low temp heating element

can someone please point me to some information for constructing a low temp heating element. I need something that will maintain about 290 degrees F for periods of an hour or two. i need some guidance on the type of element to use and how to control it's temp.

the element needs to be about 12 inches long and should probably be flat on one side. width can probably vary from 1/2 to 1 inch. It should take its power from standard 120v household service. TIA

Reply to
Rich
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To design one would need a lot more information that this. Here are some thoughts:

You could just take an aluminium bar and bolt some wirewound resistors to the underside of it. Wire them up to a simple bimetallic switch (set to the correct temperature) also bolted to the underside and then pot the whole underside in appropriate potting compound. If you want more precise temperature control, any electronics project book on the use of triacs/scrs will have circuits. The dimensions of the bar, choice of resistors, potting compound (if needed), in fact just about everything, will depend on information you haven't yet provided.

Reply to
RSPCWD

constructing a low

about 290

guidance on the

probably be

inch. It

TIA

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would be you best bet...call em.. they will fix you right up.

To locate this sort of thing do this: go to

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click on the 'web' tab thats on top of the blank box.

when that comes up... enter yer key words.. 'electric heating element'

a good skill to know.

Reply to
Phil Scott

If you just want to buy one,

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might have something suitable.

This depends on what you're trying to heat up (for instance, air is easier to heat up than water) and how fast it needs to heat up. The main control over this is the wattage of the heater.

If you can stand some slop in the temperature setting, a control like

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is simple and relatively inexpensive. For finer control, you might use a more expensive switch like
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. For better control than that, you probably need an electronic controller like
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.

Standard disclaimer: I don't work for Omega or otherwise get money from them.

Matt Roberds

Reply to
mroberds

Sue, you need to define what you need to keep heated to that temperature, I will point out the obvious.

you really need to specify how much power the thing will need to supply,

think of heating a thimblefull of water and then a bathtub full of water or indeed to the extreme, a Swiming pool full of water. All three would have different power requirements. Even if you do not know, the power , if you talk to the resistor manufacturers as suggested in other posts it will help them enormously if they know what you are trying to do with it

Tom

Reply to
Tom Grayson

Did you perhaps mean to address that to the OP rather than me (Sue)? I did advise the OP that, "in fact just about everything,will depend on information you haven't yet provided".

The OP did say that he wanted advice on " constructing a low temp heating element" - hence I didn't point him at sources of pre-built ones but gave some suggestions on making his own. "Normal" metal-encapsulated, bolt-on resistors are a relatively cheap and widely available solution to many small heating projects.

Reply to
Palindr☻me

Yep I read it wrong, thought you were the OP

You see , I do not need a lot of exercise, Am Always jumping to conclusions :o)

Take Care

Reply to
Tom Grayson

Sorry for the lack of info. Here's what I'm trying to do:

I want to build a book binder that uses thermal binding tape. I think this is basically a strip of ribbon w/ hot melt glue applied to it. Heat the strip, stick in a stack of paper, and fold the edges of the tape around the spine of the document to the front and back cover. The tape actually becomes the spine of your book. I'm not worried about the mechanical parts (for folding the tape), but the heating parts are not something w/ which I'm really familiar.

I'm guessing power needs are less than 1000 watts. All I want to do is control the temp so that the glue melts w/out burning up the tape or taking 20 minutes to melt. Commercial machines operate at about 290 degrees F, but they cost several thousand dollars for something which is essentially a desk mounted hot glue gun with an integrated clamping device.

The AL bar thing you described sounds interesting but I'm not knowledgable enough about the items you describe to put one together w/o more info. I am knowledgeable enough about general electrical safety to NOT electrocute myself or burn my house down.

Lemme know if you need more info. Thanks.

Rich

Reply to
Rich

So lets take an aluminium bar, about 1" wide by 12" long. You won't need many watts at all to heat that to 290F, when in its holder (say an aerated concrete building block, if you have them in the USA, with a suitable hole drilled through it longways (say 2" diameter). As soon as you start using it, it will need power to keep to temperature - but I am guessing that there is a gap of at least minutes between each binding operation?

