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So I have my own project that I want to make. In this Project i want to light up 7 led diodes (1 contains 3,0-3,6V each).

To light up 7 led diodes, how much voltage do i need (min to max) and how big of a resistor is required (min to max)? Any sort of assistance would be appriciated!

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  • \$\begingroup\$ what voltage supply do you have? \$\endgroup\$ – Wesley Lee Nov 30 '15 at 19:41
  • \$\begingroup\$ Since you are a beginner, and since you said, "soldering", be careful not to overheat the LED's while soldering them. \$\endgroup\$ – gbarry Nov 30 '15 at 20:11
  • \$\begingroup\$ By all means calculate this out. However, before you solder something together it would be great to try out your plans on a solderless breadboard with a resistor assortment on hand. That is, assuming you are using through-hole LEDs. If you were using SMT ones you might just skip to a PC board - changing out SMT resistors isn't all that hard. \$\endgroup\$ – Chris Stratton Nov 30 '15 at 21:42
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Take a look at this calculator:

http://ledcalc.com/

You can find V forward (voltage drop) for your LED color in tables such as these:

enter image description here

If your LED looks like the following image then the typical current would be 10~20mA.

enter image description here

enter image description here

You'd want to find your LEDs current vs Vf graph such as the one above, then plot and find the Vf for the current you want it to operate on, then use the formula:

Vsupply - Vf*(number of LEDs) = current * resistance

Then round "resistance" to the closest standard value. (I'd round up for lower current).

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  • \$\begingroup\$ Thank you alot! These Pictures and explanations is exactly what i should study! \$\endgroup\$ – Engineer Dec 2 '15 at 18:04
  • \$\begingroup\$ Just a quick question what is TYP ? Does it mean typical aka average ? \$\endgroup\$ – DollarAkshay Feb 7 '16 at 4:01
  • \$\begingroup\$ yes you could interpret it like that \$\endgroup\$ – Wesley Lee Feb 10 '16 at 7:38
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It is important to note that LEDs are primarily current driven devices - that is, the current, and especially the maximum current, is what you need to control. This should be well documented in the specification (or data) sheet for the specific parts you have.

For high output LEDs, thermal management is also important, and will also be discussed in the data sheet and possibly also in application notes.

As such, a sophisticated approach to LED operation is to build a current source. The typical voltage source + resistor is a crude and often perfectly adequate approximation - but you need to design those pessimistically or you may find that the LEDs expire somewhat before you anticipated it. So, if you are running 7 LEDs in parallel which have an operating voltage range of 3.0-3.6 volts, you design as if they were 3.0 (or 2.9) volts when operating and select a resistor (one for each LED) that will pass the appropriate current (maximum or somewhat less) based on that assumption. So if you have a 20 mA LED and a 5 V supply, you subtract 3V from 5V to get 2V across the resistor, and 100 ohms comes out for the resistor.

If your supply is 12V you either get 450 ohms, or you stack a couple LEDs in series and use 300 ohms, or 3 in series and use 150 ohms.

It's always safe and usually advisable to use a bit more resistance (less current) - LEDs die quickly when the current is too high, and live a long time when it is low. If your LEDs have a different current rating, just apply R=V/Ito determine the correct resistor size.

Or, look into simple current sources (such as the ever popular LM317 adjustable voltage regulator in current source mode) and drive your LEDs with a current source.

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  • \$\begingroup\$ Thanks for the good and specific explanation! I'll definitely have a look later on! :D \$\endgroup\$ – Engineer Dec 2 '15 at 18:02

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