PWM + transistors + Led arrays + power supply + vo

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello,

Just got my Arduino 2009 kit, and I passed the blinking led test!!! Hurray!! ::slight_smile:

Now, I intend to use the Arduino to control multiple led arrays, with PWM fades - aquarium lighting - and I would like to know if this is doable or if I have missed something:

  1. Single power supply 32V 500mA
  2. Voltage divider + current regulator to power the arduino (7V - I have to check the required mA??)
  3. The PWM outputs will connect to the base of 6 transistor or mosfets to power led arrays - 8-10 leds each, from 20mA to 300mA, individually regulated; there may be more than one array connected to each mosfets, individually regulated

This means the PWM will signal the base (at 5 volts I guess?), then the load at the collector will be at 32 volts, with mA regulated for the type of leds being driven. I know all must connect to the same ground in order to the transistors or mosfets to work.

Am I on the right direction?

Thanks for all the help!

2

Don't use a voltage divider for power supplies.

For high current LEDS (anything over 80mA) you need a constant current driver rather than the resistor current limiting.
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1234273497
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1258412419

32V is too much for the front end of the arduino, this needs regulating down first.

8-10 leds each, from 20mA to 300mA,

That looks a bit much for a 500mA supply, have you done your sums?

3

Thanks for the reply.

I have LM317L and LM317T regulators available, so I can supply 7V to the arduino, plus I can regulate the current for each led array on the ground side - there will be 20mA, 100mA and 300 mA led arrays.

Each array has 8-10 leds, but 20mA for the entire array, from the 32V supply.

If my sum gets above 400-450mA (it probably will), I have a 32 or 36V power suplly with 1A, no problem there.

I'd like to get your opinion regarding the PWM - transistor base connections. Will it work?

Thanks.

4

regarding the PWM - transistor base connections. Will it work?

Yes you can PWM it, but as I said the larger LEDs require better regulation than a simple resistor.

5

I don't use resistors for led current regulation.They ideally work by current intensity not by voltage.

The LM317s will take care of the mA, one regulator per array.

If I can make it work, I'll be a happy man!

Thanks.

6

The LM317s will take care of the mA

The LM317 is a voltage regulator not a current regulator. Are you using it in a constant current configuration as described on page 19 of the data sheet?

one regulator per array.

I am not sure what you mean by array? Assuming you mean a matrix with no current limiting resistor a constant current or constant voltage regulator is not the way to drive a matrix.

7

Correct, I use the LM317 as constant current regulator to get whatever mA I need for each led series (this is what I mean by array) in question, i.e., 20mA, 100mA or 300mA.

8

Ok, the PWM can be done by putting a transistor in the ground connection. You confused me with the term array, it's not the right word, you have more of a string.

So you have
+ve supply --- LM317 current feedback resistor, LEDs, collector - emitter - ground.

9

Are you using it in a constant current configuration as described on page 19 of the data sheet?

Sorry to ask folks, but which copy of the datasheet are you looking at?
I don't see anything about constant current source on page 19 of http://www.national.com/ds/LM/LM117.pdf only current limiting.

GB

10

OK page 18 on that link 1A current regulator. Current limit / regulator is the same thing as constant current, it just depends on how you look at it.