Hello guys i want to know what kind of resistor i have to use for my hp led's i have 28v charger witch one i want to use for my LEDS so i have
16 hp leds 1w
Forward Voltage : 3.2V~3.4V
Forward Current: 300~350mA
Color temperature : 3300k ~ 3500k
Intensity Type: 60~70Lm
oh and i want to make then in one row or maybe two ..!
Your resistor value is:
Vsupply - VLED / ILED
Vsupply = supply voltage
VLED = LED forward voltage
ILED = Current the LED usese
If your putting more LED's in series calculate the total VLED
This is basic ohms law R = V/I
For the power of the resistor it's ILED x Vsupply - VLED
It's normally not recommended to use simple resistor current limiting for higher power leds, but rather drive them with proper constant current driver circuits.
Lefty
why not ? although i can imagine that the power of the resistor can easily become big but ultimately i can't see why now the resistor if the voltage is constant enough.
current sources will be very useful if a variable supply voltage is a problem
can u please show your example because when i try to calculate it i just get some extreme numbers ...
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sparkylabs:
Your resistor value is:Vsupply - VLED / ILED
Vsupply = supply voltage
VLED = LED forward voltage
ILED = Current the LED useseIf your putting more LED's in series calculate the total VLED
This is basic ohms law R = V/I
For the power of the resistor it's ILED x Vsupply - VLED
This is the size you need: http://www.ebay.com/itm/321062309076
Assuming forward current 325mA (0.325A) and voltage 3.3V:
28V - 3.3V = 24.7V drop on the resistor
24.7V / 0.325A = 76ohms
24.7V x 0.325A = 8W !!!!!!
I agree with Lefty. You loose/waste quite a bit of power & heat in the resistor, and variations in supply voltage or LED voltage will result in variations in actual vs. calculated results. Power supply effects are actually exaggerated... The more voltage you drop across the resistor, the less the "exaggeration" (but more power is wasted).
Your 28V charger may not be constant & regulated at 28V... You might have to measure it with the load and adjust the resistor values accordingly.
can u please show your example
It depends on how you you wire-up the 16 LEDs. If we configure 4 LEDs in series with a resistor (and repeat that 4 times) You'll have...
3.3 x 4 = 13.2V across the LEDs.
28-13.2 = 14.8V across the resistor.
14.4V / 325mA = 46 Ohms. (47 Ohms is a standard value)
14.4V x 14.4V / 47 Ohms = 4.4 Watts (To be safe & reliable, use 10W resistors.)
With 4 of these circuits each drawing about 325mA, your power supply will have to supply 1.3 Amps. (About 36 Watts, with about half the power going to LEDs and about half to the resistors.)
if the current source is a SMPS type then yes it is a very good idea for that sort of current, If your source voltage is stable enough then you can put them in series and may end up with a small voltage difference to soak up.
fungus:
This is the size you need: http://www.ebay.com/itm/321062309076
As a side effect of not using a proper regulator, you'll be able to warm your feet on it in Winter.
Hint: Using 3.2V out of 28V is only 11% efficient. The other 89% of the power gets converted directly to heat.
And where i could find and how i can know what kind of driver to use for the led's there is allot of drivers over the internet .!
retrolefty:
It's normally not recommended to use simple resistor current limiting for higher power leds, but rather drive them with proper constant current driver circuits.Lefty
you need one that delivers the current you want or that can be adjusted to that current and that can take your supply voltage, although I personally have never used one
sparkylabs:
why not ? although i can imagine that the power of the resistor can easily become big but ultimately i can't see why now the resistor if the voltage is constant enough.current sources will be very useful if a variable supply voltage is a problem
The reasion why it is not a good idea is the the forward voltage on a power LED is not stable over time and temprature. Also the small resistor size you have to use will not approximate a constant current supply like a higher resistor will.
Yea I suppose so