current-sourcesimulation
For a project I am working on (related question here) I need to model a nonideal variable current sink in spice.
Simply reversing the orientation of a current source or inverting its value is not sufficient because it is still sourcing current, just in another direction.
I am fairly certain there is a 'trick' using some built in components to simulate a variable resistance since as far as I understand, that may be the most accurate model for this component.
You can, rather easily, hand translate it. The biggest headache is making sure you've got the correct variables defined.
Going off of this: http://www.youspice.com/ys/bjtfromdatasheet.3sp
And comparing it to one of the LTSpice models that is in this: LTSpice
It looks like LTSpice follows the same conventions as PSpice models, so my assumption is that the model information housed in the LTSpice link is correct for more than just LTSpice (haven't tested against NGSpice, but it's just a Berkley Spice program so it shouldn't be any different), you just need to add a new BJT with those model parameters that are outlined in the links.
EDIT: Looking even further, even AIM-Spice has the same model setup for a BJT. I'm extremely confident that as long as you translate parameters correctly you can take any spice model and move it from one spice program to another, assuming it's at least using the basic Berkley Spice setup
If you are selecting a column via a 4515 de-multiplexer, ONLY one column is active therefore it is impossible drive more than 1 LED in the same row at any one moment in time: -
This means, if 25mA will drive 1 LED sufficiently then that's all you need.
EDIT Passerby has made a very good point about the drive capabilities of the 74HC4094 - basically, in a nutshell, the max supply current is 50mA which means that if all 7 row outputs were selected at once, the maximum current deliverable per line is about 7mA and this is somewhat less than what the OP was expecting. There is another serial device that could suit and this is the 74HC595 and its maximum supply current is 70mA; this will deliver 10mA to each of the 7 LEDs but the OP has to decide if this is good enough.
I think the 74AC164 may just do the job however - it has 25mA outputs but I'm struggling to find the max supply current spec - anyone any ideas? Anyway, here's a small diagram that shows it driving a 7 seg display with about 13mA per line: -
Best Answer
If you want a current sink that can't drive a node below ground, just add an ideal diode as a voltage clamp.
simulate this circuit – Schematic created using CircuitLab