Is this a safe way to add extra current control for a DIY tDCs device?

Question

I have put together the well known DIY tDCs-device Inthinkerator MK1, and wanted to add an extra safety feature, but I dont know if this is the correct way to do it. The original design is without the 2N4393, but I've added it.

tDCs stands for transcranial Direct Current stimulation. The concept is nothing new, but has gained alot of attention lately. Alot of research is being done, and there is alot to read about it. https://www.ncbi.nlm.nih.gov/pubmed/?term=tdcs is one source.

(You can also search for snake oil, but it does'nt seem to have the same medical application potential... https://www.ncbi.nlm.nih.gov/pubmed/?term=snake+oil)

My plan is to set a max current on the 2N4393, so if something went wrong with the original setup with 2N3904, it still would'nt go over set current limit. That's my plan at least.

By setting the trimpot at 2N4393 to never let it put thru more then 2,5mA, and then use the 5k pot to 2N3904 to be able to change the output from 0,36 to 2,0mA.

Can someone say if this is a bad idea? The simulation in Qucs "says" it will work as intended.

Would appreciate all help!

Answer 1

I built one of these using an LM334. One of the things I discovered while doing these experiments is that even with a "raw" 9V from a battery shorting direct to the electrodes the current barely exceeded 2mA through the head (with large area saline electrodes). So anything powered from just a battery (NOT mains PSU) is reasonably safe.

Answer 2

It will work, but not the way you think. What you have is a constant current source (J2N4393) driving a constant current sink (2N3904).

Since each one is trying to 'fix' the current to be a specific level, the one with the lowest current setting will dominate the flow of current.

For the 2N3904 to work properly, the 2N4393 MUST supply a bit more current than the 2N3904 needs. The voltage at the collector of the 2N3904 would be close to 9 volts. If the source current is a tad too low, the 2N3493 will dominate and output very little voltage.

Ideally the 2N4393 should be set up as a current limiter, instead of a constant current source.

One option is to lower the value of the 400 ohm resistor, and remove the 680 ohm resistor.

The following current limiter works from 1 mA to 100mA.

^{simulate this circuit – Schematic created using CircuitLab}