Which is safer and preferred: constant current or constant voltage?

• A. Constant voltage
• B. Both equally safe
• C. Neither is safe
• D. Constant current

Answer: (D)

The important idea is that safety in electrotherapy hinges on controlling the amount of current delivered to tissue. A constant current source keeps the same current through the body regardless of how the skin-electrode impedance changes. It automatically adjusts the voltage to maintain that fixed current, so the dose—the charge delivered per pulse—stays predictable. If impedance goes up, the device raises voltage to keep current at the set level; if impedance drops, it won’t suddenly push more current than intended. This limit on current helps prevent overheating, burns, or nerve irritation, making treatment safer and more reliable.

With constant voltage, the current varies with impedance. If contact improves or skin conditions lower resistance, a higher current can flow, increasing the risk of tissue damage. If impedance increases, current drops and the therapeutic effect can become inconsistent. Since safety and therapeutic effect are tightly linked to the actual current and charge delivered, constant current is the safer and preferred approach.