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Nichrome wire heating project. help please!

Discussion in 'General Electronics Discussion' started by Slowbutsure, Feb 18, 2018.

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  1. Slowbutsure

    Slowbutsure

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    Feb 18, 2018
    Hi

    I want to make a nichrome silicone heater pad. I want to limit the maximum temperature to 70 deg c. Can anyone help me how to do that?

    Power: 12v DC (car battery)
    Nichrome wire is 20 guage 0.8mm

    Thanks!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
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    That sounds like really thick nichrome wire. Do you know what the resistance per meter is?

    How much power so you need?

    What are you heating?
     
  3. Alec_t

    Alec_t

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    To limit the temperature to a set value you will need some electronics to measure the temperature and provide feedback to a transistor/switch which controls current through the nichrome element.
    Thermal insulation around the heated object will be a major factor in determining how much power is needed for the heating.
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Why bother building when you can buy this one all ready to go at Wally-World? Just add a thermostat and 20 A wiring between the pad and your car battery.

    Of course you should bone up on thermodynamics to understand how this thing will get hot, and how it will transfer that heat to the surrounding environment. And it will be necessary to transfer that heat energy somewhere if you apply the full rated power of 240 watts (12 V at 20 A), otherwise the silicone covering will become dangerously hot. Even 70 C is dangerously hot to human skin, so some sort of power control will be necessary to limit the temperature rise.

    You have a wide choice of temperature sensors ranging from platinum resistance thermometers (very accurate but pricey) to thermistors (fairly accurate and cheap) with thermo-couples somewhere in between. All temperature sensors, except for bi-metallic switch thermostats, require signal conditioning and power control electronics.

    Since you are operating your heating pad with low voltage, high current, DC there are only two reasonable ways to control the power input: on/off or bang-bang control, and pulse-width duty-cycle modulation or PWM. Bang-bang controllers are usually simple bi-metallic thermostatic switches that complete the circuit when the temperature is too low and open the circuit when the temperature is too high. PWM or pulse-width modulation is similar to bang-bang control, but with precise control of the on-time versus the off-time as a function of temperature.

    Either approach works well with DC because there is little or no power developed across the switching element whether it is closed (completing the circuit to power the heating pad) or open (preventing application of power to the heating pad). PWM is preferable in almost all control applications because it allows a much more precise control of the average power input to the heater. And PWM modules are widely available at dirt-cheap prices from Pacific Rim countries.

    With a bit of research (Google is your friend) you might find a PWM module with a temperature controller function and appropriate sensor. Just make sure it can switch the maximum current that occurs when the heating pad is cold.

    BTW: 20 AWG nichrome is waaay too large. And ribbon nichrome would be a better choice for making a heating pad. Buy the Wal-Mart pad and blow it up first before trying to make a DIY pad.
     
    Externet likes this.
  5. Slowbutsure

    Slowbutsure

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    Feb 18, 2018
    I wish I could go to Wally world and buy that one! unfortunately its a few thousand miles away from me in the UK :) Ive ordered some Ribon. Thanks for the suggestion.

    My concern is it getting too crazy hot. I just want to know that in even the most conducive conditions it wont go over 70deg c.

    Would it work just putting a 50w resistor 6ohm? Along with a fuse and a switch.

    Thanks.
     
    Last edited: Feb 18, 2018
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Quit wishing and learn how to make Google your friend. Try this link to get stuff from Wal-Mart shipped to your address in the UK by using a parcel forwarding service.

    BTW: It's a world economy now in the 21st Century. Buy from anywhere, ship to anywhere with sufficient incentive and imagination... barring restrictive import/export regulations imposed by governments.
     
  7. Slowbutsure

    Slowbutsure

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    Feb 18, 2018
    Yeah, ive just ordered some 0.4mm ribbon instead. Im heating a metal fuel filter on a car. I just want to limit it from getting crazy hot e.g >70c. I dont need a specific temperature though.
     
  8. Slowbutsure

    Slowbutsure

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    Feb 18, 2018

    lol, its a pleasure being schooled by you my friend. I will check out the link and have a look at the time scales. I did look at getitng one of those from China, but it would basically arrive when the winter here is over, unless I paid more for shipping than the product itself. And on google, I get information overload to be honest. its like a super intelligent friend who gives you 1 million answers to a straight question.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    No. You need a temperature sensor to reliably prevent a temperature rise above 70 C. Here is one that opens at 180 F (~82 C). Just wire it in series with the heater and it will remove power when the surface reaches the switching temperature.

    This sensor is based on the Texas Instruments patented Klixon bi-metallic switch sensor. The patent is long expired so anyone can make them now. The device consists of two dissimilar metal coin-shaped pieces, concave stamped, and welded at the edges, thus forming a bi-metallic switch. It can be depressed by a spring on the convex side to allow it to "snap" to the other position at a certain temperature, where the formerly convex side now becomes concave and the formerly concave side becomes convex. It then remains in this switched state until the temperature decreases a sufficient amount to cause it to "snap" back to its original state.

    Some Klixon sensors do not reset. These are used in appliances as fail-safe cut-outs. If you decide to acquire and use one, make sure it automagically resets back to the closed-circuit condition when the temperature drops below the trip point temperature.

    Years ago at trade shows, Texas Instruments kept of bowl full of these gadgets to hand out free. They were designed to transition states at nearly body temperature, so you held one tightly in your hand to warm it up and then, using your thumb, manually switched it to the now-stable "hot" state. Then you laid it on a flat surface and stepped away, waiting for it to cool down to room temperature. This took several seconds, sometimes as long as a minute, but eventually it went "snap" and bounced up into the air when it cooled sufficiently... often startling passersby, which no doubt was the intent. Fun toy, but I have never seen them commercially marketed and the few I had have long since disappeared.
     
