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

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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!.

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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.

 

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.

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stevez

 

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

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gerty

 

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

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When life gives you lemons... make a battery.

 

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

 

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

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When life gives you lemons... make a battery.

 

How are you planning on handling the heat transfer ??

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gerty

 

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...

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Jeff
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needs to be.

 

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.

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stevez

 

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:

https://secure.farnell.com/jsp/endec...U=343675&N=401

By the way:

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

FoxyRick.

 

[/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

 

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.

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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.

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