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I have a school project where I have to test a sensor and then measure the output with a multimeter.

I'm using a LM35DZ piece which gives as output itself a linear voltage of 10mV/C degree.

However, one requirement of this project is to have at the very ending a current between 4-20 mA.

The temperature in my hometown is about 25 degrees, so I'm willing to set my 0% (4mA) as 20 degrees, while 100 degrees would be my 20mA output. I've seen a couple of solutions involving transistors and potentiometers.

My rough set is receiving 5V at +Vs' leg of the LM35, GND grounded and the Vout goes directly to non-inverting input of a u741 operational amplifier, while I have a classical non-inverting amplification configuration.

The big deal here is I am struggling to transform this 0.2V into 4mA and further the 1V voltage into 20mA.

Any ideas/explanation attached (as I am also writing a report about it) of how to do it?

Appreaciate the attention and hope to hear something soon!

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  • \$\begingroup\$ Have you got a better IC than a u741!? They are terrible! \$\endgroup\$
    – KyranF
    Commented Apr 1, 2015 at 0:16
  • \$\begingroup\$ Have you tried any of the 4-20mA current ouput circuits available online? \$\endgroup\$
    – KyranF
    Commented Apr 1, 2015 at 0:16
  • \$\begingroup\$ it's pretty simple stuff,check here allaboutcircuits.com/vol_3/chpt_8/7.html \$\endgroup\$
    – KyranF
    Commented Apr 1, 2015 at 0:18
  • \$\begingroup\$ Why are some schools spending their time assigning projects using 45+ year old technology? (µA741 - 1968, 4-20 ma current loop - 1950's) \$\endgroup\$
    – tcrosley
    Commented Apr 1, 2015 at 0:43
  • \$\begingroup\$ Sex is around since the dawn of man, and it appears not to have gone out of fashion. 4-20 mA loops are still widely used in the industry, despite all the fancy new buses and protocols available. Mostly for legacy, but also for robustness. And you can add a Hart to it, if you want. Sadly the 741 is way way out of fashion, but it can be useful to use a piece of junk (with all due respect for Fullagar and - in a way - Widlar) to study on. Its limitations are a good school after all. \$\endgroup\$
    – Sredni Vashtar
    Commented Jul 14, 2016 at 1:20

1 Answer 1

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An op-amp driving the gate of a MOSFET with feedback from a resistor connected between the MOSFET Source and ground is the basis for a voltage-to-current converter. If the source resistor is 1k ohm, drain current 4-20 mA corresponds to 4-20 volts at the source (the gate takes no current). When your temperature sensor delivers a 75mV swing over the range of interest, you need the op-amp circuit to provide 21.3x inverting gain. Standard resistor values 10kohm (sensor to op-amp -ve input) and 220kohm (drain to op-amp -ve input) get you close. I leave as an exercise adding potentiometers for trimming exact gain and offset, and you need to look into minimising the input current offset error of the u741. By all means embrace the old u741 for school work because it is unconditionally stable in simple feedback circuits like this.

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  • \$\begingroup\$ I can follow the description, but to make it easier to understand and more useful in future you should draw a quick circuit with the tool provided. \$\endgroup\$
    – tomnexus
    Commented Apr 1, 2015 at 9:07

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