We have investigated the dc current–voltage characteristic of high temperature superconducting microbridges. When a dc voltage is applied to a microbridge, it switches to a lossy state due to the formation of a hotspot in the bridge. We have measured the length and temperature of the hotspot as a function of the applied voltage, and have developed a thermal model to explain its steady state behavior. The hotspot has a flat‐topped temperature profile, with the maximum temperature independent of the applied voltage. The length of the hotspot, and hence the bridge resistance, increases linearly with the applied bias, so the current is independent of the applied voltage once switching has occurred. © 1995 American Institute of Physics.