52. LED Resistors

LEDs always require something to limit the amount of current (amperage) going through them. Without some kind of current limiter, an LED acts like a short circuit, which will make it overload itself or its power supply. So it's always necessary to put something in the circuit that sets a safe limit on the LED's power consumption. The simplest and most common way to do this is by placing a resistor in series with the LED.

Which LEDs need resistors?

All LEDs require something to limit current, but some LEDs come with suitable resistors already built in. You don't need to add anything external for devices with their own built-in resistors.
How can you tell if resistors are built-in? A rule of thumb is that any LED that you buy as a bare component requires an external resistor, whereas finished products that happen to contain LEDs usually have any necessary resistors built in.
Here's a handy guide to which is which, for the parts commonly used in pin cabs:
  • "Star" LED: external resistor required
  • Common small LED: external resistor required
  • Light strips, standard type: resistors are built in
  • Light strips, addressable type: resistors are built in
  • LED #555 replacement bulbs: resistor is built in
  • LED #161 replacement bulbs: resistor is built in
Special exception: You don't need a resistor with any type of LED when using it with a "Small LED" output port on the Pinscape main expansion board. The Small LED outputs are special in that they have their own built-in current limitation, which makes an external resistor unnecessary. This exception doesn't apply to the Pinscape flasher output ports or any other output ports; it only applies to the "Small LED" ports.

How to wire the resistor

Here's the basic wiring plan for connecting a current-limiting resistor to any LED feedback device:
As you can see, this doesn't change the basic wiring plan for an output device, it just adds to it, by inserting the resistor into the wiring between the LED and the output controller.
Note that it's equivalent to insert the resistor into the positive voltage wire (the one that connects the LED to the power supply) instead of the negative wire (to the output controller port). You can do that if you prefer it for some reason. But it's almost always more convenient to put it on the negative side, as shown, because it's usually easier to daisy-chain all of the positive voltage connections together. In fact, for some RGB LEDs, the negative side is the only option because the positive connections for the three color channels are sometimes wired together inside the LED. I'd recommend keeping things simple by adopting a uniform rule that the resistor always goes on the negative side.
For an RGB LED, each color channel needs it own resistor. In fact, the resistor values for the three channels might even be different. When you're calculating the resistor values (which we'll come to in a moment), you should do a whole separate calculation for each channel. Even though an RGB LED looks like one LED, it actually has three physically separate LEDs inside, each with its own current and voltage specs.
Basic resistor wiring plan for an RGB LED. This type of LED has separate connections for the three color channels. Each color channel must be wired to its own resistor. Note: the order of the pads shown here won't necessarily match your LEDs.

Choosing a power supply

In most cases, you can use a 5V supply for LEDs.
LEDs should always list a "forward voltage" value, sometimes written VF. This is usually somewhere between about 2V and 4V. Red LEDs tend to be at the lower end of that range, and blue and green are at the higher end. A common point of confusion for new cab builders is what this voltage means for the power supply. The forward voltage isn't the same as the supply voltage: you don't have to use a power supply that exactly matches the LED's forward voltage. Rather, the forward voltage tells you the minimum required supply voltage.
The basic rule is that the supply voltage has to be higher than the forward voltage. Ideally, it shouldn't be too much higher, because the higher it is, the more power will get wasted as excess heat burned up by the resistor.
5V is usually the ideal choice, because it's the lowest common supply voltage that's high enough to work with most LEDs.

