# Which Resistor for 9V Battery and 2ma Current?

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#### NathanielZhu

Joined Dec 5, 2011
50
Hey, so I'm trying to make a simple device that will look similar to this:
http://oi42.tinypic.com/wgyl8i.jpg It's just a picture I drew up.

My Main question is for choosing the right Resistor.
My goal is to reduce the current of the 9V battery to 2mA by the time it reaches the sponge (see picture) and pretend all the wires are insulated copper wires.

What is the specs of the resistor I should get?
Also, do I need to put a resistor on both the wires to the cathode AND the anode or just one of them

#### MrChips

Joined Oct 2, 2009
29,236
Use Ohm's law.

#### T.Jackson

Joined Nov 22, 2011
328
What are you trying to do exactly?

#### Wendy

Joined Mar 24, 2008
23,276
Assuming a short, and using Ohm's Law,

R = V / I
R = 9 / 0.002
R = 4500Ω ≈ 4.7KΩ

The real current draw will be much less, since your sponge will also have resistance.

Joined Dec 26, 2010
2,148
The link in the first post refers to "transcranial DC stimulation" aka passing electric currents through the skull (to stimulate the brain???).

A picture shows wires terminating in what look like soldering iron tip cleaner sponges, possibly soaked in saline. Presumably these would be placed on the victim's head. This really does not sound like a good idea, in my view.

#### SgtWookie

Joined Jul 17, 2007
22,230
Since the resistance of the load will be an unknown, and will vary considerably, you would need an active constant current circuit to limit the current, rather than a fixed resistor.

#### tommydyhr

Joined Feb 3, 2009
39
2 mA wouldn't be too detrimental though. With the resistance through the skin and the skull, the current amounts to microamperes, or perhaps even just a few nanoamperes.

#### Audioguru

Joined Dec 20, 2007
11,248
Why would anybody want to put DC current into their brain??
EDIT: OK, I read Wikipedia about how it is used to help brain disorders, but it might trigger a seisure. ZAP!

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#### tommydyhr

Joined Feb 3, 2009
39

#### joeyd999

Joined Jun 6, 2011
4,974
This is one of those topics I'd be wary of commenting on. Someone is going to try this at home with 120 VAC mains when they don't get the results they expect with a 9V battery...

#### bertus

Joined Apr 5, 2008
22,219
Hello,

I am closing this thread as it violates AAC policy and/or safety issues.

Quote:
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• Any kind of over-unity devices and systems
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• Devices designed to electrocute or shock another person
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• Transformer-less power supplies
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