I have a project where I want to be able to cook food (very similar to a crock pot). What would anybody here recommend to use to convert AC to useable DC and what should I use to heat the food? Should I use cartridge heaters? How many watts would be OK? Also, I would need to turn on/off the heating element so what should I watch out for?
nearly all simple cookers use AC power to directly run a heating element. The elements are usually just wire coils, or sometimes flat resistance wire types. since the quantity of food being heated varies it is nearly always necessary to incorporate a thermostatic controller in order to vary the power. On toasters this is accomplished with a simple variable timer to allow you to adjust how long the bread is exposed to the heat. On crock pots and the like there is a variable thermostat which keeps the contents being cooked at a steady temperature. If you do use a heating element with exposed wire remember to keep it well insulated. The coils are live power and can be lethal if you come in contact with them. Also remember to use a thermostat which can handle the load presented to it by the heating element. If you can't easily find a thermostat for the current needed, then use a relay to handle the power. The amount of power needed will be influenced by several factors. The quantity of food to be cooked, the size of the oven, the final desired temperature, how fast it needs to cook, and other factors. If you look at similar sized units you can find out a good approximation of what you need by taking notes of their power needs.
Another good choice is an old 2 gallon type coffee pot. it's temperature setting works off of a thermo-switch at about 180 degrees F. They can be had for pennies at your local thrift store. I have used them from such sources my entire life and they seem to last decades in hard use. If you do build your own, consider using a solid-state type relay from your local electronics supplier. if you get one which is slightly overkill it should last forever and it can be switched on and off using low voltage sources.