# How do I get a constant current of 2mA; output voltage of 9V dc?

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 are you trying to achieve exactly? – vicatcu Mar 9 '12 at 17:05
@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
Why are you trying to put 2mA through someone's head? Most people would notice 2mA as rather unpleasant. Just 10mA is a violent shock. This is not a good way to measure brain waves. – JustJeff Apr 22 '12 at 2:04
I wonder who voted your comment up... Based on the 2 ma rating, I assume you are looking at building your own trans-cranial direct current stimulator (tDCS)? Your belief that it is safe because you'll be using batteries only is quite incorrect. You need to have really good current limiting circuitry, as well as safety protocols for placement, current (2 ma is the MAX recommended), and duration in addition to a battle hardened, medical grade circuit design. – Jarrod Christman Dec 21 '14 at 2:49

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.

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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

### 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.

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
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
@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
I just bought two death dealing devices in my local Argos- these lethal contraptions are on sale even to children and the mentally enfeebled, under the innocent sounding name of "9 volt pp3 batteries". The live terminals, which are maintained at a constant voltage of more than 9 volts (I just tested it, using suitable safety procedures including wearing rubber from head to toe) at well above 2 milliamps. When will somebody do something about these ubiquitous death boxes? – Ian Bland Apr 20 at 22:56

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)

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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

The following is a standard simple discrete constant current source:

R1 would be the subject's head. I simulated this with 4k, 1k, 500, 100 ohms and the current was always almost precisely 2mA.

If you want better precision a JFET and an LED makes a great precision voltage reference.

Note that you can absolutely kill somebody with 2mA or with 9V. A 500V 2mA shock would stop your heart. A 9V 10A shock would make you start to smoke.

And believe it or not you actually can produce high voltages and high current with just a 9V battery.

If you take a 9V battery and touch the terminals across a biggish inductor (those big metal coils in power supplies) and then watch carefully as you pull it away, you can see a spark. I'm not sure what voltage you can get from that sort of hack but it's a lot more than 9V.

If you take a 9V battery and connect it to a thin enough piece of wire, it can get red-hot which in the wrong situation might cause a fire.

So you might want to get a better understanding of the relationship between current and voltage before you hook yourself up to a DIY brain stimulator. The circuit above would not generate 9V across R1. It will generate R1 * 0.002 volts because V = R * I.

But if you connected the collector of Q1 in the circuit to your head and then attached ground to another part of your body, it would produce at most 2mA as desired.

Note that the other circuits posted are current sinks which is different but could achieve the same effect depending on how you connect the "load".

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Yes, you can build one, and it is not complicated. Yes, you use batteries. I think it was a bit unfair to pick on your question. You should have asked, how do I build a tDCS device?

But first, the disclaimer and caution.

Disclaimer and Caution: Electrical devices can kill you. If you build a 2ma device, rather than buy one, you risk death.

OK, now that you understand the risk, here is info that you need to get started.

If you are going to build your own device, begin by doing a lot of research, and make sure that you understand what you are doing. Your device must be able to measure your output to ensure that your current is no more than 2ma.

It is simple to build a 2ma output device. The device is referred to as a transcranial direct current stimulation (tDCS) device. Go To for a design: http://speakwisdom.wordpress.com/2013/04/02/a-very-simple-current-regulated-tdcs-device/

Currently, 2ma devices are used commercially for a variety of reasons. The military is currently experimenting with such devices to improve cognitive performance in a number of areas. Commercially similar devices are sold commercially to stimulate muscle tone in the face and other parts of the body.

To start your research, go to Wikipedia: http://en.wikipedia.org/wiki/Electrotherapy_%28cosmetic%29

Good luck. Hopefully this info was helpful.

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## protected by Dave Tweed♦Apr 20 at 23:03

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