hi, im new here and to using the arduino (hasnt arrived yet)
id like to create a control circuit that takes a standard wall power supply (outputs 9v) and run it through a breadboard power supply ([like this one) and has two output terminals; one with +3v and one with -3v.
this is being used for a robotic arm that is currently run off of batteries that are set up in the -3v and +3v config.](http://www.sparkfun.com/commerce/product_info.php?products_id=114)
It will take a little more work to make a negative voltage (you need a voltage inverter) but I wonder if you could avoid it altogether.
Do you really need +3 V and -3 V? Or do you just need a 6 V difference to power the motors? Remember that “voltage” is all relative, so strictly speaking there is no such thing a -3 V in absolute terms: when one talks about “-3 V” it means there is a electric potential difference of 3 V between it and the chosen “ground” reference (the negative sign just indicates the direction of the voltage difference).
If you could provide more detail on how the robotic arm takes its power (and how the batteries are currently connected), it would be helpful.
thanks for the reply and information.
how the batteries are currently configured in the arm is in two rows. one row totals the +3v in one direction and the other row is in the opposite direction giving -3v, but as you say its relative. they are in series with a vin or vout and a gnd at either end respectively.
i believe the motors are driven forward by the +3v and in the reverse direction via the -3v according to a basic schematic of the control that came with the robot. (ill get pictures up of this when i get home)
i think for this particular case it would be best to split a 6v input into +/-3v using some type of [vground method. ie:
http://www.edaboard.com/files-eboard/vi … t_4651.png
but my experience cuts me short here as i am not totally sure what calculations need be done and if the given components in the diagram are the ones i should use for my application.
the robotic arm has 5 servo motors and a small led light that all have 2 pin connectors. one being v and the other being gnd im pretty sure, which makes me wonder about the voltage supply. could it be that the +3v and -3v are transferred through the same line resulting in two directions of movement?
i just got my arduino in the mail today so hopefully i can do some testing to get a feel for how the motors are controlled and if i can manipulate them via the arduino 3.3v line.](Virtual Ground Circuits)
I’ll be interested to hear more about how the arm power supply is wired and what you discover with your Arduino. Sounds like a fun project.
Couldn’t you just use 3v and a reversing relay or h-bridge to reverse polarity for the motor?
/mike
n1ist:
Couldn’t you just use 3v and a reversing relay or h-bridge to reverse polarity for the motor?/mike
Possibly, but what are those things? I’m just looking for whatever method is easiest to take a battery powered robot and convert it to wall power so it can be active at all times. It’s controlled by a program through a usb cable hooked up to a computer.
If you have full control over the motor and are going to be driving it directly, then the best thing to use is an [H-bridge (visit link for more info, or Google “H-bridge”). This is an arrangement of transistors (MOSFETs are best) that allows driving high currents to a motor with the special ability to switch the direction of the current. The H-bridge lets you control this high current, potentially higher voltage, load with regular microcontroller logic outputs.
There are H-bridges available as integrated circuits like the L298, or you can make your own with discrete MOSFETs. H-bridges made with discrete MOSFETs will be able to drive much higher currents than an L298.
SparkFun has several ready-made H-bridge options, including
Hope this helps!](http://www.sparkfun.com/commerce/product_info.php?products_id=9479)](http://www.sparkfun.com/commerce/product_info.php?products_id=9571)](http://www.sparkfun.com/commerce/product_info.php?products_id=9670)](Adafruit Motor/Stepper/Servo Shield for Arduino kit [v1.2] : ID 81 : Adafruit Industries, Unique & fun DIY electronics and kits)](http://www.sparkfun.com/commerce/product_info.php?products_id=9815)](Part 1 | Modular Circuits)
thanks for the info and i may very well use this at some point in the future to control the motors directly via an arduino board.
but at this point all i am looking to do is convert this battery powered robot into a wall powered robot. any specific insight on doing that?
i recently made a large order from sparkfun and should have all of the necessary components to build a custom circuit, but cannot find consistent and easily transferable information on making this conversion.
thanks again for following up.
You’re on the right track with your voltage splitter, but the 3904’s probably won’t handle the current. So that’s the first question: how much current does this robot need?
