A JFET (junction field effect transistor) conducts between the source and drain contacts without an applied voltage at the gate contact. It behaves like an ohmic resistance until the pinch-off voltage is reached. For the transistor installed here, this is approx. 2.5V. The current flow between drain and source remains constant once they are reached. A voltage at the gate contact that is negative to the source reduces the pinch-off voltage. This allows variable setting of the current flow limitation.