A light-emitting diode (LED) is a semiconductor that emits light when a current flows through it. Modern LEDs are available across the visible color spectrum up into infrared and ultraviolet, and vary in intensity from dim to very bright. LEDs present many advantages over traditional light sources including lower energy consumption, longer lifetime, improved robustness and smaller size. Since their commercial boom throughout the seventies and eighties, LEDs found their way into many applications almost entirely replacing incandescent light bulbs for low-power indicators. Over the last years LEDs have become increasingly popular for high-power general lighting and automotive lighting. For more information about LED history and technology, refer to the Wikipedia LED article.
- 1 Low-power LEDs
- 2 Using low-power LEDs
In the BMW 8 Series many instruments are still illuminated with traditional incandescent light bulbs. In comparison to the LED lighted instruments, the incandescent lights often look pale and dim. The latter is more a construction issue of the instruments than inherent to incandescent light bulbs. Today car manufacturers put effort in making all instruments appear equally bright illuminated, but during the development process of the E31 the engineers pulled whatever they had laying on the shelves – not caring much about brightness and color tone. Some instruments are well-lit while others are barely visible in the dark. Some produce a deep orange color, others a pale yellowish orange. Therefore some people may want to replace some of the light bulbs to provide a more consistent illumination of the instruments. This is an excellent application for low-power LEDs.
LEDs are available in many different shapes and sizes. The best-known and most popular packages are the regular 5 mm (T1¾) and 3 mm (T1) round LEDs but when size matters it may be necessary to look at smaller LEDs in the surface-mount technology (SMT) family of components. Popular SMT packages for LEDs are the 3.2 mm PLCC-4 and PLCC-2 packages, and 2 mm 0805 package. The size numbers may not seem to differ much from the 3 mm round packages, but a look at the picture below shows the difference. SMT packages are usually rather flat – only 1–2 mm in height – whereas 3 mm round LEDs are around 5 mm tall and 5 mm round LEDs up to 9 mm. Even smaller packages exist, but these are highly impractical to use with hobby tools. The small 0805 package is already challenging for inexperienced hobbyists. SMT components are usually automatically placed by expensive machines and soldered in reflow soldering ovens. Nevertheless, there's an increasing usage of SMT components in hobby projects.
LEDs usually feature rather narrow viewing angles. 60–80° is normal for standard-intensity 3 mm and 5 mm round LEDs, while high-intensity LEDs often go as low as 15–30°. An incandescent light bulb on the other hand emits light in all directions. That means replacing a light bulb with an LED is not always straight-forward. The LED's viewing angle may be too narrow to get the same quality of even light distribution as with the light bulb. The illumination will be even worse than with the light bulbs. Take for example the ASC button in the BMW E31's center console. A single light bulb illuminates the ASC text. It may be a bit pale and dim lighted, but the light is even distributed. If that bulb was replaced with an LED with narrow viewing angle, only a small circle in the middle of the ASC text would be well-lit and the rest of the text would be barely lighted at all.
3 mm and 5 mm round LEDs with a large viewing angle of 100° and more do exist, but are harder to get in BMW orange. The SMT LEDs have a clear advantage here. Many of these LEDs are specifically constructed for display backlights where a large viewing angle is obligatory. Viewing angles of 120° and more are very common.
The brightness of LEDs varies from dim up to intensities that can cause eye damage when stared into directly. It is wrong to assume the luminous intensity represents the amount of light an LED emits. The luminous intensity is the brightness of an LED on a single spot, thus the viewing angle must also be taken in account. Of two LEDs with equal luminous intensity, the LED with the greater viewing angle will emit more light. The luminous intensity is expressed in candela (cd).
The intensity is roughly linear with the current flowing through the LED. The lower the current, the lower the light output and the higher the current, the higher the brightness but when the maximum current is exceeded the LED's semiconductor will overheat and burn-out.
|Low||lV < 100|
|Standard||100 < lV < 1000|
|High||lV > 1000|
Never stare directly into the beam of a high-intensity LED!
