Converting analog input types

[jonnies]

Hi everybody,


My question isn't directly PLC related this time, but I have a feeling some of you could answer it anyway

I have an board up here that takes in a passive, 0 to 90 ohm input from a fuel level sensor, and converts it to a 0-100% fuel level. The customer wants to switch to a more accurate fuel level sensor, which can output either 0 to 10 V or 4-20 mA. Is there a simple circuit we could add that would trick the board into thinking it is getting a 0 to 90 ohm input from a 0 to 10 V or 4-20 mA source?

Also: when the passive resistance input ranges from 0 - 90 ohms, the input is acting as a constant current source, with current constant at 2.5 mA through the resistor, and the voltage is varying between 0 and 225 mV. If it goes higher than about 150 ohms it shuts off with a broken fuel sensor alarm. Tomorrow my manager wants to try just hooking up a 0 - 225 mV source up to it and slowly increasing it, but I have a feeling the constant current source won't like being in parallel with our constant voltage source one bit. Any suggestions?

 

[bernie_carlton]

You aren't getting 'ohms' as an imput. It's either current or voltage. The resistance of the sensor (0 - 90 ohms) is affecting the current or voltage presented to the measuring device. If you can show the connections, the appropriate measurement at the input (either current or voltage) and the relationship between the input and the percent fuel level then we can work with that information.

 

[jonnies]

Thanks for the reply. I know-- it's like I said, it is a constant current source no matter what the resistance is. The voltage ranges from 0 to 225 mV based on the resistance...this is all in my last paragraph.

 

[danw]

Is there a simple circuit we could add that would trick the board into thinking it is getting a 0 to 90 ohm input from a 0 to 10 V or 4-20 mA source?

There are no commercial volt/mA to resistance converters that I know of.