4~20 mA current generator

[rinchan6]

Hi guys. Our Instructor gave us a project he called "Simulator", and it is a Power Supply he said. Features is it can output 4mA to 20mA digitally ranging 10 V - 30 V. I asked him what range of impedance are we supplying and he said we will have a variable load to supply.

With limited knowledge about the project, we can't surely finish this project. I need help identifying this type of power supply and maybe a schematic diagram, and we will try to build it from scratch with a little redesign. Thanks.

 

[KrisBlueNZ]

This sounds like a homework project so I have moved the thread to the Electronics Homework Help section. I have also renamed it.

4~20 mA current loops are used in control systems to transfer analogue quantities such a temperature, pressure, current, voltage, etc from transducers to controllers, and to transfer analogue control quantities from controllers to output devices such as motor controllers.

So you need to create a 4~20 mA current source that can drive a load with a voltage drop from 10~30V.

If it's a resistive load, at 0 mA there will be 0V across the load, so the real output voltage range (called the compliance voltage range of the current source) is 0~30V.

There are various circuits that will convert a voltage into a current output. Have a Google around to find them. You can feed a voltage in the range 1~5V into a current regulator using a 250Ω shunt resistor to make a 4~20 mA current generator.

What will your circuit be powered from? If it's powered from a fully isolated supply such as a battery pack, you can insert the current controlling circuit in either the positive or the negative side; if the supply is not isolated from the load, you may have to insert the current controlling circuit in the positive side, so the load connects between your output and the 0V rail.

So this current needs to be controlled digitally, so you will use a microcontroller, right? How do you plan to generate the control voltage? DAC? PWM? Some other way?

Do you have any other background information?

 

[rinchan6]

Honestly I have no idea or any other information to add but microcontroller and PWM might do since we're told to design one. About the load, our instructor insisted that the load will be anything and nothing in particular. I will do some study about current regulators so I could have some feedback.

Mind if I ask for a simple diagram? a block diagram would really fill me up.



EDIT 1:
found this on google. http://sub.allaboutcircuits.com/images/03039.png

So basically, this can be it but a few question, with any load impedance, is the output current still lie between 4 to 20 mA?

EDIT 2:
I did some simulations, and found out any load will still have 4-20mA

but, the output voltage is ranging 1 - 5 volts, i tried to change the input voltage range and the shunt resistor value, but did not get the 4 to 20mA range.

What im thinking is the current range will be (input voltage range)/(shunt resistance), is that correct?

EDIT 3:
after checking my simulation, i forgot to increase op-amp supply above 30 so i've got it working now.
my shunt reistance is now 1.5k to generate 20mA with a 30V input.
if this is not controlled digitally, this is all we need right?

Suppose that I would not use microcontroller, what are the other options?

 

[KrisBlueNZ]

I don't think your instructor should be asking you to design a circuit if you haven't learned how!

That circuit is not what I was thinking of, but since your instructor was so vague, he should accept that circuit as an answer.

You're right that that circuit can't drive 4~20 mA through any load resistance; that's just because it has a limited output voltage range. This is a real-world limitation on any current sourcing circuit. For example, to push 20 mA through a 100 kΩ resistor load, you need to apply 2000V across it! This is the reason why your instructor specified a maximum output voltage of 30V.

Yes, that's right. With a resistive load, the voltage across the load for a given output current can be calculated using Ohm's Law.

If you don't want to use a microcontroller, you would have to use a parallel-input, voltage-output DAC (digital-to-analogue converter). A parallel-input DAC takes a digital input in the form of a number of digital inputs - from 6 to 24, usually - and converts the value they represent into an analogue output within a given range. Your instructor didn't specify the resolution you need, so an 8-bit DAC should be enough.

Since your instructor just told you that the output current had to be "digitally controlled", you can probably just use a DAC. Voltage-output DACs need a reference voltage, and the output voltage range from 0V to the reference voltage depending on the digital input value.

Here's a Digi-Key product filter to suggest some suitable DACs. http://www.digikey.com/product-sear...nSort=1000011&stock=0&quantity=1&pageSize=500

 

[rinchan6]

Does DACs need to be programmed? I see alot of programmable outputs. I'm not familiar which DAC is common but I surely cant find an 8bit DAC with just a single output.

So resolutions are set by programming the DAC? I wanted a resolution of 1.5v and an offset output voltage of 6v to have a current range of 4mA to 20mA.

I have learned how to make a DAC using op-amps and i'm quite satisfied, should i still use single package DACs?

 

[KrisBlueNZ]

No, DACs don't need to be programmed. You provide a reference voltage, and a digital representation of the output voltage you want, and it produces a voltage that you can feed to your voltage-to-current converter circuit.

You can't find an 8-bit DAC with a single output? Did you look at the link I posted? There are lots of them!

You don't want a resolution of 1.5V. Do you know what resolution means? You can add an offset voltage with an external circuit using an op-amp, or if the DAC has two reference voltage inputs (one for top of scale and one for bottom of scale), you can offset the bottom-of-scale value that way.

You can make your DAC using op-amps and resistors if you want. It's up to you.

You should be able to figure out the remaining details of this project with a bit of thought, if you look at your notes, and use Google and Wikipedia.

 

[rinchan6]

What i meant is i cant find DAC in our local sellers, like DAC0808, they said they never sell like one, so I assume its not a common DAC. Ordering online will be an option but will be the last.

Yes i really meant 1.5V step size/resolution. Our instructor stated he wanted 4, 5, 6....19,20, he doesnt want 4.5mA, 5.5mA. with 6 to 30 volt input and 1.5k shunt. Or maybe with a smaller resolution, i can just amplify the output using op-amp. Same thought

The project design is almost done with an op-amp dac though, Thank you so much.