As said in the introduction, LEDs are available across the visible color spectrum reaching into infrared and ultraviolet. That means there's a lot of choice – even in one color. For example, two red LEDs may both emit a different tone of red light. The color name just indicates a group of LEDs with similar color – not exactly the same color. When accurate color matching is required it is best to work with wavelengths instead of colors. The wavelength expresses the true color of an LED. Most if not all datasheets will list the LED's wavelength. Refer to the table below for the relation between wavelength and color.
|Ultraviolet||λ < 400|
|Violet||400 < λ < 450|
|Blue||450 < λ < 500|
|Green||500 < λ < 570|
|Yellow||570 < λ < 590|
|Orange||590 < λ < 610||BMW orange λ = 605|
|Red||610 < λ < 760|
|Infrared||λ > 760|
|White||Broad spectrum||White is built-up with other colors.|
The color spectrum forms a continuous color gradient. It's not like 568 nm green is very much like 569 nm green, and 570 nm yellow is suddenly a completely different color. They will be indistinguishable from each other for the human eye. There is no real point where one color becomes another – they fade into each other. The colors are just arbitrary categories to make a quick identification possible. Besides, for many applications the exact color doesn't really matter.
White light is a special case. White light is not built-up of a single wavelength, but a broad spectrum of wavelengths. An incandescent light bulb emits light throughout the whole spectrum – from infrared to ultraviolet. The vast majority of the radiated energy is infrared instead of visible light, which is why incandescent light bulbs are inefficient. Because of the dominance of longer wavelengths, a light bulb does not produce pure white but a yellowish to reddish tone. The red becomes more apparent when the light is dimmed. An LED on the other hand emits light of just a single wavelength. This is also called monochromatic light. White LEDs use some tricks to produce white light. Two popular approaches to manufacture white LEDs are RGB LEDs and phosphor based LEDs. White RGB LEDs are built-up from three single-color LEDs; red, green and blue (hence the name RGB). RGB LEDs radiate a spectrum that looks more like three spikes whereas an incandescent light bulb radiates an even distributed spectrum. Nevertheless, the eye interprets it as white. Phosphor based white LEDs are made of a blue or ultraviolet LED with a yellow phosphor coating. The phosphor coating induces a wavelength shift across the visible spectrum.
Apart from special cases like white LEDs, LEDs emit only a single wavelength – color. The light is monochromatic. This may not seem of much importance, but when changing incandescent light bulbs to LEDs it may cause a surprising effect. The instrument illumination is not white but BMW orange and in a few places other colors like blue and red to indicate cold and hot. The white light from the light bulbs is colored by color filters. Color filters block or dim most wavelengths allowing only a narrow band to pass. For example, a blue color filter blocks all wavelengths except blue from a light source and the result of white light shining through this filter will be blue light.
This induces an issue with monochromatic light sources. Put the same blue color filter in front of an orange monochromatic LED and the result will be no light passing through at all. There is no blue light in the orange LED's outputted spectrum and the orange is blocked by the filter. In practice some light will pass because color filters are never perfect, but it will be a lot dimmer than the rest. This is something to keep in mind when changing lights. Some may prefer an all-orange look while others want to preserve the other colors too.
To keep the red and blue illumination it is sufficient to use white LEDs as these are not monochromatic and will work correctly with color filters. But if these white LEDs also illuminate parts that are BMW orange another issue may raise: Many instruments in the E31 that are illuminated in BMW orange, get their orange color not from a color filter but from dimmed incandescent light bulbs. Unlike white LEDs, incandescent light bulbs have dominant longer wavelengths and thus emitt a yellowish to even reddish light instead of pure white. Replacing the light bulbs with white LEDs may cause the instruments to be illuminated in pale yellow instead of BMW orange. Hence the preference for BMW orange LEDs.
The closest match for BMW orange illumination is a wavelength of 605 nm, but anything within the range of 600–610 nm will do just fine. The problem is these wavelengths are just outside the popular ranges, so many electronic components stores do not carry LEDs of this wavelength in stock. They should be able to order them though.
