This study will investigate whether a new medical technology can help reduce post-operative pain in patients undergoing total knee or total hip replacements. The new technology is called Transcranial Direct Current Stimulation (tDCS) and uses a very small amount of electricity to temporarily stimulate specific areas of the brain. The electrical current passes through the skin, scale, hair, and skull and can temporarily increase or decrease activity in the areas of the brain that are thought to be involved in pain reduction. The tDCS sessions last 20 minutes, and patients will be asked to report pain ratings. This study in specific hopes to determine optimal tDCS dosing parameters before tDCS will become a viable clinical intervention for post-operative pain across surgical specialties.