# Electric Heater?

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#### Electric Rain

##### New Member
Ok. I'll spare the details, but, I need to build an electric "Foot Warmer." DON'T HIT THAT BACK BUTTON!!! :lol: Hear me out. I have some of it figured out. I can have a metal box, and mount some kind of heating element on the inside of the top. So when you put your foot of the top of the box, the heat transfers from the element, and through the metal box, to your feet. I need it to be warm enough to...well...warm up your feet when they're cold, (Duh) but not be to hot that it burns your feet. I also need a pot (variable resistor) to adjust the temp. The temp should be from about 95 degrees to 120 degrees (Fahrenheit). (Do you think that is a good temp range?) And... that’s about it. Schematics would be nice, but then again... it seems like it should be easy, so any other help would be nice too. (I seem to have the most trouble finding a heating element.) Can someone help? And please don't come back with something rude like, "Get some slippers." I just like building electronic... stuff. :wink:

#### Sebi

##### Active Member
I know, existiert a cable with heating function: the power depend from cable length, and the heat is self-controlled. Another possible way a ceramic heating element

#### Klaus

##### New Member
I would agree with Sebi's suggestion to use heating cable. Its normally used to keep pipes or even floors warm. Certainly the safest thing to use for you as it cannot overheat. You simply control the heat by the length of it and the way you coil it inside your box.
If you want it adjustable, I woud strongly advise AGAINST using a potentiometer to control the heat output. It needs to be quite big (and expensive) to cope with mains power and there is no thermal feedback. For a temperature control you shoud only use a bimetal switch. This switches the heating element off when the set temperature is reached and on again if it cools down too much. You can get fixed temperature ones or
you could salvage an adjustable one from an electric iron and use it at its lowest setting inside your box, eg, you must mount the bi metal switch so it can sense the heat inside your box.
Take care with mains the power and make sure there is a decent earth connection to the metal box and you use only a 3 pin mains plug.
Klaus

#### stevez

##### Active Member
Not sure who will be the user of the heater but note that it is not uncommon for people to have a loss of sensation, particularly in extremities, of temperature. On failure the device can exceed temperatures that do damage yet the person cannot sense it until it's too late.

#### tansis

##### New Member
Some years ago I was unfortunate enough to spend a couple of winters unemployed and was forced seek accomodation in the less expensive end of the housing market (slum bedsit) The landlord was becoming suspicious that somebody was tampering with the coin operated meters that regulated the mains supply. (I say suspicious out of of kindness, he was *@!X* livid that the takings fell short of the actual bill for the house even with his extortionate rate set on the meters).

I came up with a simple device of a carboard tube (6 feet long) salvaged from a skip, placed small 12volt fan inside and stood it upright in a corner of the room. With the fan running it would draw the warm air from ceiling hieght and expel it ground level. Made a big difference.

In case your wondering the culprit that was caught tampering was on the floor above me and had driven meat skewers into the mains cables entering the meter connected by a piece of tv coax....

#### mxitman

##### New Member
Just a few questions regarding yours. Did you want it to always to be turned on by the presence of someones foot only? or just by switch? You can wire a safety bi-metal switch in series with the power supply, and can be hidden away in the box, they are available in a variety of temp ranges... in the event it overheats it will cut power and can be manually/auto resetable. I would also recommend the heat cable or if you want to really go overboard build it with a peltier module, that way you can make it heat or cool by reversing polarity, and useing aluminum as it is a good conducter of heat, hope that helps.

mxitman

#### Electric Rain

##### New Member
mxitman said:
Just a few questions regarding yours. Did you want it to always to be turned on by the presence of someones foot only? or just by switch? You can wire a safety bi-metal switch in series with the power supply, and can be hidden away in the box, they are available in a variety of temp ranges... in the event it overheats it will cut power and can be manually/auto resetable. I would also recommend the heat cable or if you want to really go overboard build it with a peltier module, that way you can make it heat or cool by reversing polarity, and useing aluminum as it is a good conducter of heat, hope that helps.

mxitman

Thank you very much! That helps me allot! I don't have much time to type more, but thanks everyone. I guess the heat cable thing should work, as everyone has suggested it. Oh yeah, I was thinking I'd just use a switch, but the foot switch thing sounds really cool, so I think I'll do that. And one more thing, could you tell me how hard it would be to do the heat and cool thing? Thanks everyone.

