A plasma regulator or an EPS regulator was a component used to regulate plasma in the electro-plasma system of a starship or starbase.
On a Constitution-class starship, plasma regulators were crucial to the functioning of the warp drive. When the USS Defiant arrived in the 22nd century mirror universe, it was sabotaged by the removal of two of its plasma regulators. (ENT: "In a Mirror, Darkly, Part II")
Around stardate 50929.4, O'Brien had to recalibrate a shipment of EPS regulators to prevent them from interfering with Deep Space 9's artificial gravity grid. (DS9: "In the Cards")