2014-10-14 (Tu) Adjustable tDCS

N-Channel JFET transistors were ordered. These transistors turn off when enough power is applied but freely conduct otherwise.The idea was that when too much current was sensed the circuit would shut itself down as a safety precaution.

Pin headers and the surface mount chip were soldered to a prototyping board so it could be used on a breadboard. A picture of the soldered chip was taken with a macro lens. The circuit was constructed so that current passing through an adjustable resistor would feed the gate and when enough power was present at the gate the transistor would trigger and stop the power flow. The resistor was adjustable with the hope of finding the 2mA point to keep amperage low. No point on the resistor would cause the circuit to open or cap the current.

 Transistor and part number

Transistor soldered in place

Transistor attached to breadboard

Potentiometer being adjusted

Research was done on the transistor (MMBF5485 PDF data sheet) and it was shown that the current required to trigger the transistor was higher than the 2mA available. The adjustable resistor was removed from the circuit and a direct connection was made to the gate. The safely potentiometer in the tDCS was readjusted to exceed the 2mA safety limit but there was no interruption done by the transistor. Other transistors were researched.

To do:

  • Compare to commercial unit
  • Update part list
    • Resistor 4k57 Ohm or 5K Ohm potentiometer

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