#### mechie

##### New Member
Heat mats

Look Habistat up.
A vivarium heat mat is safe, cheap and probably easier to get than tape, any decent pet shop will sell them.
All sizes and powers are available and they are easy to control with a simple light dimmer :wink:

Even on full power they would struggle to start a fire (unles you insulate)

#### Electric Rain

##### New Member
Mxitman, I took what you said about the peltier module and started thinking. I think I'm going to use one of those and instead make a, what I like to call, "PC&HS." Otherwise known as, a Personal Cooling and Heating System. I'll use a fan to blow the heat/cold towards the user. And put everything nice and neat in a box. But I need some tips on the module. Like where to get it, what one to get, how to wire it all up, what to do about variable temp. and so on. I think someone will appreciate a box that blows heat/cold at them with adjustable temp. then a box that heats up and warms your feet much more. :? I found this web page,: https://www.marlow.com/products/productlist.asp?RootNav=MICoolers
but I don't know what one to get, if I can even have variable temp. and what not. It's kind of a mess, but can someone help me figure is out please? Thanks for the help everyone.

Later,

Electric Rain

#### Echelon 01

##### New Member
You want to go by the Q max value when dealing with heaters or in this case heaters/coolers. This is the usable heating power in watts. The MI1023T with almost 10 watts isnt bad, I would not go much over 10 Watts unless your using a very heafty aluminum frame with some steel or other less conductive metal as a base. I used four 30-watt cartridge heaters to heat a small insulated aluminum block to 175 degrees celcius in less than 30 seconds for a heated imprinting press I designed. These cartridge heaters were tapped into the block though and the block was very small (about the volume of a cigarette box). I hope this helps you get an idea of how fast you want to heat up with what amount of metal with what wattage. The same principles can be applied to the cooling.

#### Echelon 01

##### New Member
Second thought - if this is going to be a hollow aluminum box with the pelts inside against the metal, I would use two of the same one i just mentioned, wire them in parallel and place them at opposite ends of the box against parts of the box that you can not touch accidently because although aluminum disipates heat very evenly, you will still get hotspots where they are placed. Maybe even make double sides like [|____|] ya know? Have your fans force the air from the bottom of the box to the top. Put small slits against the bottom edges of the box and maybe make the top like a grill.

#### Electric Rain

##### New Member
Ok, now I've got it... for the foot warmer that is. I think I really truly found some good heating elements. Electronic Goldmine has these heating elements that they say are used by NASA for keeping everything warm on robots that they send up to... mars and stuff like that. So they should work pretty well. :lol: And, they're cheap. Now, at the bottom, I copied the part out of they're PDF catalog where the heaters are. I'm mostly looking at # 2280052-3260. But, I really need to know what I should do to control it. And how many I should use, and all of that. Or, if the ones I think will work will even work. So, I need someone to help me with this please. Like, what one should I use? How many should I use? Would it be a good idea to mount them on the underside of an aluminum box? How should I control the temp.? Should I worry about controlling the temp.? Will a wall DC adapter work for it? Will it take to much power? Will it get hot enough? Will it get too hot? I am kind of worrying because I'd really like to get this do by Christmas. :lol: :mrgreen: :| :cry: Can someone pleeeeease help me with some of my questions? Thank you very much.

Rain

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#### spuffock

##### Member
I don't think the idea of cooling is useful, you would need at least 100W of cooling to feel any benefit. The efficiency of peltier devices is not spectacular, 300W of power to obtain 100W of cooling...... This brings us to the big problem. 400W of heat to remove from the hot side, without allowing the temperature to rise much. Water cooled heatsinks? It starts to get silly.

#### Electric Rain

##### New Member
Yeah, I was thinking, and I don't think peltiers would be good enough either. :? They'd be too expensive anyway. So what do you think about the heaters I found above? :?:

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