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Apeshaft
Posts: 51
Supporter
Hi all, just connecting my annunciators and I have a problem with the white LEDs.
Connecting them serially in pairs results in a very week illumination. I don't have this problem with
the other LEDs (orange). I am using one 100 Ohm resistor per couple of LEDs.

I have the data sheet for my specific diodes here:

https://www.kjell.com/se/.mvc/Document/File?id=0e106552-348e-465f-b747-a7e300aac1a7

Your help would be very appreciated!

Best regards

Oskar
[Last edited by Apeshaft, 2018-08-24 20:00]
2018-08-24 19:38
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StephanHo
From: EDDG, Germany
Posts: 1111
Supporter
Hi Oskar,

if I understand you correctly, you switch the LEDs in series.
This doubles the breakdown voltages of the LEDs.

White LEDs have an operating voltage of about 3V and if you switch two in a row, they need a voltage of over 5V. On MEGA but you have only 5V. Then you have a resistor, over which a voltage drops, so you are missing for the clean operation of the LEDs ever at least 2-3 V.
Switch the two LEDs in parallel and use a resistor (470 - 680 ohms) for each LED, then it should work.

Orange LEDs only have a breakdown voltage of approx. 1.8V, green ones are approx. 2V. Therefore, you did not notice it immediately.

I always recommend, with only 5V operating voltage, LEDs always parallel to switch. This one needs more resistance, but never has problems when the colors change.

Look into the Datasheet:Forwar Voltage: 2.9V min, 3.2V typ, 3.6V max.
Pay Attention: the Forward current is 30mA.
Recommend is 20mA for a light yield of typ. 13000mcd.
If you take only ~10mA they are bright enough.Accordingly you should calculate the resistor.
Grüße,
Stephan
2018-08-24 23:13
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Apeshaft
Posts: 51
Supporter
Thank you Stephan. Do I "have to" change all LEDs to parallell or is it sufficient with the ones that are too dim?
2018-08-25 01:23
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StephanHo
From: EDDG, Germany
Posts: 1111
Supporter
Hi Oskar,

that depends on you and your taste.

In principle, you should, for example, use a LED tester to determine how much voltage drops across an LED (in conjunction with a multimeter). At up to 2V you can connect them in series, above in parallel.

For consistency, I would recommend using 2 LEDs to switch them in parallel and to measure the current per LED.

In contrast to others, you even used the datasheet. If the LED has a brightness of typ. 13000 mcd, then it does not have to be operated at 20 mA. Just take 10mA, that should be bright enough.

If you take a 100 ohm resistor, the current is calculated as follows:
Type. LED operating voltage 3.2V. Then you have to drop over the series resistor 1.8V (3.2V + 1.8V = 5V). At 100 ohms and 1.8V voltage, the current is calculated as I = U / R that is I = 1.8V / 100 ohms = 18mA.
If only 10 mA flow, calculate the resistance with R = U / I. That would be 1.8V / 0.010 A = 180 Ohm (3 rings: brown-gray-brown - 4 rings: brown-gray-black-black - each + tolerance ring)
180 ohm is a common value of the E24 series and can be taken directly.
You would then load the output of the MEGA with 2 x 10 mA = 20 mA. For the 100 ohms you are using, 18 mA for series connection and 36 mA for parallel connection.
So you see, when connected in series, the same current flows through each component; in parallel connection, the currents add up through each branch.

As stated above, it depends on how far you have progressed with the construction and you want to do the effort. For the LEDs, which have a higher operating voltage than 2V, it would be recommended. It's your decision. For a clean, consistent solution I would take the parallel.
Grüße,
Stephan
2018-08-25 13:32
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okansacli
Posts: 1
Hallo Stephan,

Very promising software which I have downloaded yesterday to give it a try. So far it looks fantastic. Thank you.

I have an Audrino MEGA 2560 board, which has 54 digital I/O pins. I would like to use 28 of these to power up leds. Each channel will have 2 leds (serial or paralel I am trying to figure out and the resistors) there will be whites and yellows. (plese refer to the attached sketch)

Can my Audrino MEGA board light up all the 28 channels at the same time ? similar to the attached image

https://drive.google.com/file/d/13nQ4QUvWC2ZW9EuHTy7VqMRVdg9xaazQ/view?usp=sharing
Best Regards from Istanbul

Okan Sacli
www.okansacli.com
2019-01-09 10:46
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StephanHo
From: EDDG, Germany
Posts: 1111
Supporter
Hi Okan,

welcome to Mobiflight!

If I look at your circuit, it looks like a savings version, but maybe that's how it was explained to you.
Basically you should not switch LEDs in series with an operating voltage of only 5V and with parallel LEDs not with only one series resistor.

Basically every LED should have its own resistor. This has the advantage that you do not have to check later, which resistor belongs to which LED.

For the LEDs, care should be taken to take so-called high brightness LEDs (more than 8-10,000 mcd). These already shine bright enough in a cockpit with a current of 8 - 10 mA.

Basically, an output of the MEGA can deliver up to 40mA of power. But this would be the limit, which should be avoided when building.
If you use LEDs with 8-10mA, you can safely connect 2 LEDs to each output.

If you use the High Brightness LEDs recommended by me, you can find out for yourself on which power they shine brightly enough for you. Keep in mind that on the one hand you can still see them when the sun is shining, but you do not dazzle them in the dark.

The plain yellow LEDs need the full 20mA of power for some brightness, while the high brightness gets by with far less. Give it a try.

Actually, there is no objection to operating all 28 LEDs at the same time, if care is taken not to exceed the maximum current of 500mA.
The operation of 28 LEDs is not realistic except during test operation.

You can connect up to 40 LEDs to a MEGA if you observe the current limit.
If the current is exceeded, you can distribute the load of the LEDs on several MEGA modules, but also if you need more than 40 LEDs.
For switching elements, this limit is 50.


For larger power consumers (stepper motors or servos) it is recommended to use an external power source (5V) in addition.

Look around here in the forum a little. Other users have shown very instructive circuits from which you can learn a lot.

Please think about your circuit.

I would be glad if you report on your progress.
Grüße,
Stephan
2019-01-10 00:18
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