LEDs too dim?

[Perik]

Hello HondaCop

You have to think of the output circuit of USB Master which I believe is 74HC259 or an equivalent. These have an absolute maximum current of approx. 20-25 mA pr. output.
With a "high" output of approx. 5V, one LED will survive without blowing and without a resistor due to the limiter in 259, but it will shine like a big star....

Your 2 LED in series should be OK if they are 2V LED. If 3.3V and in series you are just at the point where you may see some light.
So 3.3V need to be in parallel, but even then you will be short of current
(not able to give both 20mA - less bright than possible) and you will stress the output stage of HC259.

If you must use 2 LEDs – find some with 2V rating or add a transistor driver (eg 2N2222) and you can get whatever you want, but then be prepared to blow a few....

I think the best is to use a single LED with highest possible mcd and use a good arrangement with a diffuser as told by fordgt40.

And in general – always use a current limiter (resistor of suitable size).


Just some thoughts.

[HondaCop]

Man, such AWESOME info being put out in this thread, and I'm still confused.

Ok, let me see if I am getting this right.

Each MC output is 20mA and 5v.

So ideally, the LED attached to each output should have a 5v and 20mA rating?

Once I hook up 2 LEDs in series, what happens? Does the amperage drop? Does the voltage drop?

If I have a LED with a rating of 2.8v and 20mA, then ideally, I need to feed it 2.8v and 20mA in order for it to shine at it's brightest?

Ok, so let's say I have the following LEDs:

1.8v-2.2v @ 24mA

If I put ONE LED on the MC output, it would be too much voltage right? I guess I need to use them in series until the voltage across all LEDs drop to 1.8v-2.2v?

Will putting them in series, reduce the amperage? What if I go with 10mA LEDs in series?

This is so confusing.

[HondaCop]

WOW!!! After doing some research throughout the Internet, I think I now have a better understanding of what needs to be done and how, with regards to LEDs...

BTW, checkout this AWESOME website!

http://metku.net/index.html?sect=vie...calc/index_eng

It has several calculators which help you determine what resistors need to be use and best of all, the last one called "The Guru", lets you input what you need to accomplish and it will show you a diagram of how to create your circuit! OUTSTANDING!

[Perik]

HondaCop

So many questions.....

This is confusing, but have a look at the attachment and be even more confused. Diagram showing the relationship between applied voltage and the current running through the LED. This diagram is for a typical 3.2V white LED.
As you see the curve is very steep. At approx. 30mA constant current the LED blows so you have to keep the voltage below approx. 3.3V. You need to get rid of 1.7V (5-3.3). This you do with the resistor we talks about. Let say you want to run to LED at 20mA at 3.2V. Then the resistor need to be approx. 1.8V/20mA=90ohm. I'm using 120ohm for my 3.2V LED and they still shines very nicely.
So why does not the LED blow attached to 259; well I think each output stage of the circuit have a 100-120 ohm onchip limiter resistor. If you relay only on the onchip resistor the 259 will blow when all the output is activated in the same manner.

So ideally, the LED attached to each output should have a 5v and 20mA rating?

Correct - that would be great. They exists but I believe it's a combination of a LED and resistor in the same housing.

Once I hook up 2 LEDs in series, what happens? Does the amperage drop? Does the voltage drop?

If the LEDs are the same, the voltage (5V) will be split in two - 2.5V over each LED. Find the CurrentvsVoltage diagram for your LED and then you will find the current. If the LEDs are 2.2V, you are above the limits. If you connect your LEDs directly without a resistor to a PC powersupply with a 24A rating your LED would maybe have an early evening....
If 3.2V you would have 0,1mA through the LEDs and I believe no light.

If I have a LED with a rating of 2.8v and 20mA, then ideally, I need to feed it 2.8v and 20mA in order for it to shine at it's brightest?

Correct and without blowing.