Some may dislike the orange BMW traditionally uses in their cars. Any color can be used, but there's a reason why BMW opted for orange. The human eye is not equally sensitive over the whole visible spectrum. Furthermore, light of longer wavelengths tends to scatter less making instruments easier to read. Studies showed orange illumination in a dark environment is the easiest to resolve for the human eye. Blue on the other hand is one of the worst colors to resolve.
Below is a list of recommended LEDs that emit BMW orange light. They are selected on wavelength and viewing angle. LEDs with a narrow viewing angle are not suited very well to replace incandescent light bulbs in the BMW 8 Series and are thus not listed.
- Avago Technologies HSMD-C170: → 8 mcd intensity, 170° viewing angle, 2 mm SMT 0805 package. Very small LED with excellent diffuse and even light distribution. Rated at 8 mcd it is rather dim but it can be mounted in the smallest spaces.
- Avago Technologies HSML-A101-S00J1: → 220 mcd intensity, 120° viewing angle, 3.2 mm SMT PLCC-2 package. Small LED with average intensity. The viewing angle is less wide than that of the HSMD-C170, but still outperforms most round LEDs. Has a wide range of applications in the E31. Highly recommended!
- Avago Technologies HSML-A401-U40M1: → 1125 mcd intensity, 120° viewing angle, 3.2 mm SMT PLCC-4 package. Small LED, very similar to the HSML-A101-S00J1 but offering a luminous intensity of up to five times higher. In most cases the HSML-A101-S00J1 will suffice but when more light is required the HSML-A401-U40M1 is a good choice.
Any LED with similar specifications is fine of course, but in case the local electronic components store does not carry anything suited, the listed LEDs by Avago Technologies are readily available from large electronic components retailers like Farnell and Conrad.
Using low-power LEDs
Low-power LEDs have a power rating of 250 milliwatt or less, but with some of the circuits presented in this chapter, a multiple of the LED's power is lost in the rest of the circuit. Therefore these circuits are only useful in low-power situations. Don't try to adapt them for high-power LEDs because the total power consumption will get very high and completely outdo the benefits of the LED's low power consumption. There are much better approaches to use and dim LEDs, but they are all more expensive, more complex and occupy more space – and especially the last is often an issue when replacing light bulbs with LEDs.
The image on the right shows the symbol that represents an LED in electronic circuit diagrams. It's very similar to that of a diode – after all the LED is a diode – but two arrows indicate the emitted light. The symbol may differ a bit from diagram to diagram, but it's always very recognizable.
Just as with a diode the current flows only in one direction: from anode to cathode. The positive terminal of the power source should be connected to the anode and the negative terminal to the cathode. When current flows through the LED, it emits light. A reverse voltage (positive and negative terminals switched) sees the LED as isolator and no current flows. A difference with regular diodes is that LEDs don't handle reverse voltages very well. Some diodes can withstand reverse voltages of hundreds up to thousands of volts, but an LED usually breaks with a reverse voltage of only a few volts. This is important when working with a 12 V automotive electrical system. Incandescent light bulbs do not have polarity so it's not always immediately clear which terminal of the socket is positive (hot) and which negative (ground). Replacing a bulb with an accidentally reversed LED may damage the LED. Always respect the LED's polarity! Round LEDs normally have a flat spot on the edge indicating the cathode. If not cut-off in a previous experiment, the polarity can also be seen from the leads. The long lead is the anode and the short lead the cathode. SMT LEDs use various methods to indicate the polarity. Refer to the LED's datasheet for more information. If uncertain use a digital multimeter (DMM) to determine polarity of socket and LED.
