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LED-based lighting, Part 2: Automotive Lighting

This installment deals with automotive applications for LEDs. Here we'll cover automotive conversions to more-efficient lighting, as well as conversion of tractor systems; we'll discuss HID versus LED lighting, and include a practical example about how high-power LED headlights are designed.

Why LEDs for automotive-type uses?

In this kind of usage, efficiency really doesn’t count so much as the resistance to vibration. Rough-service survivability for LED-based lighting is excellent. When you get to LED-based headlights, one real advantage stands out: Gradual failure. No more sudden darkness in the middle of a job. LEDs will gradually put out less and less light, until they fail to light altogether. But by that time, you'll know that the unit is going bad and can have a replacement on order.

The down side

LEDs are very directional. Light comes out typically in a fairly small arc, so if you're thinking about something to replace a regular light bulb, you have to be prepared for a lot less area to be lit unless you have a specialized lens in front of the LED or a unit made specifically to be a headlight.

Here's a photo of a typical hobbyist LED.




The only way to get real lighting power out of little LEDs like these is to put a bunch of them together in a cluster; and that's how less-expensive LED replacements work. Here's a photo of one of those types of replacement units.



LED lights have to be designed correctly - with a heat sink and current limiting - if they are to last. An LED that has enough power output to be useful for something like a headlight is going to be dissipating a significant amount of power. An ordinary light bulb or even an HID bulb can just run hot and be okay. But since an LED is a solid-state device, it has to be kept cool or it will fail from the heat. Below is a typical graph of output versus temperature. Notice how the output goes down with case temperature here. It knees off at 75°C (or about 160°F), and the LED will fail altogether if allowed to rise beyond 90°C (or about 194°F). That can be easily handled with the right kind of heat sink.

See the attached graph.


About "Drop-In Replacements"

There are a lot of so-called 'drop-in replacement bulbs' out there that really aren't. Several websites sell something that will go right into an 1156 or 1157 socket. These are common applications for taillights, turn signals, and backup lights. When replacing standard bulbs with LEDs in cars, there are a couple of hidden problems.

First, the LED cluster draws so little power that the electronics in newer systems sometimes thinks the bulb is burnt out. And if the vehicle is older and uses a thermal flasher, suddenly you have no turn signals. So in these cases, you have to add a load resistor so that the newer electronics sees "there's a bulb there" or, for older vehicles, so that there's enough load current to make the flasher work.

The second thing with 'drop-in replacements' is that you have to choose an output color to match whatever lens it's going to be shining through. This is important because the lens is an optical filter: it cuts out every part of the light spectrum except the color you see. So if you have an amber lens, choose an amber LED replacement. Choosing red LEDs for an amber lens will give you an amber-ish light, but it will be very weak, since the light power will be in the wrong part of the spectrum, and so filtered out by the lens.

This is illustrated by the Figure below: Notice that LEDs are designed to have a specific light output color. When that light color doesn't match the lens color, you can now see how the output will seem really dim. It's important to realize that often the red LEDs you get as replacements don't match the red of the lens exactly, either. So you will need to accept some drop in what you see coming through the lens.

See the attached color graph of the lens filter effect.

These replacement units are generally 'clusters' of small, hobby-grade LEDs that are put together to get an acceptable level of light output. In the case of signal lamps, you buy the color of signal lamp you need, (like a red LED replacement to go inside a red taillight) and those generally work well.
Command Electronics (www.commandelectronics.com) carries complete stop- and-taillight replacement units and the cost there is more reasonable, less than $5 each. These will at least get you something to reduce your power budget, if all you're powering them with is a tractor's small alternator.

