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Manchester (England England)
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Incidentally, it occurs to me I assumed the 4.7k resistor would run to infinite resistance if it fails. I did verify it before soldering and do periodically check it but it would be nice to know if I'm right on that. (!)
No a resistor can fail short as well as failing open.

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If you look up 'GoFlow'
I did, sounds like snake oil to me.
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Whoa. A good thing to know. Thanks.

This tDCS is for real. It's not going to cure AIDS or fill potholes but results have been reproduced. Right now the field is wide open -and all over the place. The (business) students at GoFlow are not be best example of the approach but are an indication of how much interest is out there. As I understand it, they started out as/still are a student project. Again, a project of a group of business students. "Snake oil" seems entirely fair.

I really only mentioned them in particular since they've made the news a lot lately and had just released a full schematic. I was double checking their notations and looking for part sources when Google turned up this thread. (I don't know much but their 'schematic' in the Make post about them was pretty stupid.)
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The point is that any voltage sent into the head via contact pads is mainly going to get a current go through the skin from one contact to another. The brain is encased in a skull, which is made of bone which is not a very good conductor. So there is little chance of passing any current actually through the brain with this method.
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We had a saying when we were apprentices  -  I'm sure it still applies !!

"It's the volts that jolts   -  but the 'mills that kills "   
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I hear You guys. Believe me I watched progress for years before putting my noggin on the block. The highest current anyone has studied seems to be 2ma. The skull does indeed block most of the current but that tiny remainder is what does he job. Some electrode 'montages' put the contalatteral reference electrode off the skull entirely, on the arm or shoulder for instance. Their was concern this may run potentially deadly currents through the brain stem though subsequent research has shown otherwise.

I knew 7ma could stop a heart going into this, I never entertained the slightest thought of placing electrodes acros my heart...and as I said I let a number of studies pile up for years before believing it can be done on the head safely. This new info about what I thought was a safe assumption does put some runs in my safety net.

I suppose a pair of resisitors in parrellel would be best/simplest for a newb like me though clearly not ideal? It does seem odd that the safe tolerences are that wide considering what's being worked on.... I'll just have to keep checking resistance before each use until I can handle the real fixes. Fact is I probably would regardless. Funny. How many of us do a 'preflight' on our cars before every drive?
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I suppose a pair of resisitors in parrellel would be best
How come? If a resistor can fail open or short parallel is not going to help, seriese may be. But if you know that little about it then you should not be doing it.
The problem is you want so much for it to be true it will be no matter what physically happens. Placebo.
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Placebo? If that was all it took I'd be all for it. Much easier. You're right though, I can't be all that objective but that's what I'm using the published papers for. Obviously I'm not running a randomized double-blind study, I'm just following a given protocol and seeing what effect if any I notice. I've been both disappointed and also very surprised at the effects at other times.

There's nothing wrong with series, though if one resistor fails open everything stops. (Kind of the point I know but inconvenient.) If in parallel one fails short the total impedance is still the average of the two, right? So twice as much current flows rather than thousands of times as much. Since the "safe" current range presently extends more than two orders of magnitude from the minimum useful value a factor of two increase isn't much. Especially as it only has to be for the remainder of that session given I check impedance before each use. If wired in parallel I won't discover a problem to fix and find out I wasted all or part of the last session at the same time. See? No real change but much less frustrating if one does fail. I could even do that next run as planned and allow the time to fix it properly later. Resistor values need to be different in parallel vs series, of course.

I guess even one failed resistor is pretty rare if used within it's rating so while it's worth planning for I'm not sure it's worth fixing if a different arrangement allows an extra layer of redundancy with a safety margin maintained. Like maybe two serial pairs of resistors wired in parallel...so if one fails in either leg resistance is constant. Heck even if one failed short in each leg impedance would remain constant. Even one short in each leg with a third failed open would still match spec while both of one leg shorted or even that and a third from the other is not ideal but tolerable. According to present estimates at least.

Any more thought into it and it's just going to be a fused regulated circuit. It's going in that direction certainly.

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If it shorts it will still cause excessive current flow, regardless of the other one
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There's nothing wrong with series, though if one resistor fails open everything stops. (Kind of the point I know but inconvenient.) If in parallel one fails short the total impedance is still the average of the two, right? So twice as much current flows rather than thousands of times as much. Since the "safe" current range presently extends more than two orders of magnitude from the minimum useful value a factor of two increase isn't much.


Total parallel resistance is 1/( (1/R1) + (1/R2) + ... +(1/Rn) ), not the average.  With two 9k4 resistors(for 4k7 total), one shorting to .001Ω(I'm not sure if that's an accurate estimate, but I'll use it anyway) would result in a total of 0.0009999998936Ω.  That'll be a [sarcasm]little[/sarcasm] more than twice the current
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