Current generators (Part 1)

Part 1

Here starts a new series of post dedicated to current generators. As usual, the idea is to describe working experimental prototypes that feature few components and quite good performances.

Current generators can be found in various applications such as battery chargers (advanced ones, aka multiple stages or intelligent battery chargers), LED drivers and all sorts of industrial applications such as 4-20 mA current loops.

Let’s start from the simplest up to more refined designs. Tweaking the LM317 voltage regulator is probably the simplest design. It requires only one very common chip and one adjustable resistor along with a couple of capacitors if high stability is required (optional components marked with a *).

Simplicity has a price, and in this case, the circuit suffers from few drawbacks: current adjustment is not linear versus the potentiometer cursor position as I = (1.25 / R), consequently it is tricky to adjust. However, thanks to additional fixed resistors, the adjustment range may be constrained and ease the fine tuning of current .

Next is a plot of the output from a current regulator where the feed back resistor is made of an adjustable one (150 Ohm) in parallel with a fixed one (1500 Ohm) to which is added an other resistor in serial (120 Ohm). This configuration results in an almost linear output ranging from 5 to 10 mA.

A better idea would consist in designing a more elaborate – although still simple circuit. The next one features an operational amplifier, a specialized chip used for measuring the voltage across a shunt resistor plus a few extra passive components. An INA193 chip (U2) is inserted in the feed back loop of the op-amp so that the voltage setting from the potentiometer (P1) is compared directly to the output voltage of the INA193 which is itself directly proportional to the current flowing through the shunt resistor (R shunt). The operational amplifier (U1) shall comply to the following characteristics: single supply power supply voltage, rail to rail input and output and output current compatible with the required output current of the circuit. I personally choose the MCP6001/2/4 which delivers up to 23 mA  and which I used a lot in recent designs.

Building the circuit is very easy as shown from the next picture

Next is an illustration of a simple spread-sheet (Open-Office format) that I built (as in many other designs) to play around with values. You may download it from >here<. I use color conventions for highlighting values of interest: orange for input values, gray for clones and yellow for output values.

It is possible to achieve higher output currents thanks to an additional transistor as show in the next schematic. This time, the current drawn is only limited by the transistor specifications: in the present case, the 2n3904 can drive up to 200 mA. However, care shall be taken with power dissipation, as this plastic package dissipates only 1.5 W.

 

I you lack the INA193 chip, you may build a differential amplifier of your own. Next is a suggested schematics for such current regulator.

It features a differential amplifier built around U2 which gain is determined by the R1/R2 and R3/R4 ratio (assuming R1=R3 and R2=R4). In this way the differential amplifier outputs a voltage which is equal to: G * the voltage drop across R shunt, where G is the gain. In order to achieve the proper amplification, resistors shall be from the precision class, 1% or better. R5 is a voltage limiter thus limiting the current output. Next is an illustration of the components set up on a breadboard.

 

 

 

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