74HC595 +TLC5940 multiplexing 128 leds -> not enough power

[mozobata]

Apologies in advance for probably missing a lot of things. I'm pretty new to electronics.

I'm building a 128 led (monochrome) multiplexed array, driven by an Arduino. I'm using a 74HC595 as the current source and a TLC5940 for sinking and PWM. The output consists of an 8 (74HC595) x 16 (TLC5940) array of leds. Any number of the leds can be on at a given time.

Here is the

I'm doing the multiplexing by turning on a column with the 595, one at a time, and then setting the appropriate values for PWM for the TLC, lighting the rows in a columns at desired value.

Multiplexing works fine already but scale becomes a problem with the current 8x6 array i have built (6 out of 16 so far connected to the TLC). I'm pretty sure my problem is that turning on a column with the 595 can not supply enough juice for the 16 leds in it. The leds have a forward voltage of 3.3V with 20mA current. This would mean the 16 leds in series would eat up ~53V @ 320mA, which is far above what the 595 can provide and the TLC can sink. How would i go with making this work? Is there some magic i can do in software to help with this (my guess is not)?

I've been looking at adding a darlington array (something like ULN2803) to pump up the voltage to 50V per channel. This i guess would be fine if all 16 leds would be on at the same time but if only 1 is on the led will most certainly fry. Because of this i guess i need to be able to add a resistor that changes value depending on how many leds are on at a given time. I was thinking of putting in a digital potentiometer to do this but it feels a bit overkill + i haven't come up with a good solution for hooking this up (without putting a single one on each 2803 output)?

Ideally i'd keep the circuit quite simple but i'm open for ideas so any suggestions are more than welcome! Also I wouldn't be surprised if i'm totally missing the point on the problem..

If what i'm trying to do is not clear enough i can create a schematic for it

thanks

[redwudz]

Wrong forum. Better in Newbie/General Forum.
Moving you.


And welcome to our forums.


Moderator redwudz

[pandy]

All diodes in series consume only 20mA (assume your current source is TLC5940), voltage can be issue (for 595 and for TLC5940).

[johnmeyer]

Pandy is correct: when you put diodes in series, their voltages (the forward voltage drop) add, but the current require to drive them is the same. Power equals voltage times current, so if you are following what we are saying you will understand that the power does indeed scale linearly as you add more diodes in series.

To drive something at 50+ volts from a part that is more or less an old TTL equivalent, you'll have to use an appropriate transistor for each string of LEDs in series and, of course, you'll have to create a DC power supply that can produce that voltage.

[betwixt]

1. Use a UDN2850 or its equivalent circuit in discrete components (on its data sheet)
2. Wire the LEDs in parallel with each one having a series resistor to limit its current to 20mA. R in Ohms = (supply voltage - LED Vf) / 0.02 where Vf is the LED forward voltage drop. You quote 3.3V for Vf but that is unusually high for a 20mA LED, most small LEDs are around 1.6V.

That satisfies all the electrical requirements but it does mean you need lots of resistors, there is no way around that I'm afraid unless you enter the complexities of high voltages.

Note that both TLC5940 and ULN2803 are current sinks, one must be a source which is why I suggested the UDN2850.

Brian.

[Budman1]

Would led strips work? Power is direct and sequenced indiviually by arduino. Then you just need a powersource of 5v and adequate current.

[pandy]

Sink or source - current is constant - only different topology (high/low).
If i understand functionality correctly (based on description and on tlc5940 datasheet) this circuit will act as XY array capable to display some pictures with 6 bit level for every diode?

IMHO diodes shall be not connected in series but they should be connected to common line which can be connected to GND by some transistor capable to sink current from 16 LED (depend on your LED but TLC5940 is capable up to 60mA so around 1A) - some small MOSFET like NTR3C21NZ should be fine (NTR3C21NZ was cheapest N-MOSFET suitable for this circuit i've found in mouser.com), i assume you wish to use 74hc595 to save some pin's on CPU and and only 1 output will be active at the same time and it will be used to control small MOSFET.