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Sunday, January 17, 2016

Voltage to Current Source 4-20 mA

Here is a circuit that is a voltage to current converter but with a current source. That means it supplies a current from a positive terminal and can drive a remote shunt in a Measuring Instrument. The other end of the shunt returns to gnd or low.

More than one instrument can be connected in series if the the sum of the voltage drops in the instrument shunts are much less than the voltage 12V of the current source.

The 0-1V to 4-20 mA Converter published earlier is a current sink,

Voltage to Current Source 4-20 mA

You can use a LM358 or LM324. The first opamp is a Voltage to Current with a sink output. That current creates a varying voltage w.r.t the 12V DC supply, this varying voltage is mirrored by the second opamp across the source output resistor. This way a constant current is obtained with a sourcing output. The control elements are small signal high gain transistors. Any suitable equivalent can be used. Even the opamp can be chosen by the precision and application you want.

Industrial Process Control Circuits

In this form of feedback. way to understand .... "Op-Amp drives the output to maintain both inputs at the same level" and also the "Output takes the polarity of the dominant input" and lastly "dominant means, more positive". +5 is more Dominant than +3 or 0 or -2. Then -3 is more dominant than -12. See which is more positive.

Long distance of current loop may need higher voltage and lower source resistor value. Then the output transistor needs to change, if you use 24V DC then that voltage should not reach opamp. Design needs to foresee all possibilities of I/O troubles, as these are wired by a customer, mistakes happen. Hence, Industrial Designs have to be rugged.