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Topic: ULN2803A continuous current (Read 1 time) previous topic - next topic

Shpaget

Little background:
I'm working on a model railroad where I want to control the lights in the houses, train stations, street lights etc.

That means I need to control multiple LEDs (total number not defined as of now, but I'm thinking of no more than 64, probably 32 outputs per one micro controller) with various effects (again, not strictly defined as of now, but include fading, flashing, flickering and continuous light) that are partially random (for example, multiple independent LEDs simulating the flicker of a broken neon light). The LEDs in question are again, not defined, since they will be used for various effects, but will be small SMD LEDs, counting no more than 15 for sure, but most likely no more than 5 per single output (various colours, of course :smiley-mr-green: )

I guess you get the picture by now, I want a standardized device I can install on multiple locations and just use minimal reprogramming to set the effects on the outputs.
The idea is to use multiple pairs of 74HC595 and ULN2803A that can be easily daisy chained (just plug another board in) and adjust the software.

Since the code I'm writing uses randomized on/off times for flickering and other effects I can not be sure how many LEDs I will need to power on at the same time, so I guess I want to make sure to be able to drive all 8 from each 74HC595 / ULN2803A pair simultaneously. The question I have is, can ULN2803A supply enough current to do it or should I go for time-division multiplexing?
I'm having some trouble understanding how much can this ULN supply over each output and can it output that amount over all the pins simultaneously.

http://www.datasheetcatalog.org/datasheets/90/366828_DS.pdf

So, long story short, can you help me read the datasheet?  :smiley-red:
What's the maximum safe current per each individual pin I can draw from this ULN continuously over all pins simultaneously?

afremont

You can't draw any, it's an open collector output.  It only sinks current.  Each "driver" inside the chip can sink a maximum of 500mA.  The datasheet doesn't specify a maximum current thru the ground pin that I could see.  With all drivers active, I'd keep it below 100mA per driver.  With about 1V of saturation voltage, the device will get hot in a hurry if you try to sink 500mA on all 8 drivers simultaneously.
Experience, it's what you get when you were expecting something else.

Shpaget


You can't draw any, it's an open collector output.  It only sinks current. 


My bad.


Each "driver" inside the chip can sink a maximum of 500mA.  The datasheet doesn't specify a maximum current thru the ground pin that I could see.

That's my problem.


the device will get hot in a hurry if you try to sink 500mA on all 8 drivers simultaneously.

I figured that much. But how do I figure out how much can I sink safely for extended periods (several hours)? I could always hook one up and load it progressively until it releases the magic smoke, but I reckon there's a better way.

majenko

That sounds like a silly combination to me - especially when there are chips like the TLC5940 which are SPI controlled and give 12-bit PWM output on 16 channels of up to 30V each channel...

Or if you want more there is the TLC5947 which gives 24 channels...
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Papa G

The TI data sheet for the ULN2803A shows a total substrate-terminal current of -2.5A. I think power dissipation will be the limiting factor, however, if you operate it continuously.

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