BIG Led Matrix with ULN2803A at 12V

Guys, it has been a long time... :wink:

I'm currently doing some research on how to build a big 4 x 4 LED Matrix, but using 16 Arduino Pins and two ULN2803A ICs.

I'm using 12V for the ULN2803A so I can drive 3 leds in series with a 56ohm resistor. The leds are super bright white leds, with a Vf of 3.2 so I'm worried that this may break the ULN2803 when we have all 8 channels on.

6 leds per channel = 150mA * 8 channels = 1.2A @ 12V

Is this possible? I have only tested one channel so far, and it works correctly. My main worry is when all channels (96 leds total) are fully on. I don't mind if they dim a bit, just worried about heat and blowing up the ULN2803... :wink:

Another solution, I guess, would to use one transistor per channel and forget the ULN2803...

I'm also considering just using 4 leds per channel and running at either 12 or 9V, using 2 x 2 leds in series. I would them get around 100mA total per channel.

Any advice is much appreciated!

Best Regards, Guilherme (William) Kalfelz

"doing some research...."

How about telling us EXACTLY what you have actually done so far, in the way of testing your circuit ?
"doing some research" doesn't really tell us anything about what you have done so far.
Do you mean you have a pile of parts or that you have powered up and tested the circuit ?

As stated a diagram or schematic would help here but in case you do not have the spec sheet for the ULN2803 then here is a link.

this shows the current per channel and totals allowed.

Thanks, those datasheet are complete, the one I had wasn't... ok, so it does support 150mA for all 8 channels, nice!! I will work with that in mind. :slight_smile:

So far the idea is simple, 16 led squares, each with 4 or 6 leds. Tomorrow I will do the math and decide on things...

Cheers!

One question, how's the math to check the Wattage when running several leds? The input will be 12v, with the right resistor to get 22 or 24mA... as I read that the uln has power disipation problems over 1.5 watts...

So, I use the Led Vf (3.2V) or the system 12V, multiply with all channels mA or what? Thanks again, and sorry for the silly questions. :wink:

check page 6 of the specs which shows tables for max duty cycle at specific current level with number of devices running.
For example if you have 7 channels at 200ma per channel you can run 100% duty cycle.
Or say for 8 channels you get 50% duty cycle at 300ma
So setup one channel and measure the current draw if you run 6 channels then look at your current draw interset with 6 channels and see how much duty cycle you can run.
In reverse if you want 100% on time then for 6 channels limit your out puts to about 220ma per.

I'm starting to think it will be easier to just use a 200mA transistor (npn?) per channel? Again, it will be 16 sets of 4 or 6 leds, around 150mA Max each of the 16 sets... still need to understand about the power disipation watts thing...

spicetraders:
check page 6 of the specs which shows tables for max duty cycle at specific current level with number of devices running.
For example if you have 7 channels at 200ma per channel you can run 100% duty cycle.
Or say for 8 channels you get 50% duty cycle at 300ma
So setup one channel and measure the current draw if you run 6 channels then look at your current draw interset with 6 channels and see how much duty cycle you can run.
In reverse if you want 100% on time then for 6 channels limit your out puts to about 220ma per.

Thanks, I understand that, but still don't known how to calculate the watts value to see if it will be under 2watts total, for 16 x 150mA 3.3V Leds in a 12V system...

Ohm's Law

P(W)= V (V)* I (A)

raschemmel:
P(W)= V (V)* I (A)

Yeah, indeed, I'm aware of that. :wink: but what voltage I use? The Led Vf, system V, what?

Thanks again.

You would use the voltage across the “gate” output, which is an open collector output (each) of a Darlington transistor.
That could be around 1.5 to 2 volts

Ifyou run 150mA down an output then figure 1.5V * 0.15A = 0.225 W

Reading the 2803 spec sheet it states:
“The ULx2803A, ULx2803LW, ULx2823A, and ULN2823LW
have series input resistors selected for operation directly with 5 V TTL
or CMOS. These devices will handle numerous interface needs —
particularly those beyond the capabilities of standard logic buffers.”

The out puts can run up to: “the outputs are capable of sinking 500 mA and will
withstand at least 50 V in the off state.”
So output should work with 12 or 3.3 or 5.
Leaving it to your LED requirements and power supply source

Oh, thanks!! But do you mind telling me how can I measure that?

If I lower the channel needs down to 100 mA I will be using less than 2w in all ch...

But do you mind telling me how can I measure that?

P = I x V
You can use a meter in current mode to measure the current or you can measure the voltage drop across a current limiting resistor in series with the led. If the resistor is 100 ohms and led is rated for 20 mA then V = I X R = 0.020 A X 100 ohms = 2V.

Place one lead of your voltmeter on the output and the other probe on Gnd.

1.2V * 100mA = 0.12W
0.12 W * 8 = 0.96W

1.5V * 100mA = 0.15W
0.15W * 8 = 1.2 W

Ok, I got a fancy multimeter so I can test things correctly. But them I got confused again. Here’s what happens.

I got 3 ultra bright straw hat leds, rated 3.2 to 3.4, connected in series, with the (-) to the ULN2803, and the (+) to the 12V power.
I’m using the ULN2803 to drive them with a 12V power adapter rated 1A. It reads 11.5V on the output when connected to my Arduino board (the VIN connection).
When I connect a potentiometer to test the leds from the ULN2803 output, and check the mA usage, I can only go as high as 63mA for the 3 leds when the potentiometer is at zero ohms. If I remove it and just connect directly without a resistor, the same result, max of 63mA, and it fluctuates a bit. No heat, no led explosions. :wink:
The voltage output of the ULN2803 pin is 0.8V, a bit low I guess. The chip reads ULN2803APG Toshiba, in case it is important.

So I wonder if I got something faulty here or if I’m doing something stupid.

Maybe I will test with a 2N2222 Transistor instead, to get better values…

Please, advise? Thanks again guys!

Best Regards, WilliamK

If I remove it and just connect directly without a resistor, the same result, max of 63mA, and it fluctuates a bit. No heat, no led explosions. :wink:

That is a very bad idea. You should NEVER connect leds to power without a current limiting resistor . Who trained you ?

Indeed. BAD Willy! :disappointed_relieved:

I fixed it, now I'm using just one led, 40ohms resistor, 5V supply, ULN2803. It works correctly. 23mA on the led and it looks good. :wink:

Maybe you should review the resistor value calculations in this post: