ULN2803 (Load Driver - switches ground): Not going all the way to ground, I still get half a volt?

Thread Starter

Mahonroy

Joined Oct 21, 2014
339
Hello, I am using a ULN2803 load driver to activate/deactivate a 3.3V sensor. I am doing this by connecting the sensors ground to the output of the ULN2803. When sending 3.3V to the ULN2803 input, the ULN2803 should switch the sensors ground to ground.

Here is a schematic:
uln2803_problems1.jpg

When I do this, I am reading 0.6 volt difference between the sensors ground, and the PCB's ground. Similarly, the sensor is outputting a higher analog voltage than normal. I can take an alligator clip and short the sensors ground to PCB's ground and everything is normal.... so for some reason the ULN2803 is not bringing it all the way down to ground?

I read through the datasheet again and it seems this should be working, so I am not sure what is going on? Any help or advice would be greatly appreciated, thanks!
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
339
Just for my future knowledge, how did you determine this from the datasheet if you don't mind?

Should I just drive this with a standard n-channel mosfet instead in the future? The sensors use less than 100mA so not much power at all. The only reason why the ULN2803 was because I already had one onboard and I had unused channels.
 

ebeowulf17

Joined Aug 12, 2014
3,275
Here are the specs from the datasheet.
9B2CCCDB-BA21-4A84-B7B0-3B894761C9DA.jpeg
092C2676-240B-4AB3-ADD8-38AC39DFF59D.jpeg

A MOSFET with a low Rds-on spec (low resistance from drain to source - sort of the MOSFET equivalent of low CE saturation voltages in BJT components) would probably get you much closer to ground. No solid state switch will get you all the way to ground, although at low currents some MOSFETs will come very, very close.

Still, if you need great accuracy and tight tolerances, you might be better off not switching ground at all, but switching the high side instead. Then your ground reference is as perfect as possible.
 

BobTPH

Joined Jun 5, 2013
3,094
Use a high side switch. PNP transistor or P channel MOSFET. If you have a higher supply voltage, like 5V, a 3.3V low dropout regulator with shutdown is an option.

Bob
 

AnalogKid

Joined Aug 1, 2013
9,158
What is the expected current through the 2803 when it is on (switching the output to GND)? Consider a small power MOSFET as the switching device.

ak
 
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