Opinion #1 based on Hacham Ovadia Yosef
On Erev Shabbat one may put a pot of raw food on a plata which is connected to a timer so that when the plata eventually turns on the food will cook. Since there are some poskim who do not allow doing so, one who refrains even though he is not required to is worthy of blessing. On Shabbat itself one may place a pot of liquid that was cooked before Shabbat and cooled down somewhat on a plata that is currently off but will turn on with a timer.
Opinion 2 also based on Hacham Ovadia but others as well. These rules apply the same as if there were a timer, except in the cases of foods that are liquids such as soups, for that see the last opinion.
However, according to Hacham Ovadia Yosef, as he writes in his work Yehave Da’at, one may reheat fully-cooked, dry food over a “Blech,” that is, a metal sheet covering the flame. Since people do not normally cook in this fashion, placing food on a metal sheet does not resemble standard cooking, and is therefore permissible on Shabbat. Placing food on a “Blech” differs in this respect from placing food over an open flame or in an oven, which are, of course, conventional methods of cooking food. Hacham Ovadia maintains that one may place the food anywhere on the “Blech,” including the area directly over the fire.
There are other authorities, such as Hacham Ben Sion Abba Shaul (Israel, 1923-1998), who disagreed, but one may certainly rely on Hacham Ovadia’s lenient ruling in this regard.
Hacham Ovadia likewise allows reheating dry, cooked food on an electric hotplate. One may place the cold food anywhere on the hotplate, even directly over the heating element.
Summary: It is permissible to reheat dry food on a “Blech” or electric hotplate on Shabbat, provided that it is not a liquid and has been fully cooked. One may not reheat food on an open flame or in an oven on Shabbat.
According to some, you can also reheat soup as long as you place it there BEFORE the timer kicks back on.
The question arises as to whether one may set an electric hotplate on a timer, and place cold soup on the hotplate before the hotplate is activated by the timer. For example, one may wish to set the timer to activate the hotplate at 10 AM, and place the soup on the hotplate just prior to that time, so that he could have hot soup during Shabbat lunch. Since the hotplate is cold and inactive at the time he places the pot, perhaps we should not consider this person to be reheating the soup on Shabbat.
This issue is subject to a debate among the Halachic authorities. A number of recent Poskim, including the Hazon Ish (Rabbi Abraham Yeshaya Karelitz, 1879-1954), Rabbi Moshe Feinstein (Russia-New York, 1895-1986) and Rabbi Eliezer Waldenberg (Israel, 1915-2006), ruled that this is forbidden, each for different reasons. Hacham Ovadia Yosef, however, in his work Yabia Omer (vol. 10), rules leniently, noting that this situation falls under the category of “Gerama” – an indirect action. Since the person does not actively reheat the soup, but rather indirectly causes it to be reheated, this is permissible. This is also the view of Rabbi Shlomo Zalman Auerbach (Israel, 1910-1995). Hence, one who wishes to be lenient in this regard certainly has authorities on whom to rely.
IIt must be emphasized, however, that one must be careful not to mistakenly compare different cases, and draw conclusions from one situation to another. We deal here specifically with reheating soup or another liquid that has been fully cooked and then cooled, by placing it on a hotplate before the hotplate is activated by a timer. One cannot necessarily extrapolate from this case to other situations of reheating food on Shabbat.
Summary: The authorities debate the question of whether one may place cold soup or other liquids on an electric hotplate on Shabbat, before a timer activates the hotplate so it can heat the soup. One who wishes to be lenient in this regard certainly has authorities on whom to rely.