Driving LED's with PWM

[Demon]

Pretty much the same thing I observed.

Brightness requires current, I don't think you can get around that.

Maybe if you start tinkering with transformers?

Robert

[Christopher4187]

Is this statement correct?

Even though the currents are similar, using PWM without a resistor must be more efficient since a resistor isn't being heated.

Another thing that doesn't make sense is the non-linear curve in current when adding diodes onto the PCB. One LED consumes 30mA but two doesn't equal 60.

[xpa999]

An LED cannot have more than its forward bias voltage across it. For an old red LED that voltage is about 1.6 volts. Sacrifice one by driving one with a variable power supply. It will start lighting at a little over 1.5 v. It will be brightest at 1.6v. Only when it burns out will the voltage go much higher than that.

In a circuit, the current is limited by the impedance (resistance) of the circuit driving it. If you drive the LED directly from a i/o pin, the current will be limited by the impedance of the output circuit. The output transistor or FET will act as the resistor and limit the current to some value.

There are two different limits on the current through the LED, the maximum instantaneous current and the max average current. If you exceed the first, the LED will fail immediately. If you exceed the second, it will overheat and die more slowly. There should be a resistor to limit the current to below the max instantaneous current. You can then use PWM to stay below the max average current.

The instantaneous current through the LED can be calculated using i = e / r where r equals the sum of the impedance of the driver plus any external resistor. Heat will be lost by the voltage drop across the driver transistor and the external resistor.

[Demon]

...Another thing that doesn't make sense is the non-linear curve in current when adding diodes onto the PCB. One LED consumes 30mA but two doesn't equal 60.

I have a similar problem. I'm designing panels for my residential alarm system and I'm kinda overwhelmed by the possible maximum current. A LOT of LEDs could potentially be on at the same time (I'm using solid strand 24 AWG telephone wire with 12V 7Ah battery backup).

I started with LCDs, but these panels are not only for the house where it's warm, it's also for the garage (worried about cold and humidity).

Robert

[mackrackit]

A little crap to wade through.. but these two threads might help.
http://www.picbasic.co.uk/forum/show...egment+display
http://www.picbasic.co.uk/forum/showthread.php?t=3510

[Demon]

Thanks!

Just went through the 1st, great stuff!

Didn't download the dice yet. I want to see what I can do on my own first.

Robert The Very Happy Camper

[Christopher4187]

I have a similar problem. I'm designing panels for my residential alarm system and I'm kinda overwhelmed by the possible maximum current. A LOT of LEDs could potentially be on at the same time (I'm using solid strand 24 AWG telephone wire with 12V 7Ah battery backup).

I started with LCDs, but these panels are not only for the house where it's warm, it's also for the garage (worried about cold and humidity).

Robert

If it's a panel inside then you could supply 1-3mA per LED. There's no reason to supply the maximum amount of current since you won't have an audience to view them and they are inside. I recently used Screenmaster LED's and at 1mA they were really bright. I wish I had your problem. My LED scoreboard is outside and the sunlight hits it. Needless to say the LED's have to be really bright.

[Demon]

If it's a panel inside then you could supply 1-3mA per LED. There's no reason to supply the maximum amount of current since you won't have an audience to view them and they are inside. ...

I'll be using surface-mount LEDs that I have on hand (got some greens, yellows and reds). I think they require somewhere between 15-20mA to be reasonably visible in daylight inside the house (some panels could be in full sunlight).

I'll be making my own light chimneys with clear liquid plastic from Smooth-On.

Finished preliminary testing today, very low current is achievable, I just have to improve code logic and efficiency:
http://www.picbasic.co.uk/forum/showthread.php?t=18906

Of course none of the colours I have are of the same brightness so some tweaking of the resistors will be necessary.

Robert