Let's talk headlights

There is a lot of interest around upgrading the headlight output on utility vehicles like tractors. LED is a great choice especially for the power savings, with some great replacements now available. The main thing to check when looking at a replacement is the light output. It's too easy to get a replacement with only about half the light output, creating a 'downgrade' instead of an 'upgrade.' Right now, direct-light-bulb replacements just aren't bright enough. For simpler garden tractors, the bulb used in the headlight socket is often an 1156 bulb or an equivalent, with an output of about 400 lumens. That bulb is enough to be seen by others and of fair use when trying to finish up the mowing at nightfall. When you get to standard tractor and CUT headlight bulbs, you have something with roughly 55 watts in output, or about 850 lumens each. That's enough light output to be really usable. So keep this 850 lumen figure in mind.

When considering conversion of small- and medium-size equipment headlights, you essentially have three choices. You can put in a bigger incandescent light and reflector, or you can change it over to HID or LED. Dropping in a bigger incandescent light is always the cheapest way to go, since stuff is available off-the-shelf in the local auto-parts store; but if the tractor's electrical system can't handle the added power draw, then you will want to consider HID or LED lighting. Let's take a moment and discuss the differences between the two.

Differences between HID and LED

HID stands for High Intensity Discharge. The technology that makes HID work has been around for about a century. HID is an arc light, using the same technology that makes streetlights work. A glass envelope with an electrode at each end is filled with a gas under very high pressure, and an AC ballast converts the DC from the vehicle into a high voltage to strike and maintain an arc between the two electrodes. Here's a photo of an HID bulb for a modern automotive application. Note how you can see the round pressurized glass envelope and its two electrodes.




The power requirement for the ballast and HID lamp (often called a burner) is about one-quarter the power than an incandescent lamp of the same light output. HID also lasts longer. Where you have a lifetime of about 300 hours or so with an incandescent headlight, the HID will last about 2500 hours.
HID is not without its problems. The greatest hazards of HIDs occur when they are operating. Let me first say that they are perfectly safe when properly shielded. But as anyone who does welding knows, there's a lot of ultraviolet radiation in any arc. And the same is true with any HID lamp. An unshielded HID lamp is an eye hazard, as well as a 'sunburn' hazard. The UV output from these types of lamps is strong enough to cause more than eye damage - it can also cause skin burns. So the lamp must be behind glass to filter out the UV component. Plastic-type materials can only be substituted if the plastic is specially made to filter UV. Broken shields must be replaced immediately.

There is a warm-up time before the HID lamp comes to full brightness. Depending on the bulb, this can be anywhere from a minute to several minutes. The bulbs run very hot, which demands a reflector and housing that are able to withstand the heat. When replacing them, they must be handled with extreme care as the envelope can be weakened at end-of-life, especially so because that envelope is still under high pressure. There is a very high danger of explosion when handling a hot burned-out lamp.

One thing to note when considering the conversion of an automotive headlamp to HID is the reflector system in the vehicle. If the reflector system in the vehicle was made for conventional halogen lighting, then it should be changed out for one built for HID. The arc in the HID bulb will be in a very different spot than the halogen reflector's original point of focus, so a lot of the benefit in the conversion will be lost because the light will be out of focus. Also consider that you are working with a wicked-bright point source light, and you have the potential of irresponsibly blinding oncoming drivers.

One of our vendors here, MobileHID, has several HID worklights to choose from, and they have plenty of punch to illuminate your work area. Check out the view from the driver's seat on a combine on their website. These are a great fit for an industrial application like a big machine that has to work no matter what the time of day. Since you can purchase them with flood and beam configurations (and even mixed flood and beam), you can put together a setup that works the way you do.

LED

LED stands for Light Emitting Diode. These devices are solid-state, which means they're generally impervious to vibration, and much simpler in construction than HID. They also don't come with all the hazards of arc lighting. LED-based units have an incredibly small power draw for the amount of light they put out, so your electrical system won't be taxed in running them. The other thing noted by many users of LED-based headlight systems is that they don't seem to attract as many bugs. This is because the LEDs make most of their light in the part of the spectrum usable for humans, and very little in the part of the spectrum detectable by insects.

