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Author: Subject: Improvised Hotplate
evilgecko
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[*] posted on 20-1-2005 at 18:50
Improvised Hotplate


I'm thinking of making a hotplate as I can't be bothered spending $50 on a new one. The first question is how many Watt would it need to be to make water boil. I don't like the idea of using 240V mains, so I was wondering if I could get enough energy from a 12V power supply. I don't know its max current but I would say it won't be much more than 10A. Would 120W be sufficiant to boil 200g of water. I will proberly use NiCr wire and cast the coil into a small plate of plaster of paris.
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[*] posted on 20-1-2005 at 19:54


That depends on the time frame you’re looking at. If you have a while, it should be no problem. I’d estimate that this would take around 5-10 minutes, but this is just a very wild guess. If you have good heat transfer between the plate and the water, it should be quite fast. I remember that my 300W rice cooker could boil about 500mL of water in around 10 minutes, so your plate should be fine.

By the way, we don’t actually buy new hotplates here. We just buy used junk on eBay. For example, I got a $40 Corning stirring hotplate with no heat a while ago and somehow fixed the heat. Now I have to be careful not to let things sit on it too long, else they tend to melt… (lousy Bomex):D If all you need is a hot plate, it you can probably get a cheap one with good quality on eBay (Sometimes you also see new ones).
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[*] posted on 20-1-2005 at 21:18


I bought a cheap cooking hot plate at a store kinda like Walmart for $10. a 3.5 computer fan with magnets attached to 2 of the blades spins with reduced voltage under the plate. it stirs, it heats, it cost me $11 with tax and a little time to get the stirrer to work right.
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evilgecko
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[*] posted on 20-1-2005 at 22:09


I live in good old New Zealand, and buying stuff of e-bay is expensive. New Zealands version, www.trademe.co.nz, doesn't have the same range of products as e-bay. I get so jelous of e-bay sometimes. Anyway, I need it to concentrate home-made sulphuric acid (thanks axehandle). At the moment I've got the two extremes, a homemade meths burner and a LPG cooker. I tryied using the meths burner outside but the wind took all the heat away, and I don't want to be doing this inside. The LPG is too fierce and will crack the glass. I don't care if it takes 30mins till it boils, so this is starting to look like a good idea. Also, wouldn't low voltage-high current produce more heat than a high-voltage-low current. I'm basing that theory on because power stations step-up the voltage when it travells along the power lines so that less energy is lost due to heat.
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[*] posted on 20-1-2005 at 23:04


The standard hotplates for cooking food are overpowered and have bad regulation of temperature - they are not useful for much except steamdistillations. (this is true for those plates with thermostat)
There are other hotplates which allow to regulate in several steps the power, say starting with 160W, 320W, 480W and so on. This better.

A hotplate for chemistry needs not more than 400W actually, mostly 200W are more than ok.
The heating wire of an old hairdryer (400W not more) embedded in oven-cement between two tiles and connected to a dimmer is fine and easy.


/ORG




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[*] posted on 20-1-2005 at 23:09


After a few cycles oy heating and cooling the plaster of paris might loose all of its integrity and become very crumbly. I will try to find the thread where someone made a very nice mini-hotplate out of phosphate bonded cement and resistors. Here it is http://www.sciencemadness.org/talk/viewthread.php?tid=2207
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[*] posted on 21-1-2005 at 01:11


I was going to start posting in there but I then saw the title..."Simple substitute for NiCr wire in DIY heat sources". I would kinda be posting off topic if I metioned one about using NiCr. So I created this generic Hotplate thread.

Ok I admit Trademe is good for somethings. I just found a Light dimmer rated at 500VA (that's the same as Watt, right?) for $5 while in a shop they will cost me $35.

I found a old hair drier but it is 1000W. Could that still work with the light dimmer metioned above. Or does the rating of the heating device have to be the same or lower than the rating of dimmer. My gut instict tells me it does.
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[*] posted on 21-1-2005 at 02:03


Oven cement, not plaster of paris I wrote.

You cannot run 1000W with a 500W dimmer - better recalibrate your gut-instincts if you want to survive your experiments ;)

You can use the heating wire of two 1000W devices in line though, this will give about 500W and works with the named 500W dimmer, but there are many 400W to 500W decices around for free or cheap like old hairdryers, haircurlers, old toasters, travel-hairdryers....

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[*] posted on 21-1-2005 at 11:45


Where do you get oven cement from? Why not a coil made of NiCr like axe's?
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[*] posted on 21-1-2005 at 11:52


You get oven cement at any hardware store.
I cannot read your thoughts and you provided no link so I dont know what you mean with Axe´s coil.

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[*] posted on 21-1-2005 at 15:50


Sorry, I meant like the one axehandle builds for his furnace and such.
http://www.sciencemadness.org/talk/viewthread.php?tid=2171
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[*] posted on 21-1-2005 at 18:04


Halogen light bulbs and dimmer switch make good heat sources.
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