Current control for electrotransport drug delivery

An electrotransport drug delivery system having a constant current supply, the system comprising:

• a power source;
• a first patient contact connected to the power source;
• a second patient contact connected to a transistor;
• a current control feedback circuit for providing a control signal to the transistor when the connection between the first patient contact and the second patient contact is closed;
• wherein the transistor is connected to the second patient contact;
• and
• a sensing circuit configured to measure a voltage applied at the transistor when the connection is closed, wherein the sensing circuit is configured to provide feedback controlling power at the first patient contact;
• wherein the second patient contact is connected to the current control feedback circuit and sensing circuit only though the transistor.

A method for operating an electrotransport drug delivery system including a constant current supply, the method comprising:

• contacting a patient's skin with an anode and cathode to form a connection between the anode and cathode;
• applying an anode voltage to the anode;
• providing a control signal to a transistor connected to the cathode;
• detecting a voltage at the transistor with a sensing circuit that is configured to control power at the anode, wherein the cathode is isolated from the voltage detection by the transistor;
• comparing the transistor voltage to a threshold voltage;
• and
• controlling the anode voltage applied to the anode based on the comparison between the transistor voltage and the threshold voltage.