Review on 🌡️ GeeBat 10PCS TEC1-12706 Thermoelectric Cooler Heat Sink: Efficient Peltier Cooling Solution at 12V 5.8A by Durioki Cordaro

One side colder than hot. So if you just plug it in alone, you'll feel the cold side get cold after a few seconds while the hot side get warm. After a few seconds, the hot side becomes so hot that the entire Peltier element becomes hot and begins to act as a heating element. As said, it will be relatively cooler than the hot side. So if you don't cool the hot side, the cold side isn't cold. If the hot side is hot, the cold side is colder than the hot side, but not cold enough to cool anything. They require a massive heatsink and fan compared to the size of the peltier plate itself. In my application, one of them requires about 10x the heatsink to keep it cool enough to keep the cold side cold enough for my application . However, if you're using it as a heating element, it works just by plugging it in. If you try to use it to convert heat or cold into electricity, it will also work without modification. hope that helps.