oldsaltman
New member
I understand that, but I don't know if Icefire does. If they were working with 4.5v before then they will work with 4.5v now.
Help Running moonlights on batteries.
- Thread starter saltwater4life1
- Start date
Sisterlimonpot
Premium Member
It will definitely work...
But I think what is in question now is the longevity of the diodes
But I think what is in question now is the longevity of the diodes
Icefire
Seasoned reefer
oldsaitman the only way to make it work without resistor is by having batterie that are weaker in current than the leds.
Let say you have a Luxeon Led that can take 350mA. If you have a battery than can't go any higher than that, it will supply this but intensity will change drasticly when the voltage will drop when it get empty.
That's my you use constant current puck driver (mini ballast for leds).
Sisterlimonpot have it right on how it need to be wired, you need a resistor. Without it too much current will flow and the led will die. It's not something that need a specific voltage but a specific current, like Fluorescent lamp.
But if you don't believe me, try for yourself, but put some googles on because leds can explose due to high current.
Let say you have a Luxeon Led that can take 350mA. If you have a battery than can't go any higher than that, it will supply this but intensity will change drasticly when the voltage will drop when it get empty.
That's my you use constant current puck driver (mini ballast for leds).
Sisterlimonpot have it right on how it need to be wired, you need a resistor. Without it too much current will flow and the led will die. It's not something that need a specific voltage but a specific current, like Fluorescent lamp.
But if you don't believe me, try for yourself, but put some googles on because leds can explose due to high current.
saltwater4life1
Member
So your saying i will need resistor even if it's only 4.5v adapter?
Sisterlimonpot
Premium Member
I think that icefire hit the nail on the head. a resistor(s) is/are the way to go. can you read my schematic? it's pretty simple. prolly go with 20 ohm resistors per LED. So my first picture you need 1 80 ohm resistor and my second pic you would need four 20 ohm resistor
stugray
Premium Member
Something no-one has asked or mentioned:
There was a picture posted back in the thread of the board that these LEDs are mounted on.
Perhaps there is ALREADY a resistor in the printed wiring board.
Icefire,
You appear to get the current limiting feature of the LED & resistor combo however, you are not 100% correct.
I CAN drive a LED that has a forward drop of 3.5 volts with a regulated 3.5 Volt power supply even IF the supply is capable of driving 10 amps.
As I posted on another thread:
"So: If you have a LED that "drops" 3.8 Volts at 20mA, Will you damage it if you put exactly 3.7 Volts across it with NO Current limiting resistor?
The answer is NO.
So I suggested that fishoutawater put enough LEDs in series to guarantee that the maximum voltage per LED is not exceeded under operating conditions.
Here is a datasheet for a LED I use a lot.
http://www.cree.com/products/pdf/XLamp7090XR-E.pdf
Look at the data sheet and tell me:
How much current will go through the LED if I apply exactly 3.5 Volts with no current limiting resistor?
Next tell me how much current I will get through a series string of 5 of these LEDS if I apply 17.5 Volts with no current limiting resistor?
Answer: 700mA for both cases.
How do constant current drivers work? A current limiting resistor ...... NO they regulate the voltage to set current level. If the load varies, so does the voltage to adjust current."
Stu
There was a picture posted back in the thread of the board that these LEDs are mounted on.
Perhaps there is ALREADY a resistor in the printed wiring board.
Icefire,
You appear to get the current limiting feature of the LED & resistor combo however, you are not 100% correct.
I CAN drive a LED that has a forward drop of 3.5 volts with a regulated 3.5 Volt power supply even IF the supply is capable of driving 10 amps.
As I posted on another thread:
"So: If you have a LED that "drops" 3.8 Volts at 20mA, Will you damage it if you put exactly 3.7 Volts across it with NO Current limiting resistor?
The answer is NO.
So I suggested that fishoutawater put enough LEDs in series to guarantee that the maximum voltage per LED is not exceeded under operating conditions.
Here is a datasheet for a LED I use a lot.
http://www.cree.com/products/pdf/XLamp7090XR-E.pdf
Look at the data sheet and tell me:
How much current will go through the LED if I apply exactly 3.5 Volts with no current limiting resistor?
Next tell me how much current I will get through a series string of 5 of these LEDS if I apply 17.5 Volts with no current limiting resistor?
Answer: 700mA for both cases.
How do constant current drivers work? A current limiting resistor ...... NO they regulate the voltage to set current level. If the load varies, so does the voltage to adjust current."
Stu
oldsaltman
New member
I would have thought there was a resistor in the line already for it to be working properly. That is why I gave him the answers I did.
Sisterlimonpot
Premium Member
Same here.
Icefire
Seasoned reefer
stugray That might work but you need to look at the curve of the Cree led, At 3.5V it will flow 700mA a mere 0.2 volt more and you are at 1000mA.
You need a pretty damn well regulated PSU.
for oldsaltman and sisterlimonpot, not all led light have resistor, if they design they leds to use battery just under the Vf of the led, it's not required. Personnaly I wouldn't take the chance, if you use 10$ leds and a damn resistor cost less than 0.10$, why not put one?
You need a pretty damn well regulated PSU.
for oldsaltman and sisterlimonpot, not all led light have resistor, if they design they leds to use battery just under the Vf of the led, it's not required. Personnaly I wouldn't take the chance, if you use 10$ leds and a damn resistor cost less than 0.10$, why not put one?
stugray
Premium Member
Icefire,
I agree with everything you said in the post above.
It's just that I have had this conversation before that you MUST control LEDs with "current".
If you string enough LEDs together to make it such that the voltage drop across each LED more than accounts for the supply voltage then:
1 - you dont need a current limiting resistor ( which wastes power )
and
2 - you will run the LEDs at less than the rated current ( which extends life ).
One misunderstanding about talking about the "current limiting resistor" is that it does NOT "limit current" any more than it "drops voltage".
So the installation of the "current limiting resistor" really just forces the LED to run at the rated forward voltage drop.
Stu
I agree with everything you said in the post above.
It's just that I have had this conversation before that you MUST control LEDs with "current".
If you string enough LEDs together to make it such that the voltage drop across each LED more than accounts for the supply voltage then:
1 - you dont need a current limiting resistor ( which wastes power )
and
2 - you will run the LEDs at less than the rated current ( which extends life ).
One misunderstanding about talking about the "current limiting resistor" is that it does NOT "limit current" any more than it "drops voltage".
So the installation of the "current limiting resistor" really just forces the LED to run at the rated forward voltage drop.
Stu
