How to emulate a current sink fuel level sensor

[thisisnotadream]

Currently developing a PXI station to emulate inputs to a gauge cluster and monitor its responses on the CAN bus.  Not having any problems giving the user software control over any of the inputs, except a fuel level sensor, which is modeled as a current sink in parallel with a 100-ohm resistor.  The sink draws current proportional to the fuel level from 0 to 127 mA.  The only way I can find to do this cheaply is to mount a klunky variable resistor and make the user reach over and twist a knob.  This also makes verifying the instantaneous current a little less elegant too.

Is there a simple circuit or DAQ solution that would work here?  Ideally, I would prefer something that sourced mA's proportional to a voltage output from our PXI card. 

It's been too long since Dr. Asumadu's sophomore electronics course at Western!

Dan

[K C]

Hi Dan,

With two transistors and a few resistors, you can make a current source independent of the load.

You can change the output current with the 3K8 resistor. With 0-5V on the b-e of the PNP transistor the current at the output (and 38E resistor) will be 0-130mA.

The 38E resistor must be 1W or higher and the PNP transistor a 500mA version.

Let me know if  you need more help.

[thisisnotadream]

Thanks, KC. 

That will be going in my toolbelt for the future, but I think I misspoke later in my post.

Is there a similar circuit for a current sink?  I'm supposed to be emulating the sensor, which is behaving as a load on a current source of the machine gauge cluster I'm testing. 

Sorry about the confusion.  My bad.  But, at least I got an awesome current source circuit out of the deal.

Dan

[K C]

Hi Dan,

OK next try. I assume that the voltage is low (24V)

Current sink is easier. Don't burn your fingers when you touch the transistor or use a big one.

[thisisnotadream]

Wow!  That is easy (and vaguely familiar).  You are correct in assuming voltage levels.  Acceptable voltages for this system range from 20-33 VDC.  This is a much more elegant solution than the varistor.

Thanks for your expertise.  This will get me started. 

Dan

[K C]

Most of the time the 'simple' solutions are the best. But first you have to get this working.

If it fails or doesn't work like expected just say so and we will dig further.