To produce a specified DC power supply from a different primary DC source, a DC/DC converter is required. This can step up, step down or even reverse the polarity, compared to the input supply. DC/DC converters are available in many different sizes, some being designed for PCB use , where some components require different voltages to others, and larger DC/DC converters are often stand-alone units allowing equipment to be powered via the output terminals. Most DC/DC converters also provide a regulated output and various technologies are employed, including switched-mode and linear control. Heat is often produced; many DC/DC converters feature heat-sinks/fins for heat dissipation and some employ integral fans. DC/DC converters come in many different formats including DIN rail mountable, and some are completely portable, ideally suited for automotive use with the input being powered from a cigarette lighter type vehicle socket. Some offer an adjustable output voltage, and other features to consider include static and dynamic power reserves, which can allow short term excesses in rated current, galvanic isolation, and over-voltage/overload/short circuit/reverse-polarity protection. Another type of DC power supply is the uninterruptible power supply (UPS) , or an AC/DC supply may be an option.