hello guys i want to connect 4 LED's and i do not know what power supply i should be using. the Led's are 350 mA 2.1 V. What power supply would i need?

 

i forgot that its not strips, its 4 led's in series

 

2 x 2.1V = 8.4V so at least that is required. Go for a 12V supply with a suitable current limiting resistor, or a dedicated constant current LED supply. And limit the current to less that the max current. Try 250mA and see how that goes. The LEDs will need to be on a heat sink.

 

How would i calculate this? I need 4 leds of 2.1V each running at 350mA in series.
Id have to do 2.1V*4 = 8.4V and 350mA*4=1400mA
So ill need a dedicated power supply of 8.4v and 1400mA right?
What resistor would i need to go at 8.4 with a 12v supply?
How do i calculate?

 

Why not 9 volts, and use a smaller resistor, and waste less power?

Of course if you ever decide to go to white or another color that requires more voltage the 12 would be better.

No the LEDs in series will only draw the 250 or 350 mA you require.

http://www.quickar.com/starresistorcalc.htm

Plug in 8.4 in the voltage drop box and the desired current in that box and it will tell you the resistor value.

 

With a 9v power supply id need a 3.57 ohm 7w resistor. Isnt that pretty huge?

 

Your voltage calculation is right--8.4 volts. But LEDs in series all use the same current so 350 mA. If you use a 12 volt supply (one that is really 12 volts DC) then 12 - 8.4 = 3.6 volts. R = E/I so R = 3.6/.350 A = just over 10 ohms. Remember that LEDs use DC current so don't try to use an AC source. Start with 18 ohms, if it is not bright enough, use 15. If it is still not bright enough go to 12. I would not go to 10 unless you have a meter to monitor the current. Your wattage will be 3.6 volts x .35 A = 1.26 watts; use 5 watt resistors. Your LEDs are going to use 8.4 volts x .35 A = just over 3 watts; use a big heat sink.

If the seller of the LEDs offers a constant current supply circuit for them, probably you should use it.

 

So a 8.4v 1000mA …18650 battery charger would work perfectly withot the need of residtors right?

 

 

So a 8.4v 1000mA …18650 battery charger would work perfectly withot the need of residtors right?

NO!
Unless you use a correctly designed and set constant current supply NEVER run LEDs without a current limiting resistor. Unless you want to destroy them that is. LEDs are current devices, not voltage. The voltage specified is their typical forward drop when running. You can run them on any voltage above the minimum as long as you can control the current to be less than the maximum limit.
And I would not run them at their max current rating either, to increase their life.
And a 12V supply is better than a 9V one.
12V is easier to get and if you use 9V and such a low value resistor, any variation in the 9V will have a greater variation of the current. That is another reason to run under the max current rating.

 

Why not 9 volts, and use a smaller resistor, and waste less power?

Because then it might burn out the LEDs or not work at all.

LEDs typically have a forward voltage tolerance of well over a quarter volt on either side of the nominal voltage at rated current.

At a rated Vf of 2.1 V the voltage range is likely something around 1.8 V to 2.4 V. Even if the supply is a rock steady 9 V (which it probably isn't), any Vf above 2.25 V and the string won't light at all. If you size the resistor for 250 mA under the assumption of 2.1 V, you get a value of current of 2.4 Ω. If the actual Vf is 1.8 V, then you current will be 750 mA, which will seriously reduce the lifetime of the LEDs.

 

hello guys i want to connect 4 LED's and i do not know what power supply i should be using. the Led's are 350 mA 2.1 V. What power supply would i need?

Do you have a part number or a data sheet for the LEDs you are using?

 

Hello,

You can also make a special driver for the leds.
A LM3421 will likely suit your needs.

Bertus

 

It is also possible to modify boost regulators such as this one to be a constant current source for LEDs. The same principle could also be applied to step down regulators. This is a link to how I modified a boost regulator similar to the one in the first link.

Les.

 

This are the Sheets of the leds ill be using, https://www.mouser.it/ProductDetail...=sGAEpiMZZMt82OzCyDsLFFX0UwfFOHkp13FERAV0Ieg=
I think i could use a B version meanwell power supply so i can add a 100k ohm dimmer to dim them.
Problem is im getting confused on what supply to use.
Id like to be able to regulate them to their max current when needed.

 

Those star mounted LEDs have no detailed datasheet, just the "Selection Guide".
Nobody makes a "2.1V" LED because an LED part number has a range of voltages.
Mouser does not make them and Opulent also does not make them. They are Cree LEDs and the 2.1V ones are forward Voltage Group C which is from 2.0V to 2.25V. If the ones you get are 2.0V but you calculated the current-limiting resistor for 2.1V then they will burn out soon. If the ones you get are 2.25V and you have three in series using a 6.3V to 6.6V supply then they might not produce any light.

I could not find information about the Cree LED that is mounted on an Opulent "star board". Somewhere it said 350mA if the star is cooled to 25 degrees C somehow. How will you cool them so that you can use the absolute maximum allowed power?

 

Specs say 2.1 volt forward voltage at 350 ma forward current on the Mouser link.
SG

 

Yes thats what it says...

 

I think i could use a B version meanwell power supply so i can add a 100k ohm dimmer to dim them.
.

Those B version models are usually high voltage, designed for a high number of series LEDs. Small regulated 12 volt supply followed by a PWM dimmer may be in order. You will still need to figure the correct value for the series resistor .
https://www.amazon.com/LEORX-LED-Tr...597&sr=8-11&keywords=12+volt+led+power+supply
https://www.amazon.com/Leegoal-LEDw...TF8&qid=1516550634&sr=1-6&keywords=led+dimmer
SG

 

I actually changed the led setup i want. It will still be 4 leds . What i dont understand is how to pick the right power supplies. This are the leds i want to get. And the spreadsheet. Maybe somone can help me figure out what meanwell i need?
XPEFAR-L1-0000-00701
https://www.mouser.com/ds/2/90/XLampXPE-221789.pdf