Wiring a single LED
LEDs do not operate at 12 V battery voltage. Unlike incandescent light bulbs LEDs are not voltage-driven, but current-driven devices. When a light bulb is connected to a voltage source of the specified voltage rating, it will draw a certain amount of current reaching the specified power output. Decreasing the voltage decreases the power consumption and light output up to a point where the lamp goes out. Increasing the voltage increases the power consumption and light output for a short time until the filament burns-out. LEDs on the other hand are semiconductors and show completely different behavior. An LED tries to maintain a certain voltage drop over its semiconductor junction. The voltage drop is inherent on the type and color of LED and typically within 1.5–4 V range. Datasheets usually refer to the voltage drop as the forward voltage VF.
When the applied voltage is even slightly lower than this threshold voltage, the LED does not light up. If it's slightly over this voltage, the LED will try to maintain the voltage drop and increase its current consumption up to a point where the semiconductor fries. In other words, controlling an LED with a voltage source doesn't work. Especially not in environments where the voltage is not always constant like in a car. Thus, never connect an LED directly to a voltage source!
Now put a resistor in series with the LED and supply a voltage source higher than the forward voltage. Once again the LED tries to maintain its voltage drop and increases the current. However, the same current flows through the resistor. Ohm's law learns that a resistor with current flowing through induces a voltage drop too. Unlike the LED where the voltage drop is a constant, the resistor's voltage drop is linear with the current. The more current the LED tries to consume, the more current flows through the resistor and the higher its drop becomes. The effect of the drop over the resistor is that the voltage the LED sees is also lower and the LED needs less current to maintain its own drop. It's a self-regulating system. Increasing and decreasing the voltage of the source will still increase and decrease the current through the LED, but not as dramatic as without the resistor.
One might think the resistor simply decreases the voltage over the LED, but that is not correct. The resistor controls the current flowing through the LED and the LED maintains its own voltage drop. Hence it's called a current-limiting resistor. The resistor's value can be calculated as follows:
- R: Resistance [Ω]
- V: Source voltage [V]
- VF: LED forward voltage [V]
- IF: LED forward current [A]
The forward voltage and current can be found in the LED's datasheet. If the LED's optimal intensity is too bright, use a lower forward current. From now on VF and IF are renamed to VD and ID respectively, to give a better indication of which components in the circuit diagram the variables are linked to.
An example with numbers says more than pure formulas, thus assume an LED with VF 2 V and IF 20 mA that needs to be connected to a car's electrical system.
Note the calculation uses 14.5 V instead of 12 V for the battery voltage. Cars are indeed equipped with 12 V lead-acid batteries, but to charge such batteries a slightly higher voltage is required. That's why the alternator and voltage regulator deliver 13.8 V (in practice 13.5–14.5 V). The voltage of a car's electrical system is thus higher when the engine is running. That's an important fact because when the resistor was dimensioned for a true 12 V system, the LED may operate outside maximum current specifications while the engine is running! When dimensioning circuits, always start from the worst-case scenario – in this case 14.5 V.
The result of the equation is 625 Ω, but this value does not exist in the popular E12 series. The E12 series offers only a limited set of twelve values and their tenfolds. The E12 base numbers are 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2. The closest E12 values of 625 are 560 and 680. However, lowering the resistance will increase the current through the circuit and it is already calculated at maximum current! A lower resistance is out of question unless the nearest lower value is really, really close. A higher resistance decreases the current through LED and resistor. This is not an issue, but the LED may be a bit less bright than expected. It is also possible to combine E12 values to get as close as possible to the calculated resistance. For example, a resistor of 820 Ω and 2700 Ω in parallel get very close to the desired value:
Usually this kind of accuracy is not required and the next higher E12 value will do just fine. In this case the circuit would be built using a 680 Ω current-limiting resistor.
There is a voltage drop over the resistor and current flows through it. That means it is also dissipating power. A standard E12 resistor can handle only ¼ watt, thus it's important to check whether the power dissipation is within specifications:
0.23 W is just below ¼ W and thus within specifications. The resistor will become warm – perhaps even hot, but it shouldn't cause any problems. Just make sure the resistor can't touch things that can melt or are inflammable.