Although they can be purchased in systems that are much less expensive than HID-based systems, the technology still hasn't yielded comparable lighting power output. It will be some time yet before we have good, reliable, LED-based automotive headlights. Technology continues to improve them, but such technology appears to have hit a plateau as of this writing. Price will vary together with the amount of light that the unit puts out.

Let's go back to our supporting vendor, MobileHID, for a look at what they have, as their pricing is comparable with many other web vendors currently selling packaged (ready-to-use) lights. They have some good choices on their website, and none will kill your electrical system in trying to power it.
One important thing to consider when looking at LED-based lighting is the hour rating for the unit. You're going to spend all that money on an LED-based light, then you should have at least 20,000 hours lifetime out of it. Some units may have a rating as high as 50,000 hours, depending on the parts that go into them and the level of the design.

Also, the electrical design of the light should be considered. Some inexpensive LED "headlights" will have a simple current-limiting resistor in series with the LEDs and those units are guaranteed to have a short life. This is because the amount of current the LED wants to draw rises with the temperature. The LEDs also respond strongly to input voltage fluctuations, so you have to have a fairly sophisticated regulator circuit ahead of them to keep them in check. One thing that will tell you that you have an intelligently designed unit is that it will be rated for multiple voltages. Note how the ones sold by MobileHID are rated for both 12V and 24V systems.

Why aren't LED headlights made for 6V systems? The LEDs made for lighting typically have a turn-on voltage (the voltage at which the LED begins to emit light) of around 10V. So to make one work on a 6V system would take some sophisticated (and expensive) circuitry. Such a headlight would then be impractical to manufacture in quantity.

What's inside of LED headlights

Let's investigate the guts of LED headlights by building our own. We'll design a replacement for a standard 1156 bulb. And if we're going to go through the trouble of figuring out a replacement, let's make it something worth doing. So that means… the Tim Taylor Home Improvement approach: MORE POWER.

Let's first see what's available online. Take a look at this example:




The problem with this unit from Super Bright LEDs (www.superbrightleds.com) isn't that this 1156 replacement (1156-W3X1W) unit costs $17.95, but it only has about half the output power of the 1156. More money for less light - not exactly what we're after. This shows the basic fault with the 'turn-signal light-socket cluster of LEDs' method. You just can't get enough output into that small space. It's not the LED itself (because one for a lighting application would fit there), but it's the heat sink that's the problem.

The solution lies in an LED module made for lighting applications, and to put a few of them in the same housing, to get enough light power out of it to be useful.

The standard 1156 bulb has an output of 400 lumens, so (going for More Power) we'll double that. We'll figure on about four high-power LEDs built for lighting applications (an LED that is constructed with a blue emitter and a phosphor coating), and bond them to a heat sink so they'll give us good output, long life, and we'll have a way to get rid of the heat they generate. Typical single-part quantities for something like this (say an Avago or a Cree part) are between $30 and $80, averaging $50; depending on how much power you want. In high power applications like this, we're working with wattages in the 15- to 25-watt region. Here's a photo of a type of LED used in this kind of application.



Note the way it's bonded to a substrate that allows you to attach a heat sink.

So we have our group of LEDs. Now for a heat sink. A heat sink for a set of LED emitting this much power is about as big as half a grapefruit. Most common ones will have fins for efficiency. Here's a photo of the type of heatsink necessary.




Now we're at about two hundred bucks, and we still need a circuit to drive the LED, a lens, and a housing. Driver circuits run about $25 each, and manufacturers have designed them to be pretty much 'hook up and go.' You just give the regulator circuit power, and bring its output over to the lighting-type LEDs. The circuit handles everything from keeping the LEDs safe from over-temperature runaway to keeping them from reverse currents.

Once we have everything bonded together, here's a look at what the completed raw assembly would look like.



Well, we've pretty much designed this unit from MobileHID. Now you know what's inside!





Hope you enjoyed this article. Let me know if you want any other LED or Lighting subjects covered.
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Steamguy

Always do the right thing; not because it's easy, but because it's harder.

It's always a pleasure to work with good tools and the BX2370 is amazing!

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