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Electrical heater using resistors

  1. Jan 27, 2007 #1
    Hi people is my first time posting in this forum... Great Forum¡¡¡

    Every Sunday I work selling breakfast food and I got the same problem of all.

    Food getting cold, so I want to build a heater to maintain the food warn, just to keep it warn for the people be satified of my business.

    Can you recomend me an easy, cheap method to do it I will be greatfull...

    Using home current of course...
  2. jcsd
  3. Jan 27, 2007 #2


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    Why not just put it in the oven?!
  4. Jan 27, 2007 #3


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    Or buy a heat lamp? That's what ovens and heat lamps are...
  5. Jan 28, 2007 #4
    yes is a good idea


    First I want to learn something new
    Second I will be selling food outside far from my house
    Third is difficult or not to build one

    I just want to learn how to make a circuit that can maintain things hot

    The principal idea is to learn to build a heater.. thats all. Is hard the constructuion of one ????
  6. Jan 28, 2007 #5


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    How much food do you need to keep warm?
    What sources of energy do you have?
    How long do you need to keep food warm for?
    What materials and facilities do you have to work with?
    What packaging constraints would your heater have?

    An electric heating element would keep food warm. At its most basic, your design could comprise of an insulated box with a lid and a heating element inside it. The circuit is simple, just have a heating element in series with a fuse and switch. I.e. this could be a very simple project. Alternatively, it could be as advanced as you make it.

    Then of course you could look at thermostatic control, different types of heating element, better insulation, better control, safety features.....

    What do you actually need this to do?
  7. Jan 28, 2007 #6
    Sorry for the private message because I send it without seen my thread sorry, Ok I want it simply only with a swith and elements in series thats all .

    So I want to keep warn a metal tray like 11" x 22" and a swith for turn off when sufficient hot and thats all.

    Remenber that I´m using home current so how many resisitors, in series or parallel.
  8. Jan 28, 2007 #7


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    3x 20 inch long 500W heating bars. In parallel. Yeah, don't worry about a fuse. Or earthing. Or insulation. Or safety....! I'm being sarcastic by the way.

    Seriously, if you provide us with some more constraints (such as those I outlined above) we'll be able to help you a bit more! If you haven't got any, then I'd suggest you have a think about things like insulation, economy, control, safety etc. Any idiot can wire up some heating elements to keep food warm. We'd just like to help you do it safely, efficiently, innovatively and economically.
  9. Jan 29, 2007 #8


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    And on the topic of safety -- there's the food aspect to consider. Some heating methods probably wouldn't be food-safe for human consumption. Using some chemical reactions to generate the heat, for example, could have byproduct gasses that could contaminate the food.
  10. Jan 29, 2007 #9


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    That's a good point berkeman. It also makes one consider the temperature to store the food at; it will have to be above a certain temperature to resist the breeding of harmful bacteria. Storage time will also have to be limited.
  11. Jan 29, 2007 #10
    Firstable I don´t want a POWERFUL HEATER just to keep warn...

    I imagine to put the heater in a metal platform and then the metal tray on it. Remenber I don´t want to cook with this, is only to maintain warn.

    1. First, think that I want to mantain bread warn. Like 20 to 25 units.
    2. Using home current 110v.
    3. Like 15 to 20 minutes also if we use a swith we can control the heating.
    4. Materials and Facilities: Iron base, or whatever you recomend.

    I want it the most safely, efficiently and economically, not innovatily. I want it easy make.
    Last edited: Jan 29, 2007
  12. Jan 29, 2007 #11

    Then you need to purchase something. You cannot compete with what is out there existing. If you build an enclosure that is quite well insulated, you may want to consider a light bulb or two inside to keep it warm.
  13. Jan 29, 2007 #12
    Remenber that I´m using a metal tray it will maintain open the most of the time, certains time I will cover it with some kind of hermetic cover or something like this ....

    Also I´m talking about resistors ,why??????
    because I also want to learn about it I´m not only will use it for my business I will use it in another thing I don´t know exactly...

    Just think in resistors and the easiest way to build a movable heater that I can use in other places.
  14. Jan 29, 2007 #13


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    Williams16, I'm guessing that you are not located in the US, correct? If you were, the US Food and Drug administration and other agencies would probably not appreciate you home-brewing a portable food heater to use in your food sales to the public. The liability laws in the US do get out of hand sometimes, but other times they help to protect people from negligent, harmful acts by others. If you made a home-brew heater in the US, sold some food out of it and got people sick because of it, you would be liable for their damages, plus probably some punitive (punishment) damages.

    There are similar health protections in the EU for the food industry, and I would think in many other parts of the world as well. You can't just use any old plastic to sell beverages in, for example (for good reason). And you can't call any old food container "microwave safe".

