Electric Heater using 330 Ohm Resistors

[Electric Rain]

As you might have guessed from my subject line, I'm thinking about building an electric heater using 330 ohm resistors. :lol: I plan on using 12VDC, and 144 resistors, each 330 ohms @ 1/2 watt. Therefore I will be using a 4 amp transformer to power it. They will be wired in parallel with the power source, so physically they will form a ladder-shaped strip.

I understand how it's done, for the most part. It's not hard... but I need to know a couple things. One: How do I know how much each resistor will heat up when powered? Two: I understand that they can't be powered constantly. They'll burn up. But, I need to know how long they can stay on and how long they need to cool off. Will this need to be a pulse thing where I'll need a 555 or something to keep it on for 5 seconds, then turn it off for two, ect. or something? Or can they be powered for longer than that?

I can provide more details if needed, but I'm afraid I can't tell exactally what it's for. You all know me, I like trying to sell stuff. :wink: No one else shall have this idea!!! :lol: But I can tell you that it will be used to heat a 1'X1'X6" box and that I plan to put the resistors in a spiral form covering the bottom of the box. Can anyone help me? Thank you.

Rain

 

[Nigel Goodwin]

I would advise using wirewound resistors, or wind yourself an element using resistance wire. Carbon resistors are likely to have a very short life span doing this!.

 

[FoxyRick]

I agree with Nigel Goodwin, use wirewounds. They are far more resilient to getting hot. Find cement-coated ones if you can. Depends though how hot you intend it to get.

I have made three heaters like this, all 230V mains driven; two for PCB etching tanks (50W and 80W power) with slim, cement-coated wirewounds in a shrink-sleeve, and one for a wax-melting pot (100W power) for my wife using huge, aluminium-cased wirewounds glued to the bottom of a pan with hi-temp thermal epoxy.

If you can determine how quickly heat will be taken away from the heater, you can plan the how much power you need to supply to maintain temp, and how quickly it will cool, etc.. But you will still need to heat it up to this temp, so must supply more power initially than the loss. Maybe more heat will be lost as the thing heats up, you can arrange the input to be balanced by this at the desired temperature.

All this is really tricky though, and depends the mass heated, its specific heat capacity, the ambient temp, humidity, air-flow, and probably a dozen other factors. It can be done nicely in a carefully controlled environment.

This means that temperature control is really essential. You can't (imho) rely on just supplying a fixed amount of power (pulsed or not) to maintain temperature control. Simplest control may be just an old bimetallic strip thermostat, if you can find one. Next up is a thermistor, in thermal contact with the object and wired into your 555 pulse circuit to shorten or slow the energy pulses as the thermistor warms up.

Hope some of this helps,
FoxyRick.

 

[stevez]

Whatever you do you'll have to keep the temperature of the resistors at or below the manufacturer's specified limits. Power dissapation is likely given in some free air condition. Your specific application might allow heat to be removed at a faster rate - or it might be slower than intended -give that appropriate consideration. It's not a lot different than the ampacity of power wiring - a wire is rated for higher currents in free air - the rating is lower if stuffed into conduits or raceways.

 

[gerty]

Make sure your transformer is plenty big, at least 150% of load, or it too will become a heater :twisted:

 

[Electric Rain]

Eh, sounds like a bit of work there Foxy. Thanks for the info. So then, what about nichrome wire? What do you all think about that? :?: Thanks again.

Rain

 

[Klaus]

Have you done the maths on this?? A quick off the head calculation gives a combined resistance of less than 2 Ohms. So, your 12V/ 4 Amp transformer is going to have a hard time with that, being overloaded by 50% or so.

Also, while you may have your reasons for this idea, it strikes me as a bit crazy to solder 144 resistors together to get a single <2 Ohm @ 72Watt resistor. Too many solder joints to go wrong, heat up too much and cause trouble, not to mention the expense of 144 quality, wire wound, resistors.
If you want heat spread over some length, use heater cable. You can run this straight off the mains supply.

I appreciate your secrecy though it would help if you were to mention the exact shape of the heater, wether it needs to be pliable or rigid, how hot it needs to be,what enviroment its to be used in, etc. etc.
You can mention all these facts without uttering a single word of the actual USE for this heater
:wink:
It would get you much more constructive replies rather than just assumptions
Klaus

 

[Electric Rain]

It would get you much more constructive replies rather than just assumptions

Eh, true. But any more info than this and it WOULD give it away. I think I need to do some more research on my own... I wish I could trust people not to steal my ideas... :evil: :lol:

Rain

 

[gerty]

How are you planning on handling the heat transfer ??

 

[jrz126]

You cant trust me? You know I have enough projects of my own, so I wouldnt have time to steal your idea... :wink:

and where have you been, haven't seen you on AIM in awhile...

 

[stevez]

Did you realize that you can purchase small heating elements? I recall purchasing several as I helped a friend build a chamber to hatch emu eggs. Also, there are some tab (like TO-220 case) mounted power resistors that will handle more power so you'd need less of them - and they can be mounted to aluminum strips to distribute the heat.

 

[FoxyRick]

Hi Rain,

NiCr wire is OK if you can physically mount / support / whatever it where needed. If you are heating the air around it, no problem. If you are trying to heat an object, for efficiency you really need thermal contact. This may be a lot more difficult with bare wire.

Klaus is right about too many resistors. My 80W etchant heater only had 40 (2 parallel strings of 20 series R's) - and I would have used less except that they fit perfectly and it all worked out neatly.