So I would start with say 250W as being more than enough to get the bar hot quickly. At 110V, that needs a load of about

50 ohms. I would use 4 resistors in series, such as these 12 ohm ones: Description Resistor, metal cased, power, 25W, 5%, 12R RS Stock No. 160-720 Manufacturer ARCOL Part No. HS25 12R 5%

These are on the UK site rswww.com - you will have US equivalent suppliers. They are about 1.5 GBP each.

Bolt them along the bar, countersinking the bolts on the working side.

For the temperature setting switch, I would use one of these: Description Switch, temperature sensitive, bi metallic, N/C, 150degC RS Stock No. 339-724 Manufacturer ELMWOOD Part No. 2455R-98-714 L150C

These switches have limited life (10^5 operations at full rating) but you are very lightly loading them and they should last for years of intermittent use.

You need a normally closed type. Bolt this mid-way up the bar in series with the resistors. Add an earth wire to the bar and join the 3 wires (two to the resistor chain, one to earth) to a suitable cable and plug. Clamp the cable to the bar and put a strain relief bush in, bolted to the bar too. Use correct wire, solder (or crimp) and cable for the current and temperature.

Now lay the bar on a piece of scrap wood, resistor side up and build a dam (modelling clay works fine) around the bar - high enough to be say a 1/4 higher than the resistors. Cover the resistors and switch up to the top of the dam with suitable high temperature encapsulation compound. Not essential, but it makes a finished article.

110V can cause fires and kill. If you are in any doubt whatsoever at your competence, and even if you aren't, get an electrician to supervise and test that the heater is safe before you power it up.

That should do it!

Hope that helps,

Sue

Reply to
Palindr☻me

Sorry, I cut and pasted the wrong resistors in and didn't=20 spot it. You want the 75W ones - not the 25W. It's been a=20 long day. The RS site is currently down, so I can't put in=20 the right link.

-- Sue

Reply to
Palindr☻me

Sue, thanks. That's good stuff, but I still have a few questions so I can keep all my fingers and toes. it sounds like you are saying that I can take this Al bar and mechanically attach the 4 resistors in series along the back side of the bar. my question is to what do I connect the power leads? Do I connect them to one leg of the first and last resistor, w/ the remaining resistor leads connected to the bar?

That bi-metallic thermostat looks good, but I think I already have my controller. It includes a contact temp probe and the power source for my heating element. It is actually the plug-in controller for an electric skillet. it even has a temp knob to let me set my desired heat.

thanks for the help.

Reply to
Rich

Thanks for the correction Sue. BTW, I hope that the 'rest' time between binding operations is seconds, not minutes.

Rich

Reply to
Rich

Nope. Take the hot wire to the first terminal of the first resistor, then a wire from the second terminal of the first resistor to the first terminal of the second resistor and so on to make a chain. The second terminal of the last resistor is connected to the other wire of the supply and the ground wire of the supply is bolted to the bar. If you are in any doubt whatsoever about this - take it to someone competent, say your local uni and ask one of the electrical departments lab techs to wire it for you. No disrespect intended, but I very strongly suggest that you do get qualified help with this. 110 volts can kill and can burn your house down.

I would strongly suggest including the bi-metallic thermostat, wired into the chain between the second and third resistors. It will act as a safety device if your plug-in controller falls out, makes a bad thermal contact or simply goes wrong. If any of these things happen without the bi-metallic thermostat in circuit, the unit will get very, very hot and could cause a fire very easily and very quickly.

OK, lets do some physics (if I can remember back that far)...

The bar is say 12"x1"x0.5" so its volume is 6 cu ins.

6 cu ins = 6/0.061 = 98 cu cm

Aluminium is 2700 kg/cu m, so the bar will weigh 2.7 x 98 = 265g

The specific heat of Aluminium is 0.9 joules per degree c per gram. We have 250 W or 250 joules per second. So the bar will take 2 minutes to get to temperature - if shoved in the aerated block I described (approximately).

Assuming the glue is about 5 g, that will take about 2000 joules to heat and liquify. So the recovery time will be about 8 seconds before you can do the next one.

So the design should take about 2 minutes to heat initially and do one binding every 8 seconds or so.

Hope that helps.

I say again, please, please make sure that whoever wires this up knows what they are doing or that it is checked out by someone professional before power is applied. If you get this wrong, you could see the place catch on fire within seconds - if you are still alive at that point to see.

Sue

Reply to
Palindr☻me

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