The equations above show the power consumed in the LED and the total power consumption of the circuit. It's not required to calculate this but the separate power values show an interesting issue with this setup. With a total power consumption of 270 mW the circuit does better than most light bulbs, but the idiocy here is that the resistor draws 230 mW while the LED consumes only 37 mW. That's over six times less than the resistor. In other words, most of the energy is lost into heat in the resistor. There's nothing wrong with the setup but connecting a single LED to a 12 V system is just not very efficient. It should be obvious that when the difference between source voltage and voltage drop over the LED decreases, there's less voltage over the resistor, leading to less heat dissipation and thus an improved efficiency. Too bad both source voltage and forward voltage are constants in this case. Another way to decrease the heat dissipation is lowering the current by using more efficient LEDs which produces the same intensity for a lower current. The efficiency ratio between LED and resistor will not change, but the total consumed power will go down. The table below shows the power consumption of the circuit at the peak voltage (14.5 V) and designed for various currents:
|5 mA||2500 Ω||63 mW||10 mW||13.8 %|
|10 mA||1250 Ω||125 mW||20 mW||13.8 %|
|15 mA||833 Ω||188 mW||30 mW||13.8 %|
|20 mA||625 Ω||250 mW||40 mW||13.8 %|
|25 mA||500 Ω||313 mW||50 mW||13.8 %|
Wiring multiple LEDs
Wiring multiple LEDs by putting them in parallel with a single current-limiting resistor is just plain wrong. Nevertheless, the setup is occasionally used to simplify the wiring. Try to avoid it, but if you must use it, always use LEDs from the same batch because LEDs with even only slightly different specifications can cause great differences in brightness or even failure. When one of the LEDs in this circuit fails, the current is redistributed over the remaining LEDs possibly causing more failures. In other words, it's very prone to failures!
The most logical approach to using multiple LEDs would be putting single-LED circuits in parallel. Calculations are the same as with the single LED. If all LEDs are equal, the resistors are too. Thus calculating a single branch is usually enough. But this circuit suffers from the same issue as the single-LED circuit: It is highly inefficient. Four LEDs at 14.5 V and 20 mA would already dissipate – or rather waste – a combined power of 1 W in the four current-limiting resistors. That's a lot of heat for just four LEDs.
The key to getting the efficiency up is decreasing the voltage drop over the resistor. This can be done very easily when using multiple LEDs. Just put LEDs in series and the drop decreases with the forward voltage for each additional LED. Below is an example with numbers where three LEDs are to be connected to a car's electrical system. Be sure to have read the previous chapter to understand the reasoning behind some values and the general workflow.
The nearest higher resistor in the E12 series is 470 Ω. The power dissipated by resistor and LED, and the total power consumption are calculated as follows:
At 150 mW the resistor is now well within its specifications and will operate a lot cooler. The total power consumption of 260 mW is almost the same as with the single LED – it would be the same if the exact resistor values would be used instead of the nearest E12 values – but three times more energy is converted into light instead of heat. The efficiency tripled! With each additional LED in series the efficiency will increase another factor. More light output, the same amount of drawn power and less heat. No wonder this is a highly recommended and widely used setup.
It's not without disadvantages, though. The more LEDs in series, the more sensitive the circuit becomes for changes in the source voltage. In most applications this is not an issue because they are designed to operate on a single voltage, but the electrical system of a car can vary in voltage. When the engine is turned-off, the electrical system is powered only by the batteries. Depending on their condition and charge level, lead-acid batteries deliver 11.5–12.5 V. When the engine is started, the starter motor draws a huge current from the batteries forcing the voltage down to around 10 V. Once the engine runs, the alternator and voltage regulator charge the batteries with 13.5–14.5 V. The table below shows the current through the LED in function of the source voltage for one up to six LEDs in series.
