Hi everyone ,
i will start a new project in which i need an adjustable current supply.
I mean there will be a constant 28vdc power supply and i will get current from it step by step.
what should i use accompanying arduino to have 1ma , 2ma ,3 ma up to 40 ma .
i will also need - values such as -5ma,-6ma,-7ma.
thank you.
Have you heard of Ohm's Law ? (E= I x R)=> I = E/R ?
28000 ohms = 28k ohms = 1mA
14000 ohms = 14 k ohms = 2mA
Do you see where this is going ?
It is not possible to have a constant voltage, constant current power supply. The device you are powering will consume some amount of current at a given voltage. The power supply can control either the voltage or current, but not both at the same time.
@jremington,
Bear with me. I am trying to get the OP to see that he needs a REGULATED 28V supply AND resistors to connect to it and
the current through each resistor to ground (switch by anything from a transistor to an analog switch) will be equal to the value
calculated by ohm's law. I was hoping the OP would EVENTUALLY see what you are pointing out , which is it.that he needs a REGULATED SUPPLY .
Simple to do, get one of these:
Feed it a regulated 35 volts DC, with no load adjust the voltage pot for 28 vdc output. Then adjust the current limit pot for the current limiting value you wish to use. This regulator will reduce the output voltage to whatever value it has to to keep the current supplied to the load constant.
I don't think his professor is going to accept that as a solution for the assignment.
@raschemmel: my comment wasn't directed at you, but at the OP. It seemed to me that he/she was asking for the impossible, a fixed 28V supply that delivers only certain fixed currents I should have phrased my comment better.
Thanks for your replies. I think i should make something clear to be understood.
Firstly we have already used a pot to get 1 , 2 or 3 ma by means of turning it and changing the probes of power supply for - currents.
But i want to make it programmatically controlled. therefore i need to use arduino with any board(i dont know which one or is it possible) to adjust the current with 28vd power supply.
it is not a problem that if voltage lowers when getting (for instance) 20ma from it.
i meant its initial value is 28v . of course if we get current from the curcuit , voltage will decrease .
To me , i would use a step motor to control pot but it should have more accuracy. So do you know any board or something like shield which is compatible with arduino to get the precise current value ?
To sum up , i will write an application and i will write 5ma or - 5ma in a textbox . then serialcommunication will start and arduino will control the board and finally i will have the current value that written in the textbox .
i hope everything is clear , if not i will try to explain my aims to do so.
Thank you.
So you want a programmable, setable, constant current source.
You need some form of analogue output, filtered PWM or a real D/A converter.
This needs to be fed into an op amp, which is monitoring the circuit current and controlling the transistor or FET that is producing the constant current.
While not too complex it is not trivial either.
This will not do negitave currents, it needs to be way more complex to do that.
Is this an assignment or do you just want something that works?
Your not going to be able to control the current that precisely with just any pot. You would need a 1 Megaohm pot and even that would not work that well. The whole idea of using a pot driven by a stepper motor for this project is wrong. First of all , you need
a regulated 28V supply . If you get three 12V batteries to make 36V you can get a buck dc to dc converter that you can adjust to 28V and you'll have your regulated supply. Then you need to switch resistors to get your different currents.
Then you need to switch resistors to get your different currents.
No he wants to programmatically control the size of current produced by a constant current generator.
Nothing to do with switching resistors.
For a constant current supply project see this:-
Look at the first diagram. This has a pot that is adjusting the current by changing the voltage on pin 2 of the op amp.
What you need to do is to remove the pot and get your programmable variable voltage onto this pin.
Note it will have to go up to the 35V (or whatever ) supply. The simplest way to do that is to put a PWM signal through a transistor pulling up to this voltage. Then filter it with an RC filter before applying it to this pin.
However with only 8 bits from a PWM output the resolution might be too coarse for your needs.
No he wants to programmatically control the size of current produced by a constant current generator.
Nothing to do with switching resistors.
Have you heard of Ohm's Law ? (E= I x R)=> I = E/R ?
28000 ohms = 28k ohms = 1mA
14000 ohms = 14 k ohms = 2mA
[Regulated Supply]
Resistors all connected at the supply end
Output Term------> < ----[load]+
GND---------------> <-----[load] -
[Transistors or Analog switches to switch any specific resistor in] (COPLETES PATH TO GND FOR ANY RESISTOR)
GND
Turning on R1 ===> EXACTLY 1 mA
This may be more work than the OP wants to do but it is certainly a programmable power supply , whether you like the approach or not. That being said, the circuit you linked looked a good way to go with this, and certainly less work.
@OP,
Here is something to measure the current with for your stepper motor scheme
https://www.sparkfun.com/products/12040
We really have one hand tied behind our back without a DAC. Why don't you get one ?
Turning on R1 ===> EXACTLY 1 mA
This may be more work than the OP wants to do but it is certainly a programmable power supply
Sure it is a programmable power supply but all it can possibly do is to supply it's own built in resistor.
Think about it - a bit pointless is it not?
Sure it is a programmable power supply but all it can possibly do is to supply it's own built in resistor.
It's obvious from my previous post that the intent was a resistor bank supplying all the required currents up to 40mA so it is not pointless, just not elegant and a lot of work.It would definitely supply the currents specified but it is not the best way to do it and as I said, the circuit you linked is a smarter approach. Resistor networks and banks have been used in many applications butan op amp design is a much more elegant approach to the problem. It' s too bad the OP doesn't have a DAC. The biggest problem with that idea is the resistor values are problematic and as you progress through the required current values it becomes unworkable in that it is impossible to obtain the correct values. So , in short, it is not doable and therefore it was a bad idea. I retract my suggestion.
@OP,
You will need to do micro-stepping. Can you post the datasheet and vendor link for your stepper motor driver and stepper motor ?