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question on daisy chaining leds


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#1 LarryTBT

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Posted 22 August 2012 - 01:26 AM

Is this possible and if so how many on each pwm pin? Leds will be from spark fun super bright white and blue 10mm. Any info would be great thanks

#2 LarryTBT

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Posted 22 August 2012 - 01:30 AM

Writing this from my phone. This will be for a small aquarium light fixture controlled by a netduino plus for day and night mode and watch ph and temp. Am looking at controlling maybe 25 white with 1 pwm pkn and 15 blue with another. If anyonecan help me it would be much appreciated. Thank you

#3 Paul Newton

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Posted 22 August 2012 - 05:43 AM

Hi Larry,

The white LEDs need 3.4V at up to 20mA.

The "high current" Netduino pins are only rated for 16mA - see CW2's Wiki page. Unfortunately I don't believe the PWMs are even 16mA.
So you are not going to be able to drive one of those LEDs safely (let alone 25!).


What is needed is a transistor, probably a darlington transistor.

Depending on your available voltage, you will need to make several chains of LEDs to give each chain the correct voltage. Then combine the chains in parallel connected to a darlington.


I did some tests using a darlington and put the results in this Wiki page. I also experimented with an opto isolator which you may also find useful.

Hope this helps - Paul

#4 Mario Vernari

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Posted 22 August 2012 - 05:51 AM

Hello Larry, Paul already pointed the driving problem, but...it's maybe my limited knowledge of English...are you either seeking for independently driving the led strip, or just use one pwm for all? Cheers
Biggest fault of Netduino? It runs by electricity.

#5 LarryTBT

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Posted 22 August 2012 - 11:05 PM

Just looking to drive all by one pwm pin. Ill take a look at what was recommended

#6 LarryTBT

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Posted 24 August 2012 - 08:51 PM

Would a tip 120 be sufficient? If i setup an array of 24 leds using this calculator http://led.linear1.org/led.wiz Using these specs Power supply 24v Forward Voltage 3.2 Diode Current 20mA Amount 24 It gives me a reading of each 270 ohm resistor dissipates 108 mW the wizard thinks 1/4W resistors are fine for your application Help together, all resistors dissipate 432 mW together, the diodes dissipate 1536 mW total power dissipated by the array is 1968 mW the array draws current of 80 mA from the source Now do i get a transistor that can handle the 80mA from the source or Dissipated power of 1968 mW?

#7 Paul Newton

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Posted 24 August 2012 - 09:19 PM

Hi Larry If I understand correctly, you are going to use four chains of six LEDs, where each chain is taking 20mA. The transistor needs to handle (more than) 80mA. The data sheet should tell you the voltage drop across the transistor when it is on. Hopefully this will be small (a Volt or less). Multiply this by the 80mA to get the power that will be disipaed in the transistor. Check the transistor can handle it. Paul

#8 Paul Newton

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Posted 25 August 2012 - 02:03 PM

Hi again

Yes the TIP120 looks fine. The data sheet says its a 5Amp darlington pair.

Figure 2 shows that at 100mA the Voltage accross the base emitter junction is 1.25V, and the voltage accross the collector emitter junction is 0.7V. It also says Ic = 250Ib


What does that mean?

The power disipated by the transistor will be your 80mA x 0.7V = 0.08 x 0.7 = 0.056w = 56mW
Figure 4 shows this is well within the safe operation area of the device!

The current into the base will be 80mA / 250 = 0.00032 = 0.32mA
The resistor fromm the Netduino should be (3.3V - 1.25V) / 0.32mA = 2.05 / 0.00032 = 6406 Ohms
Nearest preffered value 6K2 (6200 Ohms)

I think that's everything
Just before I throw this thing accross the room at a hard wall, I would like to say that trying to write a forum post with an android tablet is the most frustrating thing I have ever done.

A rather irritated - Paul

#9 Nevyn

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Posted 25 August 2012 - 09:05 PM

Just before I throw this thing accross the room at a hard wall, I would like to say that trying to write a forum post with an android tablet is the most frustrating thing I have ever done. A rather irritated - Paul

It's not easy on an iPad either

Regards,
Mark

To be or not to be = 0xFF

 

Blogging about Netduino, .NET, STM8S and STM32 and generally waffling on about life

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#10 Paul Newton

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Posted 26 August 2012 - 08:57 AM

It's not easy on an iPad either

Regards,
Mark


I was finding it very hard to edit anything with the on screen keypad due to the lack of cursor keys. Its very hard to get the cursor where it needs to be!

So I paid out for a mini keyboard (about ten pounds - I just realised this keyboard has not got a
pound symbol. Thats pounds as in sterling, not pound as in "#"define).

I am now finding that firefox for Android does some very strange things with a USB keyboard. e.g. enter does a delete then a newline. It does it with a full size USB keyboard too. Sometimes the delete is on a different line to where the cursor is!

And don't talk to me about cut and pasting a link to a data sheet - I had to type it in manually.
:( sorry to go off topic. I needed a rant.




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