Ok, so let's say I have the following LEDs:

1.8v-2.2v @ 24mA

If I put ONE LED on the MC output, it would be too much voltage right? I guess I need to use them in series until the voltage across all LEDs drop to 1.8v-2.2v?

Correct or add a resistor to take care of the voltage difference

The clue is the diagram(s) if you want exact numbers and a problem is;
no LEDs are the same. It all varies from Color, diameter, material etc. how they behave. If you always remember to add this limit resistor you are mostly safe. For 3.3V I'm typical using 120ohm, for 2.2V - 330ohm.

Still some thoughts - and don't shoot me if I'm wrong....

[Perik]

Hi

Forgot the attachment.

CurrentvsVoltage.jpg

[HondaCop]

HondaCop

So many questions.....

This is confusing, but have a look at the attachment and be even more confused. Diagram showing the relationship between applied voltage and the current running through the LED. This diagram is for a typical 3.2V white LED.
As you see the curve is very steep. At approx. 30mA constant current the LED blows so you have to keep the voltage below approx. 3.3V. You need to get rid of 1.7V (5-3.3). This you do with the resistor we talks about. Let say you want to run to LED at 20mA at 3.2V. Then the resistor need to be approx. 1.8V/20mA=90ohm. I'm using 120ohm for my 3.2V LED and they still shines very nicely.
So why does not the LED blow attached to 259; well I think each output stage of the circuit have a 100-120 ohm onchip limiter resistor. If you relay only on the onchip resistor the 259 will blow when all the output is activated in the same manner.




Correct - that would be great. They exists but I believe it's a combination of a LED and resistor in the same housing.



If the LEDs are the same, the voltage (5V) will be split in two - 2.5V over each LED. Find the CurrentvsVoltage diagram for your LED and then you will find the current. If the LEDs are 2.2V, you are above the limits. If you connect your LEDs directly without a resistor to a PC powersupply with a 24A rating your LED would maybe have an early evening....
If 3.2V you would have 0,1mA through the LEDs and I believe no light.



Correct and without blowing.



Correct or add a resistor to take care of the voltage difference

The clue is the diagram(s) if you want exact numbers and a problem is;
no LEDs are the same. It all varies from Color, diameter, material etc. how they behave. If you always remember to add this limit resistor you are mostly safe. For 3.3V I'm typical using 120ohm, for 2.2V - 330ohm.

Still some thoughts - and don't shoot me if I'm wrong....

LOL No bud, I will not shoot you! hahaha

Ok, you have basically confirmed what I just found out on google aswell.

One last question... Let's say I have a LED with a rating of 2.2v and 24mA... If I supply it with 2.2v and 10mA, it will not be as bright as it can be, because it's only getting 10mA of current, even though the voltage is right?

[HondaCop]

Ok, I think I found the answer to my own question. Yes, at 10mA, a 20mA LED will definitely not be as bright as it can be.

For this reason, I have gone ahead and ordered a USB Outputs card from OC. Since I will be using a pair of LEDs in parallel per output, at a target amperage of 24mA per LED, the whole circuit will have a load of 43.9mA... Well above the limit for a MC output.

With the USB Outputs card, I will be just fine and finally have some BRIGHT annunciators.

[fordgt40]

Efrain

I purchased some of those flat top orange 5000mcd leds that you suggested earlier in this thread and they work fine! By having 2 leds in parallel with a single current dropping resistor of 220 ohms I only use 14ma (which is well within the mastercard limit of 20ma) and the lighting level is both even and sufficient on the annunciator with the widest lettering ie "Speed Brake Extended"

Regards

David

[HondaCop]

Efrain

I purchased some of those flat top orange 5000mcd leds that you suggested earlier in this thread and they work fine! By having 2 leds in parallel with a single current dropping resistor of 220 ohms I only use 14ma (which is well within the mastercard limit of 20ma) and the lighting level is both even and sufficient on the annunciator with the widest lettering ie "Speed Brake Extended"

Regards

David

Glad you liked them! Those leds are great indeed.