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I need a device that can run on batteries (probably one or two 9V batteries) that will provide an output voltage of about 9V but, more importantly, is constant current of 2mA. This will be dc output.

Yes, that's a low current.

The output will go to electrodes which then go on a head. This means the load resistance isn't really known. Resistance of human skin varies between people and time of day. It's quite a large variation, between 100kohms and 1kohm.

What words do I need to include in a search to find a suitable circuit? Or what ICs will be suitable as a constant current source for 2mA? Or does anyone have a handy circuit diagram?

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You can't have constant current and specify a voltage. What you can do is specify a constant current and then apply a voltage limit at which the current will change once you hit that voltage. Also, by saying the range is 100kohms to 1kohm you are saying to voltage will range from 20 Volts to 2,000 Volts. So the specs you have given are not physically possible. I suggest you figure out what you are doing before connect your device to a human. –  Kellenjb Mar 9 '12 at 16:35
    
... what in the ... –  vicatcu Mar 9 '12 at 16:58
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what are you trying to achieve exactly? –  vicatcu Mar 9 '12 at 17:05
    
Are you sure it's 2 mA and 9 V? That's barely 18 mW of power, something a microcontroller consumes in sleep mode... –  Count Zero Mar 9 '12 at 17:12
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@CountZero the I needs to be close to 2mA. More than 1.5mA and less than 3mA. The voltage is, I think, less important, but people seem to say 9V. Certainly varying the voltage (within safe ranges) is preferable to varying current. The device needs to be battery powered, so that component failure is not dangerous. (Because without stepup transformers or diode capacitor ladders etc you're not getting enough voltage and current to cause harm.) 18mW is low power, but this if for human brain, where EEG signals are in the order of uV measured at scalp and 20mV with sub-dural electrodes. –  martin johanssen Mar 9 '12 at 18:53
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Be aware that you could badly damage or kill your subject and that this should only be done with suitable ethical, moral and legal oversight.

There circuits velow from question (not answer) here give a starting point BUT for something involving anatomical subjects, something with a bit more control of safety would be very very very advised.

The circuits are ground (negative) referenced but equivalent "high side" circuits can be created if desired. The control voltage here is shown as PWM, which would need to be filtered if it was used, but a DC input voltage would be even better.

enter image description here

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What constitutes suitable moral oversight, and how does it differ from ethical oversight? –  Nick Johnson May 25 '12 at 0:27
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That doesn't make sense!

As others have said, you can't have a constant voltage and constant current with varying resistances. That's such a basic mistake that it casts doubt on your technical ability and clouds the rest of the discussion. This short post does not count as 'due diligence'; you're messing with dangerous stuff here and need to know what you're doing.

What are you doing?

With that said, what are you doing? 1.5-3mA through cranial electrodes doesn't do anything useful or interesting that I'm aware of. There aren't many sensory neurons or muscles on the scalp, and there are far easier and safer places to test reflexes. The skull and cerebrospinal fluid are conductive; you won't get any current into the brain this way.

Here's some recommended reading

I suggest that you thoroughly read and make sure you understand section IV, Electric and Magnetic Stimulation of Neural Tissue, in the free online Bioelectromagnetism book at http://www.bem.fi/book/ by Malmivuo and Plonsey. I also suggest reading the rest of that book. Don't ignore the references cited at the bottom of each chapter - If you want to build a device for this, you should be very thorough in your research and look up some or all of the references.

And here's what you really need to do:

Finally, I'd like to posit that you might not have the skill required for this project. You're dealing with human lives; a mistake on your part could kill someone. You don't want to risk that. The wiser move would be to purchase a system for this purpose designed by people who know what they are doing. I've used ADI Instruments BioLab data aquisition modules before; they include a stimulator module with which you can apply electrical stimuli of various currents and durations. I've applied electrical stimuli with this system to my own body, and I'm still typing this. Had I done the same experiments with low-impedance electrodes, a 9V battery, and a tiny little wiring or circuit design error, I might not be typing right now. Be careful.

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I'd be interested to see any circuit that is capable of killing someone with 2mA. Or any reports of death caused by < 2mA, especially at about 9v. –  martin johanssen Mar 13 '12 at 11:44
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I think his point is not that you can kill someone with 2mA,but that the person doing this is not knowledgeable enough to design this safely. 2mA may not kill you if applied externally, but a fault in a circuit that may apply more voltage/current could cause burns or other injury. Just because he expects 2mA doesn't mean his circuit will always work they way he wants. –  lyndon Apr 21 '12 at 16:59
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@martinjohanssen, I agree with Lydon here, If the user does not realize a varying load requires either Voltage or Current to vary it represents such a fundamental error that I would not be willing to let them hook electronics to my body without auditing the circuit myself which the general populace cannot do. –  Kortuk Apr 21 '12 at 22:06
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You know,

V = R * I

If you have a fixed V (9volts) and a fixed I (2mA), you just can have a fixed R.

If your R is a human skin, head, whatever, that ranges from 1k to 100k ohms, then you can't have a fixed I or V, one of them will have to be variable.

To get a constant I, you'll have a circuity that automatically increases or decreases V, according to the resistance R.

And take a great care when doing that on a live person, animal, etc... It's not that simple and easy, you might hurt someone.

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"Electrodes to go on a head" -- is this trans-cranial DC stimulation?

(http://flowstateengaged.com/img/kickstarter-images/public-schematic.pdf)

Go careful. IT'S YOUR BRAIN.

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That looks like an interesting schematic.... although the big capacitor really defeats the purpose of having a constant current source. I wouldn't count on that fuse to do anything fast either.... –  W5VO Oct 20 '12 at 2:30
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