Is this a safe way to add extra current control for a DIY tDCs device?
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 &ndash; Schematic created using CircuitLab