Well, a flicker preventer is a double on-line UPS, so the market has a device, albeit an expensive one. You could purchase a double on-line UPS that has small

Well, a flicker preventer is a double on-line UPS, so the market has a device, albeit an expensive one. You could purchase a double on-line UPS that has small batteries and that would serve as a 'flicker preventer' while providing only a small bit of runtime. It would be possible to build this kind of on-line conversion with a very very small battery that could only provide power for a few seconds, and that kind of device would serve the goal you mentioned. I suspect the cost for this device would quickly approach a UPS in cost, so it might not be the 'cheap' solution.