ULN2003A spec.


Senior Member
looking through the ULN2003A spec and its says:

500 mA rated collector current (single output)

Am I right in thinking this is 500mA (0.5a) per output so can run 7 - 0.5a devices. So could drive 25 LEDs which have a current of 20mA each per output so 175 in total?


Senior Member
You need to bear in mind all those 500mA added up flow through on one common pin, which may have an overall current rating less than 7 x 500mA. This is not mentioned in the ST datasheet, but in the Texas equivalent datasheet
there is "Total emitter-terminal current" limit quoted as 2.5A


Senior Member
so from the Texas details I could have 5 of the outputs on. OK so if I only needed 2 on max at any one time and only ran say 5 20mA LED's all should be fine. :D

Thanks for the help guys


Senior Member
I was thinking 5 20mA LEDs would draw 100mA so if I had 2 outputs live the draw would be 200mA. total.
Figure 14 on the Texas Instruments datasheet is the most important information to refer to. There is a limited current available and an issue with overheating if too much current is drawn. That is why the x axis of the graph is the "duty cycle" - the % time that the current is flowing. If only one output is on and it is properly on, that is, not being driven by p.w.m. or suchlike, then the graph recommends that it should not draw more than about 330mA. If you have two switched on, the total current flow should not 200mA per pin. And so forth. So you should be fine with 4 outputs at 100mA each, all on at the same time, according to the graph.

I have always thought it a little misleading that the introduction section states 500mA per pin - as if the chip could drive that through every pin! Figure 14 also shows that the 500mA figure is for 50% duty cycle, not 100%. I think a lot of people run these chips well over the rating and it seems that in many cases they get away with it, but you can certainly burn them out if you chance your luck. Figure 14 shows the parameters for safe and long-term operation.


Using the TI ULN2803 Datasheet as a guide
Under absolute max ratings:
Total substrate terminal current is -2.5 Amps.
So in that case 500 mA peak per channel or with all 8 channels loaded, About 300 mA per per channel on average and some for internal loads/losses.

So albeit to a slightly different "chip", clearly power/heat losses and duty cycle as mentioned above also come into play.
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