    So if you insist on making the food heater your own project, please also make learning about the food safety laws a part of that project. What country are you located in? What agency regulates the safety of food and food containers in your country? What can you tell us about those regulations? I'll bet that the materials used in common power resistors (like the ceramics and paint coatings) would not be considered suitable for food heaters.

    Quiz Question -- What material is used in toasters to heat bread? How does a toaster work?
  15. Jan 30, 2007 #14
    I´m from Panamá, and I will not maintain a constance selling all the days of the week, is only for Sundays from 5:00 am to 12:00 pm ´o clock. Is not a lot of time also we share the business with other groups that put on list to sell any Sunday they want. Is in a Church, thats why we can´t sell all the Sundays just only when we get in list.

    The food will not stay a lot of time in the heater because we will maintain selling and cooking at the same time, the reason for the heater is to maintain the first group of food we cook warn like the last group we cook and the people will feel greatfull for our work.

    I want to learn how to do it safety also I imagine not to put the food directly to the heater I want the heater in some kind of platform and then the metal tray that I´m going to use on it and then the food on the tray.

    I´m using some plastic containers but, I buy it for the purpose in a Supermarket and they are for food storing.

    Seriously I want to make the heater using resistor, I want you and people to teach me how to do it safety and easy.

    ... So what materias I need? Whats the values for the resistors? How much resistors I need? Which is the safety manner to build the heater? What do you recomed in matter of resistor for a heater?
    Last edited: Jan 30, 2007
  16. Jan 30, 2007 #15
    Not you think that 500 watts is a lot of heat for warming???

    What values in the resistor I need?
    How much resisitors with this values I need???
    In parallel or in series???
  17. Jan 31, 2007 #16
    Check out http://www.kanthal.com/ They sell all sorts of products that you could use to make an element. I would take a look at "Resistance Heating Alloys" where you can wind your own element. It's the same wire used in toasters and hair dryers.

    The link below has all the different kinds of alloys available and specs on each.

  18. Jan 31, 2007 #17
    If you are serious about doing this you need to:

    1. Get yourself booked on a short course on electrical safety - This is very important cos you are dealing with lethal voltages.
    2. Get yourself booked onto a course on short course on electrical design - This is needed because to do this sensibly you need to use high power semiconductors.
    3. Find a supplier of ceramic heater bars (Very hard to get these days).
    4. Get a semiconductor controlled voltage control circuit that you can modify to match the heat power you want to generate.
    5. Bolt it all together.

    Honestly, good on you for wanting to learn about electrical design, but don't learn on mains voltage, it really hurts, I know and I'm supposed to know what I'm doing.
    Most people on this forum would have had quite a few years experience before attempting to design anything powered by voltages over 50V.
    If you can think of a safe way to do this powered off a couple of car batteries we may be able to help you.
  19. Jan 31, 2007 #18
    Ok lets talk about 12 volt, I want the most heat we can make using 12 volts and around of 10 ampers. I will use a computer PS, so whats the configuration of reisistor to reach a good amount of heat.

    People please help me this is for Saturday, so for the next post I want to see a VALUE any NUMBER, precaution , ideas for the heater, how much resistors please, I have a bachelor in electricity, here in Panamá, but I´m beginning my University...

    So using 12 volts i need .................. for the heater?'
  20. Jan 31, 2007 #19


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    What power is your power supply rated at? That might give you a clue as to what heat you'll be able to get out of your heater.

    12 volts, 10 amps? Let's start with this:

    Power = voltage * current

    Ideas for the heater? You've had plenty. How about wire up a circuit with some heater coils, a switch, a fuse, a thermostat, and your earthed power supply.

    If you have a bachelors in electrical engineering, perhaps you'd have an idea of how much power you can get out of a 12V, 10A supply?

    Finally, you appear to be ignoring comments people have bothered to type out for you. Panda particularly has some excellent ideas up there.
  21. Jan 31, 2007 #20
    around 120 watts, of power, also I can get more power supplies, but the question is this, Is 120 watts sufficient for maintain the tray warn???

    Is not necesary a fuse, a think that a swith and the circuit will made it, also a termostat will made economically and a little more complicated.

    I heard about 330 Ohms 1/2 watts configuration, like 240 resisitors of this measure will do a 120 watts or I´m wrong?

    Thinking seriously 120 v is really a lot and I don´t have that experience to work with. So thats what I reconsider the 12 volts.

    The problem is this 240 resistors costing 10 cents each one is around 24 dollars. So any idea of another configuration that can be cheaper.

    I´m really learning about what you are saying me, and the recomendations.
    Last edited: Jan 31, 2007
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