# [SOLVED]Voltage Controlled Resistor

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#### skatamatic

##### Newbie level 2
I need to design a circuit that uses some sort of transistor to control resistance.

Here's some background info...

I have a DAQ system that monitors a wireless pressure sensor. The information received from the sensor is digital, and we use it in our software to record charts/etc. Our customer asked that we supply an analog output with our system to allow him to hook it into a 3rd party monitoring system if need be. The 3rd party monitoring system is looking for a 4-20mA signal. I used a digital to analog converter module (an ADAM 6024) to output between 4 and 20 mA based on the 0-10000PSI reading from the pressure sensor. It has come to my attention, however, that this signal will not work for them. There are 2 terminals that they are to tie into on our system (AO+ and AO-), but I found out that on the AO+ terminal they are actually sending us 12VDC (as if they were trying to plug into/power a pressure transducer) and expecting a signal on a return/ground wire. This does not work, the ADAM6024 does not like getting voltage pounded into its AO+ terminal (as you can tell, I am no electronics guru haha).

So here's what I need:

A device that takes a -10 to 10V control signal (or anything within that range) and when there is 12VDC connected to its output 'supply' the device's resistance is changed (based on the control signal) to make the output current 4-20mA. Note that we do NOT have access to their ground, only a 12VDC power in and a signal out.

I looked into using a photocell design to do this, but would much prefer to use a transistor of some sort. I hope this is possible. ANY help or a push in the right direction is welcome. Keep in mind I am a bit rusty on my transistor physics (although I am not electronically inept) so keep things in laymen terms if you can!

#### SkyHigh

I hope I get your description correct.
Input Voltage (+/-10V). Ouput Current (4-20mA).
Operating Supply Voltage (12Vdc).

I suggest you can use a simple circuit called Howland Circuit (or better known as Voltage-Current Converter)
Circuit Idea/Op-amp Inverting Voltage-to-Current Converter - Wikibooks, open books for an open world

Alternatively, you can look for Improved Howland Circuit (with added adjustments to make it better).

You want to control voltage to control resistance. In principle, controlling resistance is controlling current.

Hope this helps. Good luck!

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#### KerimF

Do you have an idea on the highest rate that the monitor current should be changed?
I asked this because instead of the analog varying resistor (current), a PWM may help since it is simple to implement (with isolation) but its response is relatively slower due to the low-pass filter in the 4-20mA loop. I hope you got the idea.

Edited:
Their 12V is relative to which node?
I mean it seems you need also a sort of electrical isolation to isolate their 2 wires from your circuit like using an optocoupler.

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#### skatamatic

##### Newbie level 2
Thanks for the timely responses. I ended up going with a optical isolating circuit, since it was the easiest for our shop to put together. Basically it's a LED that is connected to our analog output module that is glued to a photocell. The photocell's resistance changes based on the LED brightness, and the LED brightness is changed based on the voltage output of our analog output module. Hence, a voltage controlled resistor! It's non-linear but that can be accounted for in the software side of things.

I like this solution since it requires no research on my part into transistor theory, costs about $2.50 to build, and does the job exactly as expected. Also, it does not need to be mounted onto a board, something that is a good selling point for my boss. The$2.50 build and about 15 minutes of tech time per unit costs us about 12.50 to make and we are selling them for \$175. Gotta love our free economy haha

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