LED (light-emitting diode) turn signals offer several advantages over incandescent, not the least of which are reliability and long life. Incandescent bulbs can burn out or break at the most inopportune times and LED bulbs are virtually immune to these issues.
LEDs also draw far less power than incandescent bulbs but remember, they only draw any power at all when they're on and, as you'll see later, you'll need to add resisters to effectively increase the power consumption back to that of incandescent bulbs anyway.
There are several options available for LED signals. You can replace your stock 1156- or 1157-type bulbs with similar bulbs, you could add run-turn-brake functionality with something from Custom Dynamics, or you could replace the entire assembly for a completely new look. This article will assume that you're looking for the simplest approach and will replace only the bulb.
When you're selecting a replacement bulb for your stock signal housing you need to remember that the housing is designed for a bulb which throws light in all directions and makes use of the reflector. Some LED bulbs found in auto supply stores are highly directional and not well-suited for this purpose.
http://superbrightleds.com is a recommended vendor for bright LEDs, but this one is not suitable for use in a motorcycle turn signal housing. In the case of the Mean Streak the bulb is mounted sideways in the housing, making matters even worse.
You should look for this type of bulb, which throws its light in all directions, and get the brightest that you can find and that will fit your housing.
If you're replacing your stock bulbs with LEDs in the same fixtures you simply need to replace the bulbs and move on to the "Fast Blink" section below. If you're replacing the fixtures you'll need to cut and solder wires. Just remember that LEDs are polarized, meaning that they won't work if you mix up the positive and negative connections. LEDs also must have some load (resistor) in series with them. If you get a bare LED and connect it to a battery you will burn it out. Bulbs and kits typically have these built in but remember this if you're building something from scratch.
Fix the Fast Blink
Once you get your new LEDs installed you'll quickly find that your signals blink far faster than normal. This is a feature of your flasher designed to let you know when a bulb is burned out, which is exactly how the flasher sees the LED due to its much higher resistance.
The easiest option (if not the cheapest) might be to get an LED flasher for your bike. However, this may take away the auto-cancel function of your signals if you have that.
The other fix for this is a pair of load-equalizing resistors that will shunt current to ground, making the resistance and power draw seem normal to the flasher.
Amazon and Custom Dynamics offer ready-made resistors, or you can make your own (thanks to BareAssChoppers.com). If you want to figure out exactly what you need, check out the next page (Scary algebra warning). Otherwise find some in the 6 - 8 ohm range, around 50 watts, and just keep on reading...
A little care must be given when installing the load equalizers since they can get hot. If you've ever held onto an illuminated bulb you know what happens; the resistor is doing the same thing. That additional power that it's drawing is being dissipated as heat.
A nice place on the Mean Streak is right next to the battery, where it's also easy to tap into the rear signal wires and the frame serves as a heat sink. Look for someplace with some air flow and avoid tucking them into the headlight bucket where they'd be likely to melt the insulation from other wires.
Once you have a good location selected, locate the positive and negative wires. According to the schematic to the right (from the Mean Streak service manual), the negative wire is black/yellow and the positive is green (right) and grey (left). Strip off a bit of insulation from each wire and solder one lead of the resistor to the positive and one to the negative, for each signal. You can also use a frame ground in place of connecting to a negative wire, as long as you locate a good ground point.
Resistors are not polarized so the direction doesn't matter.
Remember, "heat the material, not the solder" to get a good connection. Using a heat-sink clip, or just an alligator clip, on the wires and resistor leads will help protect them from excessive heat.
When is a Diode Needed?
If your bike has a single indicator light on the dash for both left and right signals, the resistors can cause that indicator to become a path between the two sides causing all four signals to flash. To fix this problem you need to add a pair of diodes into the circuit to ensure that current can't flow the wrong way through the indicator lamp. This diagram shows the installation of the diode kit from SuperBrightLEDs. If you purchased a "load equalizer kit", or built your own from BareAssChoppers, then you've already added diodes and this issue should be fixed.
Note that diodes are polarized, unlike the resistors, and won't work properly if they're installed backwards. They must be connected with the positive termal toward the turn signal power and the negative terminal (indicated by the line around the end representing "-") toward the indicator bulb. In this diagram the stock setup has the indicator bulb grounded through the other signal circuit. The change is to ground it to an actual ground and combine both positive signal leads to the same indicator lead.
(An interesting thing about incandescent bulbs... 1) they're not polarized and will work with current flowing in either direction; and 2) they have very low resistance such that a bulb can serve as a ground connection for another circuit.)
Calculating the Resistor Requirement
A standard incandescent bulb is typically rated for around 25 watts at 13.8 volts DC. Applying Ohm's Law to find the resistance, we first determine the current draw in amps:
I = P / E
I = 25 / 13.8
I = 1.8 amps
Now solve for the resistance of the bulb:
R1 = E / I
R1 = 13.8 / 1.8
R1 = 7.67 ohms
With that information we look at your LED bulb. This one draws 165mA, so let's see what its resistance would be. Again starting with R = E / I:
Rled = 13.8 / .165
Rled = 82 ohms
So, from the flasher's perspective we need to make it see about 7-2/3 ohms instead of the 82 ohms of the LED. The way to reduce the overall resistance of a circuit is to add another load in parallel with the primary load, so we'll put a load equalizer in parallel with the LED. In other words, we'll effectively short-circuit the LED with the load equalizer. Total resistance becomes ( 1 / R1 + 1 / R2 )-1, so to find the resistance that we need to add:
1 / 82 + 1 / R2 = 1 / 7.5 (rounding a little)
7.5 / 82 + 7.5 / R2 = 1
.09 + 7.5 / R2 = 1
7.5 / R2 = 0.91
7.5 = 0.91R2
7.5 / 0.91 = R2
R2 = 8.24 ohms
Adding an 8.24 ohm resistor in parallel with the LED bulb will make the flasher think there's an incandescent bulb installed and fix the fast-blink. If you check online for load-equalizers you'll find them varying from about 6 ohms to 8 ohms, and any of these will work. Remarkably, even if the calculation is changed to include both front and rear signals, the result changes very little. The above calculation returns 9.15 ohms, only a couple of ohms different.
Of course, you don't really have to do the math yourself...
The last thing to do is figure out the wattage rating of the resistors. The component resistors found at Radio Shack and the like are usually rated in fractions of watts and are completely inadequate. Remember that the goal is to pull around 25 watts per bulb and the LEDs are drawing very little of that. The resistors must be able to handle virtually the entire load, so get resistors that are rated for 50 watts or more. The ones mentioned above meet that need.
LEDs, Bulbs and Resistance
One source of confusion seems to be the idea that LEDs offer lower resistance to current flow than a typical incandescent bulb. This is completely true when talking about bare LEDs; they show virtually no resistance when measured. For this reason, unless you're dealing with very low voltage, you cannot use a bare LED in your application and must add a resistor in series with it. This resistor is what makes the resistance higher for the LED assembly than that of a bulb.
When you buy an LED lamp that is ready to use, there will be a resistor already installed. If your're building your own lamps from component parts you have to add the resistor yourself. A good explanation of Ohm's Law, with a focus on resistors for LEDs, can be found here and here.
Connecting a bare LED directly to your motorcycle battery will immediately destroy the LED.
The short version is:
- Low current draw = high resistance (LED)
- High current draw = low resistance (Incandescent)