A biphasic power source for use in an iontophoretic system, which system includes a transdermal drug delivery device attachable to a patient and having a positive electrode and a negative electrode, includes a constant current source and a voltage limiting circuit. The constant current source has two outputs and provides a constant current on the outputs. The outputs are connected to the positive and negative electrodes of the transdermal drug delivery device. The voltage limiting circuit is connected in parallel with the outputs of the constant current source to limit the voltage across the electrodes to a predetermined voltage. The voltage limiting circuit may be a zener diode.