|VD||2 V||4 V||6 V||8 V||10 V||12 V|
|R1||625 Ω||525 Ω||425 Ω||325 Ω||225 Ω||125 Ω|
|13.8 V||18.9 mA||14.5 %||18.7 mA||29.0 %||18.4 mA||43.5 %||17.8 mA||58.0 %||16.9 mA||72.5 %||14.4 mA||87.0 %|
|10.5 V||13.6 mA||19.0 %||12.4 mA||38.1 %||10.6 mA||57.1 %||7.7 mA||76.2 %||2.2 mA||95.2 %|
|11.5 V||15.2 mA||17.4 %||14.3 mA||34.8 %||12.9 mA||52.2 %||10.8 mA||69.6 %||6.7 mA||87.0 %|
|12.5 V||16.8 mA||16.0 %||16.2 mA||32.0 %||15.3 mA||48.0 %||13.8 mA||64.0 %||11.1 mA||80.0 %||4.0 mA||96.0 %|
|13.5 V||18.4 mA||14.8 %||18.1 mA||29.0 %||17.6 mA||44.4 %||16.9 mA||59.3 %||15.6 mA||74.1 %||12.0 mA||88.9 %|
|14.5 V||20.0 mA||13.8 %||20.0 mA||27.6 %||20.0 mA||41.4 %||20.0 mA||55.2 %||20.0 mA||69.0 %||20.0 mA||82.8 %|
All six situations are designed to deliver maximum brightness at 14.5 V and 20 mA. That way it can never run outside specifications. However, most of the time the circuits will operate at 13.8 V – a drop of less than a volt. Up to three LEDs in series, the current decrease caused by this is within 10 % and will not affect the light output very much. From then on, the difference grows bigger and the six-LED circuit is down almost 30 % at 13.8 V. When the source voltage sinks further, the difference becomes even more pronounced. This is important when mixing the circuits. If some instruments are illuminated by series circuits of only a few LEDs and others up to six LEDs, they all have the same brightness at 14.5 V, but will differ a lot on lower voltages. Below 12 V the six-LED circuit won't even light up because the voltage drop over the LEDs is higher than the source voltage.
The idea of replacing the light bulbs with LEDs was to improve the consistency of the illumination, but it can quickly go wrong this way. A few factors minimize the effect:
- It's recommended never to create a forward voltage higher than 60 % of the peak system voltage – that's just under 9 V and would mean four LEDs for this example. If more LEDs are needed put the series circuits in parallel branches as shown in the image.
- In most cases maximum brightness is not required and the circuit can be designed for a lower current. Lower currents also mean lower differences when the source voltage changes and thus also less effect on the brightness. When working with lower-than-maximum currents, it's a good idea to design the circuits for 13.8 V instead of 14.5 V. That way the brightness is always equal at the nominal voltage. Of course, the 14.5 V peak situation must be checked to avoid exceeding the specifications.
- The 10 V drop during engine start is very short and negligible. Further, how often are the instrument lights on while the engine is not running? In practice, the instrument lights will almost always operate at 13.8 V. That further minimizes the impact of the issue.
Calculating the current-limiting resistor from a certain current
This is a summary of the steps in determining the value of the current-limiting resistor as described in the previous two chapters. Don't forget a 12 V automotive electrical system is actually not 12 V! When designing for the maximum brightness, use the worst-case scenario peak voltage of 14.5 V in the equations. For lower-than-maximum currents, it's advised to use the nominal voltage of 13.8 V instead but always recalculate for 14.5 V too to make sure the maximum specifications are not exceeded.
- In case of a single LED, VD is the forward voltage of the LED as found in the datasheet. In case of multiple LEDs in series, VD is the sum of the individual forward voltages. VD should be less than 60 % of the system voltage – that's just under 9 V for a car's electrical system. For BMW orange LEDs with a 2 V forward voltage this means maximum four LEDs in series. For more LEDs add parallel branches.
- Calculate the resistance. For maximum brightness, use the LED's maximum current as specified in the datasheet for ID. For a lower brightness, just use a lower current value.
- Pick the nearest higher value from the E12 series with base numbers 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2.
- Calculate the E12 resistor's power consumption and make sure to take one that can handle the power. A standard resistor is only ¼ W.
