I am looking for advice on the existence and application of a component that will drop exactly 10 V under any conditions within a given range. I am tasked with designing a system take the response of two thermistors, linearize it, convert the signal to 4-20 mA DC, and, ultimately, control a pump system based on the difference in temperatures of two reservoirs. At the moment, I am struggling with an aspect of the transmitter design.

I have successfully modeled an op amp configuration that will linearize the voltage across the thermistor and developed a transconductance amplifier to convert an input voltage into the appropriate current range for the control loop. The problem is that the transconductance amplifier requires an input of 1 V to 5 V, but the output from my thermistor amp is nowhere near that.

To address the issue, I developed a non-inverting amplifier that will give a change in voltage of exactly 5 V over the sensing range of my thermistor; however, the output voltage there is still too high. The perfect solution would be a component that would allow me to drop exactly 10 V regardless of the output voltage of the op amp, but I'm not sure what component I would be looking for. Any help would be greatly appreciated.

 

I am having trouble understanding the last paragraph--what is the 10V going to do for you?

If your non-inverting amp puts out a max of 5V, why is that too high to drive your 1-5V transconductance amp input?

 

I'm kind of designing from both ends toward the middle. Sorry if that's a bit confusing.

The non-inverting amp puts out a range of 5 V, but the actual output values with respect to ground are between 11 V and 16 V. The transconductance amplifier requires an input value between 1 V and 5 V with respect to ground. The 10 V drop would allow those values to be met. I have a vague notion that I should be able to do this with some kind of diode or MOSFET device, but my understanding of these things is in its infancy.

 

Oh, you want this to be loop-powered--right? and the total loop voltage is in the order of 24V or so--and you do not want all that voltage across your linear components

Tricky stuff indeed!

One way to do it is to put in op amps that can support the voltage--many can support 30V--and then you must select one that has a common mode input voltage that can go to down to 0V or a little below (e.g LM324)--that way you do not need a negative rail (below common) as that REALLY complicates things--then the current consumption of all circuitry must not exceed 4mA--then all linear circuitry must be immune to changes in voltage. Difficult problem!

You could put in an LM317 voltage regulator to provide a +10V power rail, but the voltage regulation may be a little poor if the load current drops significantly below 4mA--I have applied the LM317 down to 3.6mA, but I think 5mA is the actual min load spec--there may be a better 3 terminal regulator for the application--perhaps someone else out there can make a recommendation. At any rate, the power rail load current may never exceed the instantaneous loop current requirement.

Take a look at the thumbnail--if your transconductance amp looks like this, you, can use this approach--now there are other ways to create a transconductance amp, but they may complicate this problem.