**0-1V to 4-20 mA Converter**

Ensure +5/-5 dual supply for chip TL062 IC3. Gnd is common

ps ground, let grounds radiate from ground plane in one

side of PCB. R3-R8 is an attenuator that may need to be

designed or modified.

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**1-5 V to 4-20 mA Circuit**

The input voltage to this circuit is assumed to be coming from

some type of physical transducer/amplifier arrangement,

calibrated to produce 1 volt at 0 percent of physical measurement,

and 5 volts at 100 percent of physical measurement. The

standard analog current signal range is 4 mA to 20 mA,

signifying 0% to 100% of measurement range, respectively. At 5

volts input, the 250 Ω (precision) resistor will have 5 volts applied

across it, resulting in 20 mA of current in the large loop circuit

(with Rload). It does not matter what resistance value Rload is, or

how much wire resistance is present in that large loop, so long as

the op-amp has a high enough power supply voltage to output the

voltage necessary to get 20 mA flowing through Rload. The 250 Ω

resistor establishes the relationship between input voltage and

output current, in this case creating the equivalence of 1-5 V in /

4-20 mA out. If we were converting the 1-5 volt input signal to a

10-50 mA output signal (an older, obsolete instrumentation

standard for industry), we'd use a 100 Ω precision resistor instead.

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**Howland Current Source for Grounded Load**

Тhis genius idea is implemented in the famous Howland current

source. In this clever circuit, the excitation voltage V and the

resistor R form a basic current source. It produces a current

I = V/R - VL/R, which passes from the left hand side through the

load RL.

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**Constant-current circuits**

Constant-current circuits are usually implemented with an op

amp and a discrete external transistor.

Circuit E is a current source, which requires close matching of

the R2-R3 and R4-R5 resistor pairs to ensure insensitivity to

changes in the supply voltage.

Circuit (E) Iout = Vin/Rload

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is needed, which is controlled by a computer and referenced to

circuit ground. The circuit below converts a zero to 5v

signal from a computer’s analog output into a current, with

a full scale of 100ma. The circuit shown requires a 9v DC

supply but any voltage from 9v to 12v will work.

the R2-R3 and R4-R5 resistor pairs to ensure insensitivity to

changes in the supply voltage.

Circuit (E) Iout = Vin/Rload

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**Computer Controlled 100ma Current Source**

Often in industrial control systems a constant current sourceis needed, which is controlled by a computer and referenced to

circuit ground. The circuit below converts a zero to 5v

signal from a computer’s analog output into a current, with

a full scale of 100ma. The circuit shown requires a 9v DC

supply but any voltage from 9v to 12v will work.

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**Constant current source circuit**

We can use a voltage reference turn into a Constant current

source circuit and use pnp-transistors for current-boosting