Choosing the resistor

The exact resistor you need depends on the LED. To figure the right resistance value, you need to know two electrical specs for the LED and one for your power supply:
  • The LED's Forward Voltage, sometimes written as VF
  • The LED's Forward Current, or IF
  • Your power supply voltage, which we'll write as Vsupply
You should be able to find the LED figures on the LED's packaging, the manufacturer's data sheet, or the eBay seller's page where you bought it. If you buy from eBay, be sure to write down those figures when you place your order, since eBay pages don't always stay around indefinitely.
The power supply voltage is simply the voltage of the power supply you're using to power the LED. It's best to use the lowest power supply you have that's above the VF voltage. For example, if VF is 3.2V, any power supply higher than 3.2V will work, so you should use your 5V supply.
For an RGB LED, you'll need three resistors - one per color channel. What's more, you might need different resistors for each color. In the specs for the resistor, you should find separate VF and IF values for the Red, Green, and Blue components. Calculate each resistor value separately, as though it were for a separate LED (which it really is).
Once you have those three figures, plug them into this interactive calculator to figure the required resistor type:
LED Forward Current (IF)in milliamps: mA
LED Forward Voltage (VF) in Volts: V
Power Supply Voltage (Vsupply) in Volts: V
Recommendation: -
Important: Pay attention to the reported wattage value! Choose a resistor with the specified wattage or higher. If you use a resistor that has too low a wattage rating, it could overheat.
Please pay close attention to the "ohms" units reported by the calculator. If the calculator says 5.6Ω, it truly means 5.6 Ohms, not KΩ. If the calculator comes up with a value in in KΩ, it will say so. This might seem too obvious to belabor, but this is a frequently asked question! If you've done some electronics work, you can get so accustomed to only seeing resistors in the 100s or 1000s of ohms that something like "five Ohms" looks like a typo. But it's not - there really is such a thing as a 5.6Ω resistor, without the "K". And that's about the size you usually need for the high-power "Star" RGB LEDs. So if the calculator comes up with 4.7Ω or 5.6Ω or something else in the single digits, don't assume there's an implied "K" that was accidentally omitted. 5.6Ω really means 5.6Ω.
The calculator rounds the resistance value up to the nearest standard resistor value that's commercially available, to make it easier to find a matching part to buy. It also builds in a safety factor for the wattage by assuming that you don't want to go above 60% of the rated wattage for the resistor. The "raw results" are the exact mathematical results of the calculation before those adjustments, but those exact sizes usually aren't available for purchase anywhere, thus the adjustments.
In case you're interested in the details, the calculator figures the resistance value based on this formula:
Once you have those three figures, you can simply plug them into this formula to calculate the resistance required:
R = (Vsupply − VF) / IF
By the way, if you bring up the on-line version of this chapter in your Web browser, you'll find a handy interactive calculator that does this calculation for you, and automatically applies the adjustments described below to round to a standard size. It also calculates the required wattage.
Note that formula takes the Forward Current (IF) value in Amperes, not milliamps. The LED will probably specify IF in milliamps. To convert from mA to Amps, simply divide by 1000. For example, if IF is quoted as 20mA, use 0.02 Amps in the formula; if IF is 350mA, use 0.350 Amps.
The result R is in Ohms (Ω). It's not in KΩ or any other multiplied value - it's in plain old Ohms. For a high-power "Star" RGB LED, you might be surprised to get a value in the single-digit Ohms range, like 5.2Ω. That's perfectly normal, so don't second-guess the formula and think the result needs to be in a different unit. As long as you entered the inputs correctly in Volts and Amps (not milliamps), the result is always in Ohms.
The mathematical result of the formula tells you the exact resistance you'd need in theory to get the desired current. But you can't buy resistors in just any size; they only make them in certain standard sizes. The standard resistor sizes are 1Ω, 1.2Ω, 1.5Ω, 1.8Ω, 2.2Ω, 2.7Ω, 3.3Ω, 3.9Ω, 4.7Ω, 5.6Ω, 6.8Ω, and 8.2Ω, plus each of these values multiplied by 10, 100, 1000, 10,000, 100,000, and 1,000,000.
So after you figure the theoretical R value with the formula, round up to the nearest standard size. For example, if the formula says you need a 4.9Ω resistor, round up to the next standard size of 5.6Ω.
We're going to call this rounded-up value the adjusted resistance. (Sorry if this is starting to feel like filling out a tax form!)
After you figure the resistance value and round up to a standard size, the next step is to calculate the required wattage for the resistor. This is important because resistors generate heat, so you need to buy a resistor that's capable of handling the amount of heat it's going to generate.
To calculate the wattage, plug the adjusted R value (the value rounded up to a standard size) into this formula:
Iactual = (VS - VF) / Radjusted
That will tell you the actual current you're going to get with the adjusted resistance value. Now plug the result of that formula into this formula:
W = (VS - VF) * Iactual / 0.6
That yields the amount of power the resistor uses in Watts, and adds a safety margin so that we never operate above 60% (that's the 0.6) of the maximum rating for the resistor. That's just to make sure we're not pushing our luck, and accounts for any manufacturing variations in the LED or resistor that make them a little outside of their rated specs.
As with the Ohms value, they only make resistors with certain wattage ratings, so you'll have to choose the next available larger wattage. Common wattage values for resistors are ⅛W, ¼W, ½W, 1W, 2W, 5W, and 10W.
Now you're set to go out and buy resistors! Go to an electronics vendor like Mouser and enter the Ohms and Watts values you just calculated. You should be able to find matching parts. If you have any trouble finding something with the required Watts value, it's always safe to use a larger rated wattage. For example, if the formula says you need 5.6Ω at 1W, it's fine to use a 2W resistor instead. Larger wattage resistors are physically larger as well, so it'll take up more space if you have to use a much higher wattage than the formula gives you, but it will be perfectly functional and safe.