Re: Driving a large number of LEDs with ULN2803A

Peter S. May wrote:
Anthony Fremont wrote:
Anthony Fremont mistakenly wrote:

If you run 10 parallel LEDs @ 20mA off each output, you will be
flowing 250mA thru each output driver. If you had enough voltage to
put 10 LEDs in series, you would only need to flow 20mA thru each
output. This would reduce the power dissipation of the package by a
full order of magnitude. Taking the worst case scenario of a 1.6V
Vce (from the data***) multiplied by 4A (all 8 channels on flowing
250mA each), youd be looking at over 6W of power dissipation. Your
ULN2803 will be roasting. See if you can't scrounge up some more
voltage for the LEDs so you can put your power supply to work making
light instead of heat.

Oops
1,$s/20mA/25mA/

First of all, I appreciate the vim-formatted correction. :-)

I would have used FRED syntax but, hard as it may be to believe, there is
likely noone here that has ever even heard of the "FRiendly EDitor" or GCOS
for the matter.

Secondly, all right, at another location in this thread I found that
increasing the voltage would simplify things a tad. I don't have any
experience with DC-DC converters, but it's something I could learn.
It just happens that the power supply I'll be using has a 12V tap, but
that's still too little to be of much use.

Might be of more use than you suspect.

So, what if I were to do something like this:

+15V <---+-->|-->|-->|---/\/\--+--< (collector)
| |
+-->|-->|-->|---/\/\--+
| |
+-->|-->|-->|---/\/\--+

If I'm running the LEDs at 20mA, would the above circuit only be
pulling 60mA? That would certainly drop Vce to a more convenient
level...

Yep. :-)

It even seems possible to do something like

+38V <-->|->|->|->|->|->|->|->|->|->|---/\/\--< (collector)

And only draw 20mA, period.

Exactly. The total power disspated by the LEDs themselves will still be the
same, but the power wasted in dropping resistors would be less.

The big question, then: Would a DC-DC
converter overcomplicate the thing? So, I guess that's what I figure
out next.

That's up to you to decide. You can certainly build your own voltage
booster with very few components. You can use a regulated switcher IC, or
use the TV design approach and tweak the whole system so that it just
happens to supply the right amount of voltage/current for the LED string,
and pray that nothing changes (like an LED going short circuit). You could
even use the the PIC to PWM an inductor (using a MOSFET of course) and use
the built in ADC to track the generated voltage. You could use a simple
resistive divider to reduce the 38V being generated to something in the
range of 0 to 5V. Or you could use the ADC in the PIC to measure the
voltage accross your series resistor and adjust your duty cycle accordingly.
Or you could skip the PIC and use a comparator to turn the PWM on and off.
Or you could........... ;-)


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