engifineer
01-28-2007, 04:18 AM
I posted this on another forum and thought I would paste it here as well to answer some of the questions people ask. The first part gets into a mild amount of technical detail (not too deep, but deep enough that many may just want to skip to the end) and the end has some examples:
With everyone doing so many led mods these days, and asking so many questions, I thought I would put up some quick info on leds and thier operation. As always, I dont mind answering questions, but these days it is hard to keep up with the pms I get for that and led swap stuff, so maybe this will help. Forgive me if it is already there, I didnt see it when I searched.
LED concepts:
An LED is simply a diode that emits photons of a particular frequency when a current is passed through it. The various frequencies related to different colors. The color is dependent on the material the internal junction is made from. Which brings us to how a diode works.
This is a simplified description, but should get the basic principle across. The junction within a diode is what makes it work the way it does. There is a P and N side to the diode. If the P side (anode, or positive) is connected to a positive voltage, and the N side (cathode, or negative) is connected to a negative charge in relation to the anode, current will flow. This is why they are polarity dependent.
An led can be thought of for the most part as a constant voltage device. There is a minimum voltage required to start current flow. Once this is met, the device acts almost as a short. That is why it is more accurate to design around current with these devices rather than voltage. The voltage needed depends on the junction materials (which, if you think back, relates to the color).
Blue, super green and white require a higher voltage to start current flow than do red and amber leds. This is why we have to mod some circuits based upon the color used.
When designing with leds, you can think of them as a point of constant voltage.
For example, lets say you have a blue led requiring 3.2V @ 15mA to provide the output you need. IF you are connecting to a 12V source, you effectively have 8.8V left after accounting for the LED voltage drop. That means the resistance in the circuit has that much voltage applied across it. Using ohms law, you see that if you have no resistor in line, you basically have a short. Current equals voltage/resistance, so here you have (8.8V/nearly zero ohms).
To state ohms law, with I equalling current, V equalling voltage and R equalling resistance: V = IR . So you can see you can re-arrange to get what you need.
In our example, you have 8.8V (after taking out for the LED) and you need 15mA. So, you have: 8.8 = .015 * R => R = 586 ohms (or the next closest value you can find).
If you have leds in series, their voltages add up.
Here is one warning. If you place LEDs in parallel, ALWAYS design so that each has its own load resistor. NEVER connect leds in parallel to one another and tie them all to one resistor. A failure of one can cause the others to operate improperly, or damage one of them. And due to the characteristics of leds not being 100% matching, they will not operate exactly the same way. That is why I do not reccomend the HVAC mod that simply jumpers some leds in parallel without adding a resistor for each one.
I realize that those with no electronics experience may not get all of this or want to for that matter, but the basic understanding will help you choose what mods to do and how they should be done, as well as explain why the HVAC needs to be modded for the other colors.
A note on that part. Stock, the HVAC has every 3 leds in series with an 8V supply from the board powering them via a load resistor. When you switch to blue (requiring about 3.2V) the issue appears. If you take the added voltage (9.6V) of the LEDs and subtract from the source voltage, you see there is nothing left to provide current flow. So you end up with them terribly underpowered and dim (or not working at all). The purpose of the mod is to re-arrange things to make it work.
One last example to make it clear:
Lets say you have some blue leds rated for 3.2V at 20 mA typical. You need to run a bunch of them off of your 13.8 V battery source. If you run each of them separately (in parallel. usually if they are not going near each other) then you figure this way for EACH led:
Subtract the led voltage from the source: 13.8 - 3.2 = 10.6V
Divide the remaining voltage by the current needed: 10.6/.020 = 530 (size of resistor needed)
Chose nearest common resistor size UP from value. This is 560 Ohms in this example. This means each led has power connected through a 560 ohm resistor, then to the LED, then to ground.
If you are placing the leds near each other (like on a project board) and can run some of them in sets (series), then you first need to determine how many in series you can run. I like to leave at lease 1V under the source voltage, but if they need to run when the car is off (12V) then you may want to leave more room for variation. So for me I would put them in sets of 3. So every three can be in series with a connection to battery and ground.
So: 13.8-(3*3.2) = 4.2V
Current is the same throughout each series branch, so if you need 20mA, then you have 4.2/.02 = 210 Ohm or nearest typical match UP in value. Then connect battery positive to a 210 Ohm resistor, then the other side of the resistor to the first LED in the series, then from the third led cathode to ground. Each branch of three will have its own resistor.
I will try at some point to make up some schematics to demonstrate if needed.
Anyway, I hope this will help some in the modding they do, or at least help some understand what is involved when they have thiers modded.
With everyone doing so many led mods these days, and asking so many questions, I thought I would put up some quick info on leds and thier operation. As always, I dont mind answering questions, but these days it is hard to keep up with the pms I get for that and led swap stuff, so maybe this will help. Forgive me if it is already there, I didnt see it when I searched.
LED concepts:
An LED is simply a diode that emits photons of a particular frequency when a current is passed through it. The various frequencies related to different colors. The color is dependent on the material the internal junction is made from. Which brings us to how a diode works.
This is a simplified description, but should get the basic principle across. The junction within a diode is what makes it work the way it does. There is a P and N side to the diode. If the P side (anode, or positive) is connected to a positive voltage, and the N side (cathode, or negative) is connected to a negative charge in relation to the anode, current will flow. This is why they are polarity dependent.
An led can be thought of for the most part as a constant voltage device. There is a minimum voltage required to start current flow. Once this is met, the device acts almost as a short. That is why it is more accurate to design around current with these devices rather than voltage. The voltage needed depends on the junction materials (which, if you think back, relates to the color).
Blue, super green and white require a higher voltage to start current flow than do red and amber leds. This is why we have to mod some circuits based upon the color used.
When designing with leds, you can think of them as a point of constant voltage.
For example, lets say you have a blue led requiring 3.2V @ 15mA to provide the output you need. IF you are connecting to a 12V source, you effectively have 8.8V left after accounting for the LED voltage drop. That means the resistance in the circuit has that much voltage applied across it. Using ohms law, you see that if you have no resistor in line, you basically have a short. Current equals voltage/resistance, so here you have (8.8V/nearly zero ohms).
To state ohms law, with I equalling current, V equalling voltage and R equalling resistance: V = IR . So you can see you can re-arrange to get what you need.
In our example, you have 8.8V (after taking out for the LED) and you need 15mA. So, you have: 8.8 = .015 * R => R = 586 ohms (or the next closest value you can find).
If you have leds in series, their voltages add up.
Here is one warning. If you place LEDs in parallel, ALWAYS design so that each has its own load resistor. NEVER connect leds in parallel to one another and tie them all to one resistor. A failure of one can cause the others to operate improperly, or damage one of them. And due to the characteristics of leds not being 100% matching, they will not operate exactly the same way. That is why I do not reccomend the HVAC mod that simply jumpers some leds in parallel without adding a resistor for each one.
I realize that those with no electronics experience may not get all of this or want to for that matter, but the basic understanding will help you choose what mods to do and how they should be done, as well as explain why the HVAC needs to be modded for the other colors.
A note on that part. Stock, the HVAC has every 3 leds in series with an 8V supply from the board powering them via a load resistor. When you switch to blue (requiring about 3.2V) the issue appears. If you take the added voltage (9.6V) of the LEDs and subtract from the source voltage, you see there is nothing left to provide current flow. So you end up with them terribly underpowered and dim (or not working at all). The purpose of the mod is to re-arrange things to make it work.
One last example to make it clear:
Lets say you have some blue leds rated for 3.2V at 20 mA typical. You need to run a bunch of them off of your 13.8 V battery source. If you run each of them separately (in parallel. usually if they are not going near each other) then you figure this way for EACH led:
Subtract the led voltage from the source: 13.8 - 3.2 = 10.6V
Divide the remaining voltage by the current needed: 10.6/.020 = 530 (size of resistor needed)
Chose nearest common resistor size UP from value. This is 560 Ohms in this example. This means each led has power connected through a 560 ohm resistor, then to the LED, then to ground.
If you are placing the leds near each other (like on a project board) and can run some of them in sets (series), then you first need to determine how many in series you can run. I like to leave at lease 1V under the source voltage, but if they need to run when the car is off (12V) then you may want to leave more room for variation. So for me I would put them in sets of 3. So every three can be in series with a connection to battery and ground.
So: 13.8-(3*3.2) = 4.2V
Current is the same throughout each series branch, so if you need 20mA, then you have 4.2/.02 = 210 Ohm or nearest typical match UP in value. Then connect battery positive to a 210 Ohm resistor, then the other side of the resistor to the first LED in the series, then from the third led cathode to ground. Each branch of three will have its own resistor.
I will try at some point to make up some schematics to demonstrate if needed.
Anyway, I hope this will help some in the modding they do, or at least help some understand what is involved when they have thiers modded.