2 SMD LEDs on 1 resistor

[johnnyrails]

Hello all,

Will 2 smd leds work off of the same resistor?

Or does each led need its own?

[gmpullman]

As long as the LEDs are nearly identical in their current rating I don't see a problem with using multiples on one resistor.

These common LED strips have three LEDs sharing one resistor with the LEDs in series:

Regards, Ed

[johnnyrails]

Ed, 

So the Led's MUST be in series to work?

Thanks John...

[mfm37]

Series or parallel will work. The resistor value will be different for each version. Series would be best for different led's on one resistor. You will need to allow a large enough resistor (watts) depending on how many led's you use.

Martin Myers

[gregc]

if the supply voltage is greater than the sum of the LED voltage, then an appropriately sized resistor can be used and the same current flows thru both LEDs.  For example, 5V is sufficient to supply two 2V LEDs.

If the LED operating voltages are the same, a single resistor in series with the LEDs in parallel, will pass the sum of the current for each LED.  For example, if each LED draw 15 ma, 30 ma will pass thru the resistor.

If the operating voltage of the LEDs is different, then the LED with the lower voltage will limit the voltage across the 2nd LED and it may not light.

[richg1998]

From a Google search.

http://led.linear1.org/led.wiz

Below might be helpfull.

http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm

Rich

[rrinker]

 Parallel LEDs with one resistor are more likely to be a problem than series LEDs with ooen resistor. With parallel LEDs, if they are not exactly matched, one will almost always be brighter than the other as it gets the lion's share of the current (not our favorite subway modeler). In series, all LEDs get the exact same current, because Kirchoff says so. (and you can measure this to be so, if you don't believe a 19th century German fellow) As mentioned above, this is how the common LED strip lights work withotu needing some big power resistor to handle the high wattage - groups of 3 LEDs and one resistor all in series, each group in parallel across a 12V supply line. That's also why they are specifically marked where to cut them to length.

 Also proof the order of the components in a series circuit makes no difference on the current - if you look at the strip LEDs, it is often LED, LED, resistor, LED, <cut mark>

 I still prefer 1 resistor for 1 LED, just to keep things neater. Since SMD LEDs are usually run at very low currents, the resistors can be small SMD types as well, if space is a concern. To make that easy, Ulrich (now closed) used to sell a resistor board with SMD resistors attached and larger human-size soldering pads to attach the wires to. I don't know if anyone has picked up making thiese or not - check Streamlined Backshops or similar. It WILL work find in series - if this is a DCC application, you have enough voltage at the function wires to put up to 3 LEDs in series. Resistor to use is calculated the same as for a single LED - just add the two LED voltages together for to forward voltage value, and the current is the same as a single LED.

 Single, 3.5V LED, 12V supply:

   12-3.5 = 8.5V to drop in the resistor. 1K resistor = 8.5V/1000 ohms = 8.5mA

Two 3.5V LEDs, 12V supply:

  12-3.5-3.5 = 5V to drop in resistor. 8.5mA = 5V/?  ? = 58.8 ohms, in the common 2% tolerance types yoou can find all over, you can get a 59 ohm.

                    --Randy