Measuring the current-limiting resistor from a certain brightness
When changing light bulbs to LEDs in instruments, all instruments should light up equally bright. The type and amount of LEDs required for this and the current flowing through them, differs from instrument to instrument depending on its construction. There's no way of knowing how bright an instrument will appear when powered with a certain current. Thus, instead of designing the circuit starting from a specific current, it may be more interesting to start from a certain brightness.
One way to achieve this is building a single- or multiple-LED circuit with adjustable current-limiting resistance. Without knowing the exact value in advance, the circuit can be accurately tuned in brightness, for example, to match another instrument. Once the brightness is at a satisfactory level, the adjustable resistance can be measured and this measured value is the sought-after current-limiting resistance.
- Test fit the LED or LEDs in the instrument and attach long leads to the outside.
- In case of a single LED, VD is the forward voltage of the LED as found in the datasheet. In case of multiple LEDs in series, VD is the sum of the individual forward voltages. VD should be less than 60 % of the system voltage – that's just under 9 V for a car's electrical system.
- Calculate the resistance where the maximum current flows at the peak system voltage of 14.5 V. The maximum current ID can be looked up in the LED's datasheet.
- Pick the nearest higher value from the E12 series with base numbers 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2.
- Put a 1–5 kΩ linear trim potentiometer in series with the E12 resistor and wire it up as shown in the circuit diagram. Connect Y and Z to the LED's leads respecting the polarity.
- Apply 13.8 V to the circuit. A standalone 13.8 V or adjustable power supply comes in handy here.
- Turn the trim potentiometer to adjust the brightness to the desired level. It's best to compare the brightness to another instrument of which the brightness is satisfactory – the reference. Of course the reference instrument should be powered with the same voltage.
- Disconnect the power source and measure the resistance between point A and B with a digital multimeter. Never measure with the power still connected – this may damage the multimeter.
- Pick the nearest value from the E12 series. It doesn't have to be the next higher value this time, but the value should never be lower than R1.
Now the LEDs can be permanently fitted inside the instrument together with the optimal resistor determined by measuring. It may be a bit time-consuming and not always easy to wire the test version or compare to a reference instrument, but this is the best way to make sure all instruments get equally bright illumination.
A word of caution here: trim potentiometers are low-power variable resistors and most can only handle up to 150 mW. That's even less than a standard ¼ W resistor. That means this method is not suited very well for circuits with higher currents where the power dissipation in the resistor may exceed this value. For higher currents look at the solution in the next chapter.
Adjusting the brightness with a simple potentiometer as current-limiting resistor faces a problem: Regular potentiometers can only handle a limited amount of power. High-power variable resistors do exist, but are more expensive and more difficult to get. A better alternative is to use the potentiometer to drive a transistor which on its turn drives the LEDs. Transistors are cheap and available in all kinds of power ratings.
The image shows a so-called emitter follower or common-collector circuit. In an emitter follower the voltage at the emitter (E) always equals the voltage at the base (B), minus a small voltage drop over base and emitter. When adjusting potentiometer P1, the voltage at the base changes. The transistor then adjust the current through emitter and collector (C) until the voltage at the emitter is back equal to the base voltage – Ohm's law in practice. Just as in the previous chapter, R1 limits the current and protects the circuit against exceeding the specifications of the LED or LEDs.
Where V is 14.5 V, ID the LED's maximum current rating and
The transistor acts as a high-power potentiometer – driven by a low-power potentiometer. However, it's not possible to disconnect the power and measure the resistance as in the previous chapter. The only way to determine the total resistance is by measuring the current through the emitter.
An LED's brightness is controlled by the current flowing through it – not by the voltage. That means that in this case the source voltage is of no importance. As long as the circuit is powered by a voltage higher than the combined voltage drop over the LEDs and the small drop over the transistor's base and emitter, it can be used to measure the current accurately whereas the circuit from the previous chapter must operate at the nominal value to get reliable results.