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Mixing different colored LEDs in series-parallel arrays

I've been doing lots of research, both on other internet sites and the LED projects posted here, but can't find out how to mix colours in LED displays. My plan is to build better Christmas displays from LEDs than the rope light ones you can buy. I've found out how to drive LEDs from a power supply both in series and parallel, with a current limiting resistor. What no one seems to discuss though is driving sets of LEDs off the one power supply where they use different coloured LEDs of different voltages. If I want to wire up lots of red LEDs (2.0V drop) in series and some green LEDs (3.3V drop) in series for example, can I just wire the two sets in parallel to each other (assuming both sets of LEDs in series don't require more voltage than the power supply? Thanks!!!

110100101108 years ago
yep just connect each with its own resistor if the current of all leds is the same you may connect them all in 1 series string
FutureCow (author)  110100101108 years ago
Thanks for the reply - the problem is that I want to connect about 200 LEDs and don't want to lose them all through failure of one LED.
Therefore I was planning on
  • running a 12V power supply
  • Having 4 LEDs per group of the same color LED- wired in series with their own resistors
  • Having 50 groups (different groups with different colors so different voltages) in parallel to each other
I don't know how you calculate the resistors required for this - or if I'd need additional components.
you connect the leds in series strings. few leds and one resistor in each. then you connect the strings in parallel to the 12 V

each string is on its own and not affected by others. this applies for all strings of same or different or mixed types of leds

you can mix different types of leds in one series string

if all the leds are for the same current (mA) then they'll work perfectly same as if they were powered each on its own. different voltage of the leds does not matter

if the leds are for different current then you should run the mixed string at current thats ok for all leds (the max of the smallest one) and the larger leds will be dimmer

how we determine the shape of strings and resistor ? (i should make this an instructable !)

first - a note. if you power it all from wall xformer you should keep in mind that its extremely inaccurate in its voltage. a 12 V one may give 20+ V if loaded just with few leds and significantly less than 12 if loaded close to its maximum. computer power supply may be better and at loads of few watts or more is more energy efficient than wall xformer

lets assume now that you have a 12 V suply which is trusted to give volts close enough to 12 V

lets try to make a string with 2.2 V 20 mA leds

first lets see the max size of the string. 2.2 V fits 5 times in 12 V. if we want there to be enough voltage for all the leds we should put no more than 5 leds. we can put less leds if we want

if the leds were 2.4 V it'd still fit 5 times in 12 V. but for 2.4 V leds its better to use 4 leds in string. why ? if the supply drops a bit of a volt below 12 V the string will not light. with 4 leds the circuit is more tolerant to low voltage. assume the power supply to be ~0.9 - 0.95 of its true value. for 5 V aim at 4.5 - 4.6 V. for 12 V aim at 11 V.

now we need to choose resistor. from now on lets treat the supply as its true voltage (if it says 12 V its 12 V). in the 5 X 2.2 V string the leds use up 11 of the 12 V. there is 1 V remaining

we dont vant this volt to go to the leds and burn them. (and although it can be less than a volt it can still be harmfull for the leds). so we need something to discard the remaining energy. a resistor uses electricity and wastes it as heat so its the solution for our problem

we want the current thru the leds to be 20 mA (in series circuit the current thru the string is same as thru 1 led of it). so we want the resistor to allow 20 mA at the 1 V it gets

lets use ohms law : V = A * R (and with units : 1V = 1A * 1ohm or 1V = 1mA * 1Kohm). 1 V / 20 mA = 0.05 Kohm = 50 ohm
the closest resistors that exist are 47 and 51 ohm and they both are ok

lets find how much the resistor heats. it depends on how much power it gets. W = V * A (1W = 1V * 1A or 0.001W = 1V * 1mA). our resistor gets 0.02 W = 1 V * 20 mA of power. a 1/16 W resistor (the tiniest size before smt) is more than enough

if you want the resistors to be near cool dont load them above 0.5 of their max watt. if you want warm but not hot dont load above 0.8 of max watt

for string mixed of different voltage leds its allmost the same. for example you want the string to be of red 2.2 V and blue 3.4 V leds of which all are 20 mA. you want it for a string of xmas lights colored like this R R B B R R B B

lets start from the beginning. we can fit in there R R B B before we run out of voltage. 2.2 + 2.2 + 3.4 + 3.4 = 11.2 V. (its close to 12 but usually works reliably on computer power supply which is quite accurate)

we have remaining 0.8 V which we leave to the resistor. 0.8 V / 20 mA = 0.04 Kohm = 40 ohm. the closest resistor is 39 ohm which is ok. (47 ohm is ok too. 33 ohm will allow 24 mA which won't cause immediately burned leds but will overheat them and shorten their life)

if we wanted string like R R R B B B R R R . . . . we would make parts like this
R R R B = 10 V
B B R R = 11.2 V
R B B = 9 V
B R R R = 10 V
B B B = 10.2 V
R R R B again and so on

or

R R R B
B B R R
R B B _ R (add the next R to it with longer wires)
B _ R R B (the B we skipped)
B B R R
R B B _ R again (note that the repeated pattern is not from the beginning)
B _ R R B (and so on)
B B R R and so on

and the ohm and watts of the resistor are calculated for each type of string the same way

btw - a failed led won't harm other leds (unless it fails in a short circuit way - which is rare). mixing different leds in a string does not put leds at higher risk. dont connect leds in parallel to the same resistor
FutureCow (author)  110100101108 years ago
Thanks a lot 11010010110!!!! I'd worked a lot of that out, but you answered the question I didn't know about getting different voltages in parallel. MANY THANKS!!!!