    Last edited: Feb 18, 2018
  10. duke37

    duke37

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    Jan 9, 2011
    I do not think you can measure the temperature of the wire directly since the thermocouple will affect the reading.
    You could measure the resistance of the wire and control it to a standard value. Some complication here.
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    No. The heated object will lose heat at a rate determined by its surroundings, so the temperature won't be limited to a fixed value.
     
  12. Chemelec

    Chemelec

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    Jul 12, 2016
    The Temperature of the Wire is Not so Important.
    You can determine the Current you want to supply and than determine the Length of wire needed for this current.
    Than you need to put the Nichrome Wire around the filter and Insulation around the whole thing.
    You also need get or make a Sensor to determine the 70 degree Cut Off Temperature.
    (It is Not too difficult to Make one.)

    The actual power required is mainly determined by the Mass of the Filter, The Outside Air Temperature and the Heat Loss through the Insulation.
     
  13. Slowbutsure

    Slowbutsure

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    Feb 18, 2018

    Thanks you have been very helpful. Could I ask one other thing? Could you give me a link to the actual sensor on that website? I can't find it on there. Many thanks.
     
  14. Slowbutsure

    Slowbutsure

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    Feb 18, 2018

    I presume you mean the temperature of the filter not the wire? Sorry if thats a dumb question.
     
  15. hevans1944

    hevans1944 Hop - AC8NS

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    The link was to a specific product offered at grainger.com, but perhaps that page was filtered out before it reached you because of your location in the UK. Here is another link to an alternative sensor that automatically resets and is less expensive. I am also going to attach a PDF copy of the page the original link pointed to, for whatever that might be worth to you.

    The good thing is, unlike Wally-World, Grainger sells world-wide. If you go to their international website for Grainger and register there, they will show you which items they sell that cannot ship to the UK. Hopefully, not very many, although I believe the Ajax Atomic Bomb Kit may be export restricted, if it is even listed anymore (or ever). But I would guess, given how long the patent has been expired, that these thermostatic switches would be widely cloned by Pacific Rim countries.

    As for Google giving too many results... it's just a damned machine. If you can't find what you are looking for on the first Google results page, it is likely that you used a poorly crafted search string. Try again using different words or a new phrase. Google uses AI software, but it isn't terribly bright yet. Give it a few years and it will get better. Google also tracks your web activity and uses that as input to the Google search engine to increase the likelihood of a meaningful (to you) result. And there are other search engines available too, but none IMO as good as Google. Give a few of them a try.
     
  16. Slowbutsure

    Slowbutsure

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    Feb 18, 2018

    Awesome, thanks. In terms of the Atom bomb the UK government would probably let it past customs without batting an eye lid. a thermostatic switch could be a bit harder.
     
  17. hevans1944

    hevans1944 Hop - AC8NS

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    @Slowbutsure, I am curious as to why you want to heat your fuel filter? Is this a diesel-fueled engine that is susceptible to fuel-line freezing because of the poor quality of the diesel fuel, or water condensation in the fuel tank? If it is a gasoline fueled engine, I would be worried about vapor-lock occurring with the heated fuel. OTOH, maybe heating gasoline to the vapor state, prior to mixing with air and sucking, or injecting, it into a combustion cylinder, would improve efficiency. So, please tell us WTF you are trying to DO on the other side of the pond! All of Detroit is anxiously awaiting with bated breath for news that will allow a miraculous recovery of the US auto industry. Well, all except Ford, which seems to be doing okay...
     
  18. Slowbutsure

    Slowbutsure

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    Feb 18, 2018
    I have started making Biodiesel as a bit of a hobby with a money saving edge. When the engine is warm the car runs perfectly. But on a cold winter morning you have to have pretty robust battery not to go flat before it starts. Biodidesel is thicker than normal diesel and thickens even more when cold.

    As to what we are doing here, im not sure anyone knows! I'm all for Brexit, never liked the European super state project. And I think stronger trading relationship with the US seems a much more British thing to do anyway. British people are not Europeans, unless your under 25. Only thing is if we get the US gas guzzling cars here we will go bust as we pay twice the price for 'gas' than you guys do.
     
  19. hevans1944

    hevans1944 Hop - AC8NS

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    Biodiesel seems to be a viable alternative to gasoline. You could probably run the entire British taxicab fleet using biodiesel recovered from McDonald's used cooking oil. Have you thought about also placing a "dip stick" type heater in the fuel tank to help liquefy the biodiesel fuel? These are popular in the States for heating the engine block in the winter, by heating the oil in the oil pan reservoir. Makes it easier for the engine to turn over while cranking. Wrapping the fuel lines with heating tape might also be a "good thing" to do, along with adding thermal insulation to everything.

    Bio-derived fuels have always interested me, especially plant organisms that can convert sunlight directly into electricity. I don't suppose that counts as a bio-fuel because the Sun is providing the energy, but there are other living organisms that can be cultivated and harvested for fuel. Corn isn't one of them that we should think about using in massive quantities. No need to starve Peter so PauI can drive his SUV. I hope the ethanol-added-to-gasoline fad has just about passed, because of increased production from off-shore drilling and oil-sands recovery of petroleum. Plus, I am certain that the deep ocean bottoms have been accumulating dead organisms for millions of years, slowly turning all those unused hydrocarbons into something we can use for fuel, if only some means can be developed to recover it...
     
  20. Slowbutsure

    Slowbutsure

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    Feb 18, 2018
    I use methanol a lot, and its really nasty stuff. I think they use it in racing cars. I always imagine cars blowing up!
     
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