Selecting power source for LED Tape

This may be a simple question for which I may be mocked and paraded around the town for but...

I am building a project which involves the PWM control of 20 pieces of single colour LED tape, all different lengths from 200mm to 3000mm.


I have always been a resistor user with my LEDs but the size of the larger string (3000mm) and the fact I will be using this in temperatures that vary from -10 to +25 C has made me consider whether I need to grow up and learn how to use constant current drivers.

Questions:

Each piece of LED tape will need a different amount of current, does this mean I need to buy/design an individual driver for EVERY piece?
Does it make a difference that LED tape has a few resistors built in?
Can I run a constant current source through a MOSFET based PWM circuit like I would with, say, my old 12V Battery friend?



I have searched for hours and found no simple answers so I suspect there are none but if any of you can enlighten me I will be a happy chap!


Thanks

Leo

There is a sticky thread in the LED flashlight section that talks about PWM.

http://www.candlepowerforums.com/vb...t-is-it-How-does-it-work-and-how-to-detect-it

I design a lot of electronics that use PWM to control the brightness of LEDs. Using PWM with a current limiting resistor works great in many small LED applications where heat is not an issue. Also (from the above linked thread) if you switch PWM fast enough, it is impossible (without using measuring equipment) to tell you are using PWM.

When you are driving several watts through an LED, PWM becomes problematic. As the PWM frequency goes up, the efficiency goes down (your driver can overheat). When PWM goes down too low, you can detect it (see the above linked thread).

The LEDs strips I have used all have built-in drivers and use a serial bus to set the LED brightness values.

Do you have some details on the LED tape you are using?

Thank you for your reply tgil,


I am using the tape here: http://www.ledlightszone.co.uk/led-tape/54-flexible-led-strips-non-waterproof-72w-30pcs.html

Cool white, 60 LEDs p/m, 5050.

Lengths range from 800mm to 1320mm on most outputs plus one extra big 3455mm strip.



Drivers (currently) are based on a TLC5940 and a bunch of MOSFETs (Rated for 60V 36A) sinking for each channel with a common V+.

Interesting to note that efficiency goes down at higher frequencies - I was hoping to save on power consumption by running things at around 50% most of the time and spiking up to 100% for special occasions. The TLC5940 PWMs at about 976 Hz.


Hmm! (Thank you)

Those are meant to be powered with a constant voltage source (12V DC). There's even series resistors of 150 Ohm visible in the product pic, which surely serve to control LED current @ that voltage. For example an old ATX or laptop supply could do the job, or even a regulated 12V DC wallwart.

If you can find a power source that allows regulating the voltage somewhat (eg. in 9-12V range), that would be a much more elegant way to dim those LEDs. No chance of visible PWM, no switching losses (no need to switch anything, for that matter).