Different Colored LEDs on same series circuit?

[MSgtUSAFRet]

Okay, I have a relatively newbie question about LEDs. I did a search of the site and didn't see this one asked, so...

I am waiting on some parts and tools for my Enterprise-C build so I am working on the lighting design and have a question or two.

So I know that while LEDs have differing forward voltages, they generally range in the 3mA FV average. The question is, can I (as in is it possible to) place different colored LEDs on the same series circuit?

LED Color Series Schematic

I don't know for certain and thought I should ask.

Lemme know what you all think.

Thanks!

Steve

[ImWolf]

Yes Steve, you can put diff color LED's on the same circuit. If you're wiring the LED's in series (as your diagram shows) then you'll want the LED's to all be of approximately the same voltage so you can use one resister as needed to compensate for what ever your power supply voltage is. (but you really don't want to wire in series if you can help it since one failed lamp will lead to all the rest in the series also not illuminating).

You can of course also use diff color and voltage LED's off the same power source wired in parallel as long as each LED has the resistor required to bring down the power supply voltage as needed.

Right now I have 18 diff color LED's of all about 3V on my breadboard along with a flasher LED all burning in fine off a single 3V watch battery with no resistors required.  :b

[AlW]

Steve, yes, you can operate different colored LEDs with different forward voltages in series as long as the current is limited below spec.  Standard red, orange, yellow and yellow-green LEDs have a Vf of about 1.8 V, while pure-green, blue, white, and UV LEDs have a Vf of about 3.3 V.  In your example, the cumulative Vf for the three colored LEDs is 8.4V which means the resistive drop is 3.6V.  The current thru the LEDs is then  3.6/120=30ma.  This may exceed the maximum current rating of your LEDs.  Choose a resistor that reduces the current to less than 20ma, the maximum current rating for many LEDs.  In fact, you can operate the LEDs at 10ma without seeing much, if any, reduction in light output.  So, the upshot is to choose a standard resistor value like 220 ohms (16ma) or 300 ohms (12ma) and you should be fine for the example you've presented.  Al

[simi]

Like was mentioned previously - I would strongly recommend running the circuits in parallel.  Resistor per LED takes away pretty much any calculation, allows you to adjust the light level of each LED individually (larger/smaller resistor) if that's something that's desired, and prevents the one-fail all-fail scenario.

Cheers!

Simi

[RossW]

Like was mentioned previously - I would strongly recommend running the circuits in parallel.  Resistor per LED takes away pretty much any calculation, allows you to adjust the light level of each LED individually (larger/smaller resistor) if that's something that's desired, and prevents the one-fail all-fail scenario.

Cheers!

Simi

This

[starsiegeplayer]

Yes Steve, you can put diff color LED's on the same circuit. If you're wiring the LED's in series (as your diagram shows) then you'll want the LED's to all be of approximately the same voltage so you can use one resister as needed to compensate for what ever your power supply voltage is.

Incorrect.  You want them to have the same current rating.  Every element in a series circuit will have the same current.  They can (and usually do) have different voltage drops. 

[ImWolf]

Yes Steve, you can put diff color LED's on the same circuit. If you're wiring the LED's in series (as your diagram shows) then you'll want the LED's to all be of approximately the same voltage so you can use one resister as needed to compensate for what ever your power supply voltage is.

Incorrect.  You want them to have the same current rating.  Every element in a series circuit will have the same current.  They can (and usually do) have different voltage drops.

Correct. I did miss that since both the LED(s) voltage and current are used to calculate resistance. I also could have specified more accurately above when I mentioned a "circuit" of parallel LED's, when this would actually be several circuits.

While we're on the subject, why is the output current of the power supply not considered when calculating resistance?

[starsiegeplayer]

While we're on the subject, why is the output current of the power supply not considered when calculating resistance?

That's a good question. 

An ideal voltage supply is something that will put out whatever current is necessary to always keep the same voltage drop across the supply:  no more and no less.  A 10 Volt voltage supply across a 1000 ohm resistor would generate 0.01 Amps.  If you change that resistor to 100 ohms, the power supply will change the current output to 0.10 Amps to maintain the same 10 Volts.  If you have nothing across the terminals, you need 0 amps to maintain the 10 V.

Of course, the ideal power supply doesn't really exist. 

When you have a real power supply with a Voltage and current rating, what that means is that it behaves pretty much like an ideal voltage supply until you start getting close to the current rating. At that point, the voltage will start to "sag" as it tries to keep up. For example, your "10V" power supply may actually sag down to  9.8 volts (or less) if it starts to put out too much current. 

