The poskim discuss a case where a person wants to put his Shabbat hotplate on a timer so that it will be activated on Shabbat. It has already been established that there is a Halachic basis to use timers to turn on lights. As Maran rules in siman 252, there is no prohibition to set up a mechanism that automatically causes a Melacha to be performed on Shabbat. Fundamentally, a hotplate would be no different.
The case of the hotplate poses a more problematic question. Granted that one can activate the hotplate with a timer; is it also permissible to put food on the hotplate on Shabbat, when the hotplate is still off, knowing that in it will soon go on? Hacham Ovadia (Yabia Omer, vol. 10) presents a specific application of this question. May one place a pot of cold soup on the hotplate on Shabbat, before it is activated by the timer. In this case, as opposed to lights, the person is performing an action on Shabbat that causes the prohibited Melacha of cooking to transpire. In Halacha, this is referred to as Gerama (Causing).
The classic example of Gerama in the Shulhan Aruch (siman 334) is causing a fire to be extinguished on Shabbat by means of placing water-filled vessels in the path of the blaze, before it arrives. When the flames hit the vessels, they will burst and their water will extinguish the fire. That's considered Gerama, because at the time the vessels were placed, there was no fire; although the person knew that the fire will eventually get there. The Shulhan Aruch rules that Gerama like this is permitted.
There is considerable debate among the Rishonim (early commentators), whether Gerama on Shabbat is permitted only in the cases like a fire, where there could be a monetary loss. Hacham Ovadia proves that most authorities concur that Gerama is permitted even in cases where there is no monetary loss. Accordingly, it should have been permissible to place food on a hotplate before it is activated by the timer. Nevertheless, he does not rule like their opinion because of the other authorities who disagree and permit Gerama only in the case of loss.
However, Hacham Ovadia concludes that in our case of reheating cold soup, one can be lenient. Since the soup was already cooked, there is the opinion of the Rambam that "ain bishul achar bishul" (there is no prohibition of reheating cooked foods) applies even to cold liquids. Although, we do not hold like that, Hacham Ovadia is willing to use that minority opinion in conjunction with the opinions that Gerama is permissible even without a monetary loss. Each opinion by itself would not be a solid basis to be lenient; however, when combined, they can be relied on.
None of the Poskim allow placing a raw food on the hotplate on Shabbat before the timer turns it on. Hacham Ovadia only permitted placing a cooked soup that had already cooled off. However, it seems that it would be permitted to put a raw food on the hotplate BEFORE Shabbat, and set the timer to activate on Shabbat. There is room to be lenient in such a case because everything was already done before Shabbat. It would be no different than lights on a timer.
Nonetheless, Rav Moshe Feinstein restricted the use of timers in general. He felt that it was a zilzul (disgrace) to Shabbat and that eventually people would utilize timers to run their factories and businesses on the holy Shabbat. Therefore, before using timers for other applications, one should always ask a competent Posek.
In conclusion, those who want to reheat cold soup on Shabbat using a timer can rely on Hacham Ovadia.
SUMMARY
1. On Shabbat, it is forbidden to put a raw food on an electric hotplate that will later be activated by a timer.
2. Before Shabbat, one can be lenient a place a raw food on an electric hotplate that will be activated by a timer on Shabbat.
3. On Shabbat, one can be lenient and place a pot of cooked, cold soup on an electric hotplate that will later be activated by a timer.
4. In general, one should consult with a posek before using timers to activate mechanism other than lights on Shabbat.
