This paper presents the application of semi-active control for power harvesting using an electro-mechanical energy harvester. Two semi-active control strategies are proposed in the form of a time-periodic damper and a nonlinear quadratic damper. For the periodic time-varying damper the average harvested power and the transmissibility are obtained based on the Fourier series. The semi-active periodic time-varying damper is optimised to maximise the harvested power. The performance of the optimum semi-active periodic damper is compared with the optimum passive and semi-active on-off model at a particular frequency. It is demonstrated that the periodic time-varying damper can significantly increase the harvested power at all frequencies of interest. For the nonlinear damper, the harvested power and the transmissibility are derived using the concept of the describing function. The results are compared with the linear damper. It is demonstrated that the nonlinear damper can also significantly increase the absorbed power despite having much lower displacement compared to the linear damper. This makes the semi-active nonlinear damper very attractive for mechanical energy harvesters.