Remember also that resistors increase resistance as they heat up. Again this depends just how hot you want them, but see if you can find a good datasheet for your resistors - it may have a resistance change chart or formula.

I used Phycomp resistors, see here:

**broken link removed**

By the way:

See you later -- Tu E kad y bnewa???

FoxyRick.

 

[Klaus]

[/quote]

Eh, true. But any more info than this and it WOULD give it away. I think I need to do some more research on my own... I wish I could trust people not to steal my ideas... :evil: :lol:

Rain[/quote]

Rain, whatever made you think that your ideas are actually *worth* stealing? :wink:

I much prefer to use my *own* ideas for the gadgets I'm tinkering with though sometimes, not very often, mind, I do see something described that is WORTH to 'borrow'.

I'm not in the selling business, far too much trouble involved and it would be like work :shock: , not fun

Sometimes I'm asked why I did not patent something I made (and which actually worked
:wink: ).

Standard reply: why?

to make money?

can't be bothered with all that paperwork.
:roll: :lol:

Klaus

 

[Nigel Goodwin]

Sometimes I'm asked why I did not patent something I made (and which actually worked
:wink: ).

Standard reply: why?

to make money?

There's only a tiny number of patents that actually make any money for the owner, most actually lose them money - due to the high cost of taking out a patent.

 

[TheOne]

Also a patent is only as good as the person holding it and his ability to defend it. Big companies, if you have a really good patent-idea, will try and drag you into court to scare you, claiming it is their idea or some infringement on their patents, hoping you will let go. I have seen this happen a few times.

 

[zevon8]

How about using those ceramic heaters like the ones found in a curling iron? Most are PTC, self controlling, and are ready made to be mounted on a metal surface. They are also nice and small, and run directly from mains voltage. Wost case, you buy a couple of cheap curling irons, and pull the heaters from them.

I have seen them used to heat small cabinets of electronics that are mounted outdoors.

 

[nbucska]

besides the amount of heat you may care about its spatial distribution .

I once made a 220V 1KW heater from iron wire wound on fibre-cement
tube.

In an other application I used lightbulbs as heater ( IR and conduction)

Used 4-transistor array as sensor-heater- matched diodes.

You may make a heater with PC board-like litho process with
etched resistive layers sprayed/vacuum deposited/ chemically
deposited etc.

 

[Electric Rain]

Foxy,

Ykr! Oui ghuf, E naymmo teth'd drehg yhouha eh draca vunisc fuimt ryja bmyoat dryd kysa... huf E ryja du lusa ib fedr y haf cek, oui lnylgat so luta! mum! Rao, tet oui tufhmuyt y dnyhcmydun un tet oui yldiymmo sasunewa dra myhkiyka?

Nyeh

 

[Phasor]

[Meandering slowly off topic]

Nyeh,

Ed'c hud bumeda du cbayg eh myhkiyka dryd baubma tuh'd ihtancdyht.

Brycun

PS For those who are curious, see https://en.wikipedia.org/wiki/Al_Bhed

 

[Electric Rain]

Phasor,

Famm, dryd'c dnia, pid E fyc dymgehk uhmo du Vuqo. Geht uv mega y BS. Zicd mega E's dymgehk uhmo du oui huf. Pid nakyntmacc, E'mm cdub huf. Geht uv cdnyhka druikr, cusauha amca rana udran dryh sa yht Vuqo ymcu ghufc dra myhkiyka... hud syho baubma dryd E ghuf uv bmyo dra kysa YHT ica drec vunis.

Alright fine fine. It was Foxy that started it, my sig was only a joke. Sorry to start trouble guys. Enough Al Bhed... for today. Just kiddin’.

Anyway, back to the topic. I've decided not to use resistors. I will be using nichrome wire, because I WILL be heating the air around it, like Foxy said. Hmm, let's see, lot's of people to respond to... let’s start from the beginning.

Jeff: I've talked to you since then, and it was Christmas that was keeping me busy. I'm answering your question only to make sure that everyone else here knows that I've contacted you since then and I'm not being rude and ignoring you. :lol: Also, I CAN trust you, I told you after your post, after all.

stevez: Where could I get these heaters?

Everyone else's posts before stevez second post: I've decided to use nichrome wire, but thank you very much for your help, I may need it for a different heating project, heating projects seem to be very interesting to me.

Klaus: Aw come on, that's just rude... nothing makes me think that my idea's will sell, except for the fact that... I dunno, I just think they're good idea's I guess. That and the fact that they don't seem to have things for sale that will do what my products will do. Besides, I know there are select people here that won't steal my idea's, like you and Jeff, but the real problem is people wondering into the forums from who-knows-where that WILL steal my idea. :wink:

Nigel Goodwin:

There's only a tiny number of patents that actually make any money for the owner, most actually lose them money - due to the high cost of taking out a patent.

Really? Where did you hear that???

TheOne: Bah, I'm not worried about that. Besides, I can figure out a way to say, "Well, mine doesn't do EXACTALLY what theirs does. See look, I use nichrome wire, they use..." You get my point. :lol: BTW, nice sig.

zevon8: I think I know of another project of mine where those might come in handy, thanks for the suggestion.

nbucska: How hot do you think I could get one of those PC-Board like heater's and what's involved?

Everyone: Thanks for all of your help, and again, sorry about the Al Bhed thing, I just can’t figure out what to use for my sig… *Shrugs* Bah, whatever. See you all later.

Rain