ULN2003A Darlington Transistor not behaving as expected

[geekbot2000]

Hello,

I'm feeding a 5V PWM signal (500Hz) into the base of an ULN2003A Darlington Transistor Array (Datasheet: http://www.ti.com/lit/ds/symlink/uln2003a.pdf) and using it to sink a 10V rail to ground. The problem is that when I scope the output, the amplified PWM signal looks incorrect. Also, changing the probe attenuation from 10x to 1x significantly changes the characteristic of the transient element of interest (for the better).

1. The rail is not being overloaded. Its output is regulated 10V and looks stable. All measurements taken with "open" outputs, probing the terminals of interest directly.
2. The PWM input is accurate and the GND is common across +5V and +10V rails.


The circuit diagram:


Behavior observed at 1x scope attenuation:


Behavior observed at 10x scope attenuation:


Different duty cycle at 10x scope attenuation:


What I want is a clean square wave. What looks to be happening now is a clean transition when the darlington switches ground on, but then a really lazy/capacitive rise time to +10V from ground when the darlington switches off.

Is this the expected behavior? Any thoughts? Thanks.

Geekbot2k

[c4757p]

The behavior of an NPN Darlington transistor is to pull current in (never to put it out) through from the collector (the "outputs" here) to the emitter ("common" here) when the base ("input" here) is equal to or greater than the base-emitter threshold voltage (about 1.3V). There's nowhere for them to pull current from because there is no power source on the outputs.

The rising edges of your signal are leakage across the transistor into the scope probe.

Edited because I misread the datasheet.

[Andy Watson]

Looks good to me. You say you want to "sink" the output (to ground), which the scope confirms (rapid downward transition). But, you've got no source, i.e. there is nothing to pull the output back up. Try a resistor (something around 1k) between the output and the positive rail.

[geekbot2000]

Looks good to me. You say you want to "sink" the output (to ground), which the scope confirms (rapid downward transition). But, you've got no source, i.e. there is nothing to pull the output back up. Try a resistor (something around 1k) between the output and the positive rail.

Pulling the output to 10V+ with a 10K resistor solved the problem. Thank you!

[geekbot2000]

Followup on this, is it normal for the output to be clipped - in my case I am seeing 9.6V instead of expected 10V between the rail and the output which means that the darlington isn't dropping completely to GND.

[c4757p]

Yes. That's V_CE(sat), the collector-emitter saturation voltage. Part of why Darlingtons are shit for switching purposes...

[geekbot2000]

Yes. That's V_CE(sat), the collector-emitter saturation voltage. Part of why Darlingtons are shit for switching purposes...

Thanks brah

[Andy Watson]

One minor point that I've just noticed on your diagram: You appear to have connected the scope ground to pin 9,+ve rail (?). It's more common to put the ground on 0V (just convention - makes the circuit operation easier to read). The ULA200x chips do not require a positive supply - pin 9 is the common of the cathodes of a diode on each output channel. These diodes can be used to suppress inductive loads, and/or pulled low to act as a lamp test (i.e. effectively turn all outputs on).