Gibson486:
I am starting to work on Peltiers....I am kind of confused on how they work. So, we heat (or cool, depending on your perspective) by applying DC current. To get the opposite effect (to cool when heated), we apply negative DC current.
Since retrolefty did a good job covering the PWM aspect of your post, I'll dive into the theory a bit.
Peltier junctions are devices based around the thermoelectric effect, specifically the Peltier effect, which describes how heat can effect the flow of current between two different materials. Like the Piezoelectric effect, the Peltier form of the Thermoelectric effect is a reversible process. Therefore not only can the temperature differential of a junction effect the current flow, the current flow can cause a difference in temperature between the two sides of the junction. With certain pairings of materials (e.g. dissimilarly doped semiconductors) this effect is strong enough to either cause a signficant temperature differential (i.e. heat one side of the junction, while cooling the other) if an electric current passes through it, or generate a small current at a specific voltage if a large enough temperature differential exisits accross the junction.
Does this help you understand this part a bit better?