For a real life example: If you have a 0.5A USB charger and a 2A USB charger, both will produce exactly the same voltage, but the 0.5A charger's voltage will start to sag at a half amp.  Your phone's charging circuit detects the point where the voltage starts to sag, and throttles the resistance of your phone so that it won't draw more than 0.5A.    If you try to charge a phone that can only soak up 1A of current at a time and plug it into the 2A charger, the phone never gets to the voltage sag point, and the "2A" USB charger will only put out 1Amp to charge that device.


So when you do the resistance calculations for LED, you assume you have an ideal voltage supply, and then you are supposed to go back and make sure you are actually using less current than the power supply rating.

[Tankton]

That was the best explanation I've ever heard.
That was even better than my professors explanation
when I was in college. You should be a teacher.

[Decoman]

Philosophically speaking, I guess "ampere/current" is "raw" power in theory, but isn't something that 'exist' as such outside a circuit, as if it was the same as capacitance (energy), as 'amps' is what it is when pushing voltage (electromagnetic power) across a resistance.

Edit: Uuh, let me think.

So theres the formula:
A = I x V (current = resistance x voltage)

And..
W = A x V (watts = current x voltage)

[MSgtUSAFRet]

So......the answer is 'Yes' in theroy...right?!

Probably best practice is "No, you shouldn't."

I know I shouldn't hook them in series of extended lengths. Typically, I have only put two LEDs on a single resistor rather than hook up a "string of Christmas lights". The limited room in the Ent-C's engine pods and the need for so many different light colors led me to think I could hook a single LED per color for fiber and have short fiber runs. It looks like I will have to have multiple wires running from the engines to the blinky board.

As for the theoretical extraploations and suppositions of the nature of current/amps/volts and raw power - I will leave the musings to the gurus while I sit at your feet and try to understand!

Thanks for the answers and the discussion.

Please feel free to continue!

Steve

[madmonk]

The wires back to the control board can be limited and still have LEDs in parallel with their own resistors.

If you have 3 constant LEDS, Flash 1 LED and Flash 2 LED you can create a wiring loom so that you only bring back 0V, V Constant, V Flash 1 and V Flash 2 so 5 wires back to the board instead of 10.

Hope that makes sense.

Cheers,

[scottminium]

A few errors here:
V=I*R  (voltage = current * resistance)
therefore
I=V/R 
finally, W (watts)=I*I*R

Most power supplies are constant voltage, though as Starsiege points out, voltage will sag if you near the current limit.  Most of your LEDs are going to draw milliamps, which means you could have a TON of these powered from just one power supply.  BTW, a power supply is much better than batteries because the voltage will not 'fade' and result in dark LEDs.

I typically wire most LEDs in parallel with their own resistor.  Though you can run LEDs in parallel through the same resistor, this can have odd results.  For example, if the forward voltage is ever so slightly different, one might stay dark.  This is almost always the case with different color LEDs.  For space reasons I have occasionally put two LEDs in series.

Calculating the necessary resistance is done by first figuring out what current you need.  This is typically included in the LED info.  I'd cut that in half.  Next, R=V/I (more algebra).  V=Source Voltage (battery or power supply) - LED voltage (also in the LED info).  This will be the voltage dropped by the resistor.  As you already know the current, you just divide V by I and voila. 

But there is one more important step!  Calculate the wattage the resistor will be dissipating.  If you choose too small a resistor it will likely overheat and fail.  Now that you know the current and the resistance, W=R*I^2.  Be sure your resistor has a wattage rating above what you calculate.  For most applications 1/4 or even 1/8 Watt will be good enough.

[Mstowell]

So......the answer is 'Yes' in theroy...right?!

Probably best practice is "No, you shouldn't."

I know I shouldn't hook them in series of extended lengths. Typically, I have only put two LEDs on a single resistor rather than hook up a "string of Christmas lights". The limited room in the Ent-C's engine pods and the need for so many different light colors led me to think I could hook a single LED per color for fiber and have short fiber runs. It looks like I will have to have multiple wires running from the engines to the blinky board.

As for the theoretical extraploations and suppositions of the nature of current/amps/volts and raw power - I will leave the musings to the gurus while I sit at your feet and try to understand!

Thanks for the answers and the discussion.

Please feel free to continue!

Steve

There is no