Arduino Programmable High Current Source

Other Hardware General Electronics
1

Hello,

I've bumped into this circuit problem a couple times in different projects. I want to create a constant current source (1-3 amps) that I can control from an Arduino. So I would write in code, output say 1.2 amps constant current, or output 2.4 amps. A value I can have change at runtime in the code.

Currently I have been having the arduino just turn on or off a 1 amp constant current source with a relay using the circuit below (the image is not my own), but what I really want is to be able to control what constant current I am setting.

I have heard of people using a FET as a voltage controlled (and thus arduino controlled) variable resistor in place of R1, but I have a feeling it would have to be huge to handle the current I want at out+. Also, I have used the pictured circuit in the past, but don't entirely understand how it works, and wouldn't know what sort of current is going through the ground or output pins in order to choose an appropriate FET.

Any suggestions?

Thank you,

Michael

2

For a current source I would use the LM317. It works the same as the LM7812, but the LM317 has a reference voltage of 1.25V, so the resistor can have a lower value.

To control a voltage with an Arduino is a lot easier, but up to 3 amps controlled with Arduino is not easy.
The most used way is a resistor to ground, and the voltage over the resistor is measured and the Arduino has a closed-loop code and drives a transistor.

It is allowed to have a resistor to ground ? If not, you need an high-side-current-sensing opamp.

3

Michael75:
I've bumped into this circuit problem a couple times in different projects. I want to create a constant current source (1-3 amps) that I can control from an Arduino. So I would write in code, output say 1.2 amps constant current, or output 2.4 amps. A value I can have change at runtime in the code.

Currently I have been having the arduino just turn on or off a 1 amp constant current source with a relay [...]

(Note, the 1-3-2 pin numbering in the illustrated circuit doesn't agree with the 1-2-3 numbering I've seen in regulator spec sheets.)

In constant current circuits like that illustrated, load current flows from the output pin through R1. The regulator senses the voltage dropped across R1 and raises or lowers output voltage so the drop is equal to V_ref, which is (in effect) 12 V for an LM7812 and is 1.25 V for an LM317, as Peter_n mentioned. With a 35 V max input voltage, the LM7812 also has less headroom anyway than the LM317 with a 40 V limit.

Using 1.25 V instead of 12 V across R1 produces only 10% as much heat in R1, and also reduces the input voltage that you need to supply by 10.75 V. As a concrete example, for 3 A output current, if R1 drops 12 V it's a 4 Ω, 36 W resistor. If R1 drops 1.25 V it's a 0.42 Ω, 4 W resistor.

For higher currents than the LM317's 1.5 A max, you might look at 3 A and 5 A regulators at Digikey and other suppliers. Eg, AZ1084CD-ADJTRG1 at Digikey.com is a 5 A regulator, 56¢ in singles. Note, its input voltage is limited to 12 V.

You could vary the output current several ways. You could use several relays and have several different resistor sizes to combine in parallel to make up R1.

Or, if you don't mind some experimentation and your app has no life-safety issues, it might be possible to put a current-sensing resistor (eg 0.1 Ω) on the ground side of your load, attach a PWM output to a low-pass filter (LPF) connected to the regulator's sense input, and use a feedback loop to get desired current output. The loop would read the current-sensing resistor's analog voltage; if it's too low, increase PWM setting, etc. This approach can work if your load is stable and resistive, and if setpoints change slowly. For faster-changing setpoints, DAC output rather than PWM+LPF would be needed. For dynamic loads, DAC output plus some control theory would be needed if instability is to be avoided.

Note, the PWM output might need to drive a voltage shifter (a transistor and resistor) so that voltage at the output of the LPF could exceed the 3.3 V or 5 V available at Arduino pins.

4

Hi Michael,

I am going to use this to White LED light. I think it will work. Actually I have 6 watt LED but I want 3 Watt Brightness. So I will use 48 ohm resistor for this to control the current as 0.25. What is your opinion?? Please help...

5

Controlling a resistor to control a current source is a contorted approach.

You simply output to a DAC (or low-pass filter PWM) driving a voltage controlled current source,
which is a simple application of an opamp with a pass-transistor to boost current output.
The insight is that you sense the output current via a shunt to compare with the programming
voltage.

Here's a thread about it: lm317 - Controlling high output LED's with a microcontroller - Electrical Engineering Stack Exchange

In general if you have a circuit that's programmed with a resistor, there's another
circuit for doing the same thing programmed by voltage (or current). Opamps are
versatile building blocks.

6

SERIES PASS REGULATOR WITH CURRENT SENSE