If the current draw is within the capability of an LM317 (about 1A IIRC), then that’s the simplest solution. Use two LM317 adjustable voltage regulators: one set for 3V and the other set for 6V (you can find out how to adjust the 317 by looking at online appnotes) running on a common power supply and ground.
The original ground will be the -3V supply, the output of the 3V LM317 will be the ground that goes to the robot, and the 6V one will be the +3V supply. Total cost should be around $3. Haven’t actually tested it, but I can’t see any reason why this won’t work: it’s the same concept as the circuit you showed above, but simpler and more accurate.
it is normally run off of 4 d-cell batteries (2 for the +3 and 2 for the -3). once the equipment i ordered comes in i can take some load measurements to get a better idea of how much current the motors/servos require. whats the most the 3904s will safely pass?
i actually ordered 3 adjustable voltage regulators also thinking i could do that. and some potentiometers/trimmers as well as a whole pack of resistors/capacitors and all that good stuff. so without a doubt i will be able to do it with what ill be getting, just a matter of finding the best solution.
how do i set it up to make the ground become the -3v? given the diagram, very professional a might add, how do you suggest placing the components?
i understand the logic no problem, but in terms of how to wire things up im lost as ive never actually done this myself.
Here’s a rough schematic showing how linear regulators could be used to take +9 V DC input and producing +3 V and +6 V, which along with the input power’s ground, provides three rails with 3 V between ground and +3 V and 3 V between +3 V and +6 V.
The [LM317 Voltage Calculator describes how to calculate the right resistor values.](LM317 Voltage Calculator | REUK.co.uk)
thank you so much, this is going to help immensely.
Nice diagram 
That’s exactly what I meant.
confirmed that motors can be controlled by +/- 3v signals.
going to start building that circuit and hooking things up. the adapter i have is actually +12vdc/1.25a but all that would mean is that ill need to change the resistor value for the regulators, right?
colinb:
Here’s a rough schematic showing how linear regulators could be used to take +9 V DC input and producing +3 V and +6 V, which along with the input power’s ground, provides three rails with 3 V between ground and +3 V and 3 V between +3 V and +6 V.[attachment=0]splitpower-example.png[/attachment]
The [LM317 Voltage Calculator describes how to calculate the right resistor values.[/quote]
I think I see a problem when the motor draws power between the “motor’s +3” and “motor’s neutral”. What is going to sink the current toward input ground? I assume the motor resistance is low compared to the resistors. For that 2-regulator kind of system, I think you would need a negative regulator for the U2 function. Its input would be input ground, but some other changes would be needed also.
I would not see a problem with the H system.](LM317 Voltage Calculator | REUK.co.uk)
Actually you don't want to change the resistors on the regulators. Those values are not dependent on the input voltage. The regulator simply converts the excess voltage to waste heat, so to speak, to achieve the set output voltage.Xenosis:
the adapter i have is actually +12vdc/1.25a but all that would mean is that ill need to change the resistor value for the regulators, right?
The only thing to be aware of (beside the regulator’s specified maximum input voltage) is the power dissipation that will occur (it increases rapidly as input voltage increases above the output voltage) and compared to the maximum power dissipation rating of the regulator.
analogon:
I think I see a problem when the motor draws power between the “motor’s +3” and “motor’s neutral”. What is going to sink the current toward input ground? I assume the motor resistance is low compared to the resistors. For that 2-regulator kind of system, I think you would need a negative regulator for the U2 function. Its input would be input ground, but some other changes would be needed also.I would not see a problem with the H system.
I’ll have to think about the issue you raise, and consider how circuit laws might support or disprove it. If by “H system” you mean an H-bridge, I agree, that is by far the best solution. It might be a tiny bit more work to set up, but if you’re going to all the work of setting up a couple of regulator circuits, an H-bridge IC wouldn’t be much more difficult to use and probably not significantly more costly either. I haven’t made a discrete H-bridge before so I don’t know about all the shoot-through possibilities of problems and whether some NAND gate logic before the MOSFETs is enough to made it safe. Anyway, if an H-bridge IC fulfills your maximum current needs, then it’s probably the best and simplest solution all around.
