them on a 12vdc battary"
NO!
3.0 volts * 4=12 volts
3.27 volts * 4=13.08 volts
In the case of 3.0 volts you will see light.
In the case of 3.27 volts you will not see light. (not much)
This voltage varies from part to part and over temperature.
LEDs need constant current, or nearlly constant current.
You should never run LEDs at constant voltage.
 

Sceadwian

Banned
Constant voltage is fine for 20ma varieties, for typical usage, not for power LEDs. The possible power disipation in a properyl valued 20ma LED is so low that it can be ignored if common sense is applied.
 
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conntaxman

New Member
I read the messages.I was planning to build lights with these leds for when the power goes out. I also just tried one of them just to see how bright it would be in a dark room.Pretty bright just with one 3 watt.But it was funny, I tried to just use a 1 1/2 vdc battary just to see if it would light.Nope.so I put another battary in series and it lite.I thought it would be dim with 1 1/2 vdc......I tried to figure out the amps these would use form a auto battary.I also compared it to a 100 watt light bulb.[for useage of ampridge],is this right for 5 in seires.
100 watt light bulb / 120 volts = ~.83 amps:::
5 led 3.5vdc at 700mah =3500 ma = 3.5 amp [I guess this would be per hour.]
-------------------------------
is this about correct.
And I also found out that you do NEED a heat sink.That led got hot.
Im aslo having fun doing it. ha ha ha ha .
Thanks guys
Johnny
 

audioguru

Well-Known Member
Most Helpful Member
5 LEDs in series that are each 3.5V use a total voltage of 17.5V.
If their current is 700mA (0.7A) then their total power is only 12.25W.
 

conntaxman

New Member
super leds

Hi audioguru and thank you very much for the reply . So I guess that the 5 leds wouldn't draw much out of a auto battary deep cycle. hee hee
tks
Johnny
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

I have used series resistors for driving high power LEDs with success.
I run them slightly under spec though to make up for voltage variations
and also to extend life.

If you have LEDs that you dont know the forward voltage for, run them up
to their rated current and measure it. You might need a variable voltage
supply and some resistors to do this.
 

conntaxman

New Member
MrAl

MrAl,,,tks for the reply. but with these so far I think that I will put 5 in seires seeing that the forward voltage is 3.5 or 3.7. This way I think that running off a 12 vdc auto battary they will work ,even if the battary goes to 12 or 14 vdc..Ill use a heat sink for the leds.I hope that it will work.I guess it should.
tks
Johnny
 

Sceadwian

Banned
conntax, your LED's will dim and brighten with the system voltage dramatically. Higher power LEDs for constant light level REQUIRE constant current sources. Thermal considerations creep up next.
 
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ronsimpson

Well-Known Member
Most Helpful Member
3.2 volt, 350mA LED Golden Dragon LCW W5SM
Forward voltage
min, Typical, max
2.5 volts, 3.0 volts, 3.5 volts = 150mA
2.6 volts, 3.1 volts, 3.6 volts = 225mA
2.7 volts, 3.2 volts, 3.7 volts = 350mA ******
2.8 volts, 3.3 volts, 3.8 volts = 500mA
2.9 volts, 3.4 volts, 3.9 volts = 650mA
3.0 volts, 3.5 volts, 4.0 volts = 800mA
This is at 25 C.
At -25C add 0.2 volts
At 0C add 0.1 volts
At 25C add 0 volts
At 50C subtract 0.1 volt
At 80C subtract 0.2 volts
At 120C subtract 0.3 volts

Do not drive any LED with constant voltage (even the 20mA LEDs).
In this case, you could use 4 LEDs to make a "12 volt LED".
If the voltage drops 0.8 volts the current (and light) will be 1/2!
If the voltage rises +0.8 volts the current and light will be 2X and will burn out! (This is +/-0.2 volts/LED.) This is at 25C and your part will heat causing a 2X larger change.

Look at the TPS61160 from TI.COM to see a constant current application.
 

conntaxman

New Member
Leds

Well everyone now has me thinking,My next question is .Then what would be the best way to light 3 of these 3 watt LED's with the forward voltage is 3.5 .I didn't look on the net for any IC's [regulator] with an input or 12/15 and an output of 3.5vdc.This way I could feed each one or get the The 7809 is a 9 V voltage regulator IC . and then feed 3 at once.
tks
Johnny
 

audioguru

Well-Known Member
Most Helpful Member
You do not feed LEDs from a voltage regulator. You feed them from a current regulator or from a current-limiting resistor.

Your LEDs might be 3.5V or might be anywhere from 2.7V to 3.7V.
 

conntaxman

New Member
Led

audioguru,Thanks ,never knew of a "current regulator" great,Thanks. Sooo now ha ha ha ,,,,I just have to look for aaaaaa either a 3.5 or for 2 a 6 or a 10.5 for 3 of these. lol. Ok so dose any one have a cir. that would light 3 of these from a 12dc auto battary?Or is their one on the net.
Thanks Guys.I do plumbing,heating,remodeling, ect, not making cir...oh well.Im trying ,If I had a cir. to follow no problem then I could make it.
Thanks again guys.
Johnny
 

audioguru

Well-Known Member
Most Helpful Member
The LM317 articles are not correct. They forgot to say that the LM317 needs at least 2V plus the 1.2V for the current-setting resistor. Then the total minimum voltage needed is 3.2V for the current regulator.

You want three LEDs in series that need at most 3.7V each which is 11.1V plus 3.2V for the current regulator. So you need a power supply that is a minimum of 14.3V which means a car battery will not work. Use the LM317 with two LEDs in series.
 

alphacat

New Member
You do not feed LEDs from a voltage regulator. You feed them from a current regulator or from a current-limiting resistor.

Your LEDs might be 3.5V or might be anywhere from 2.7V to 3.7V.

Assuming that you pick a current-limiting resistor that suits a 3.5V LED, but the LED connected is a 2.7V LED.
What will happen to the 2.7V LED?
Will it burn or will just have stronger illumination?

Thanks.
 
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Sceadwian

Banned
Depends on the driving voltage.
Assuming 5 volts the lower voltage LED will get 10ma more current. It'll be really bright, and probably not last nearly as long. The higher the driving voltage the bigger the resistor you'll use is which makes it closer to an ideal current source, so the over current will be less.
Moving to a 10 volt supply and a properly valued resistor for the higher LED will give you only 3ma more current for the second led
 
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