ULN2003 and Propeller

Newzed

Will the ULN2003 operate off of Propeller voltage?· Can't find anything in the datasheet that states the minimum COM voltage.

Sid

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OzStamp

Hi Sid.

We have tested the ULN2803 which is a 8 darlington output (instead of 7) and works fine.
Operated some relay's on the other side ...
3.3V from the Prop seemed to have no trouble at all driving it hard enough ..
Also there are some really neat single channel dip like the SHARP PC852 ...output not as high
but will drive small 24VDC relay's fine..

cheers
Ron Melbourne OZ

Peter Jakacki

USING ULN2003/2803s

These chips seem to be quite popular for driving heavy loads and are even used as heavy-duty inputs for sensitive logic. I hope that anyone using these chips may find these facts useful.

FACTS:
The 2003 family is basically an array of darlington transistors (cascaded current-gain) complete with resistors. On the chips there is a COM pin which appears as commoned diodes from all the collector outputs. This is in reality a by-product of fabricating circuits on a common semiconductor substrate but proves to be useful as a flyback diode when driving inductive loads such as relays and solenoids. Typically a flyback diode is connected across the coil so for the 2003 to be effective it needs to have the COM pin tied to the supply side of the load. If you are driving +12V relays then you would tie the COM pin to +12V. There are variations on this to speed up drop-out etc but this is the usual scheme. If you are not driving inductive loads or have your own flyback scheme you can leave this pin unconnected.

What is the highest voltage that the 2003 can stand? The datasheet says 50V absolute max, so there should be no problem driving +24V stuff even.

If you drive the 2003 from 3.3V logic you cannot drive 500ma loads without problems although anything up to 300ma should be fine. Remember that the 2003 is a darlington transistor and these have typical current gains of 1,000+ (guaranteed minimum) so with an internal input resistor of 2.7K plus the 1.2V drop across the base-emitter-base-emitter you end up pumping 778ua max into the input. 1000 times 778ua gives us 778ma but the transistor would not be turned on hard but would be acting more like a resistor rather than a switch. You know how linear regulators can get pretty hot compared to switching regulators? Well that's what happens to the 2003 too before it releases it's "magic smoke".

Since the 2003 is an IC it has an overall maximum substrate current limit so it is not possible to have all outputs driving 500ma loads at the same time. From the "peak collector current vs duty-cycle" chart with 7 outputs turned on and 100% duty it looks like you can only expect less than 200ma/output. This is mainly due to a thermal build-up problem and you could push it more if you immersed the chip in liquid nitrogen


*Peter*