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i am new here. First of all I am sorry for my bad english.

I want a help from those who know about nichrome heater coil. I want to make a 12v dc incubator using nichrome wire as heating element, all I need is to produce almost 40 degree celsius in my styrofoam box. For this, how much length nichrome required? Which gauge ? How much watt does it consume? ( To know the the approximate backup time from a 12v 9ah battery) . How to calculate these things? Please help me.


i think if i connect a small length (small resistance) nichrome to 12v battery, most of the volt dropped in internal resistance of the battery, that will drain battery fast. If i connect a long nichrome to the 12v battery, most of the volatge dropped in the nichrome , but it will reduce the current ( look like a 12v bulb lighting with a 6v source). So at a specific length of nichrome that will produce 40'c heat efficiently. Hiw to find that length? That is my question.

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    \$\begingroup\$ What is your ambient temperature? How does this ambient temperature change with time? What is the thermal resistance of the inside of the incubator to ambient? \$\endgroup\$ – Andy aka May 8 '18 at 9:51
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    \$\begingroup\$ How big is the box? How quickly do you need to heat it? What is the ambient (as Andy has asked)? Have you thought about how you will control it? How important will the control be? Does it need to respond OK to being opened? You need to have all of the details about the problem before we can even think to answer this in a meaningful way. \$\endgroup\$ – Rendeverance May 8 '18 at 10:33
  • \$\begingroup\$ Ambient temperature is almost 30 degree celcius, at noon it become 33 and 26 at night, i dont know what is thermal resitance. If it is the difference between ambient to inside the incubator, that is almost 7.5 degree celcius. \$\endgroup\$ – Shamnaz Kichu Mavayil May 8 '18 at 10:34
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    \$\begingroup\$ @Rendeverance last year I'd built incubator with philios 40 watt bulb and working perfectly, but that was in AC power. Now i want to make it dc. The box is 2ft length, 1.5ft w, 1.5ft hight. Material styrofoam, i have a thermostat that set to 37.5 celcius to cut off the haet source. And have a fan inside the box to spread the temperature. And it is ok to be opened. \$\endgroup\$ – Shamnaz Kichu Mavayil May 8 '18 at 10:41
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    \$\begingroup\$ Can you buy a 40W 12V bulb from an RV/camping supply store? Specifically, a 40W tungsten, NOT a "40W equivalent light" CFL or LED. \$\endgroup\$ – AlmostDone May 8 '18 at 11:42
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You don't have to deal with nichrome wire if you stay within the temperature limits of standard resistors, and 40°C stays within these easily. Buy a power resistor. The needed power depends on the heat loss. Calculate that first. $$\dot{Q} = \lambda\cdot \frac{A}{d}(\vartheta_{1}-\vartheta_{2})$$ $$\lambda: \mathrm{thermal\ conduction\ coefficient\ of\ styrofoam}$$ $$A: \mathrm{surface\ area\ (on\ the\ outside\ for\ being\ on\ the\ safe \ side)}$$ $$d: \mathrm{distance\ (thickness\ of\ styrofoam\ wall)}$$ $$\vartheta: \mathrm{temperature\ (highest\ inside\ -\ lowest\ outside)}$$

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When it comes to heat, watts are watts no matter where they come from. You've added the information that 40W has been found to be sufficient. This makes much of @Janka's answer on thermal conduction calculation (+1) redundant, but we can use the resistors as he suggests. You would need to test it anyway, since losses are often dominated by thermal shorts associated with the structure, unanticipated convection and so on, but it's never less than the conduction losses.

You know what resistance you need from \$P = V^2/R\$ so R = 3.6 ohms. You can easily buy a 50W chassis-mount resistor for a few dollars in 3.9 ohms (37W) or 3.3 ohms (43W).

enter image description here

It will need to be mounted to a large heatsink to transfer the heat to the air without overheating itself. You could also spread the heat source out by using (say) a number of similar 5W resistors, for example 10 33 ohm resistors in parallel, thus avoiding a point source of heat and reducing the risk a single resistor would fail taking all the heating power with it. The total power consumption is going to be the same in any case, by design (~40W).


A 3.3 ohm resistor will draw 3.63A so if the battery is good for 9Ah at 3.63A discharge, the backup will last 2.5h at full power (not counting the power your controller draws). A well-specified heater (in terms of watts) inside a regulation loop will typically be running around 33-66% power, so if you switch the heater on and off to control the temperature (which is what you should do), it should last more like 5 hours. If the ambient temperature happens to be very low (eg. -40'C) requiring the heater to be on all the time, the hold-up time will be more like 2.5h. If the ambient temperature equals the setpoint you won't need the heater at all.

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  • \$\begingroup\$ Thank you for the informations, now i want to test both methods. Using resistors and nichrome coil too. Still i want to know about nichrome length to be used as coil for my purpose, because later i may be try to build a huge incubator. If i know the calculations about nichrome coil that time, i can create that incubator without any problem. \$\endgroup\$ – Shamnaz Kichu Mavayil May 8 '18 at 14:47
  • \$\begingroup\$ You know the resistance. The remaining factor is watt density. It depends on construction and how hot you want to allow the wire to get. If you let it get very hot, IR radiation will become a factor. The limit is letting the wire get so hot life is severely impacted (like a toaster element which lasts only hours). It's worse than with a light bulb because it's in air. It also might burn something. \$\endgroup\$ – Spehro Pefhany May 8 '18 at 14:58
  • \$\begingroup\$ I dont know the resistance, how to know that as i dont know how much current it will drow . \$\endgroup\$ – Shamnaz Kichu Mavayil May 8 '18 at 15:47
  • \$\begingroup\$ If you need 40W and have 12V you know the resistance. \$\endgroup\$ – Spehro Pefhany May 8 '18 at 15:54
  • \$\begingroup\$ The 40 w scenario was earlier. Now i am trying to make a 12v dc incubator. Simply , my problem is.. i think if i connect a small length (small resistance) nichrome to 12v battery, most of the volt dropped in internal resistance of the battery, that will drain battery fast. If i connect a long nichrome to the 12v battery, most of the volatge dropped in the nichrome , but it will reduce the current ( look like a 12v bulb lighting with a 6v source). So at a specific length of nichrome that will produce 40'c heat efficiently. Hiw to find that length? That is my question. \$\endgroup\$ – Shamnaz Kichu Mavayil May 8 '18 at 16:09

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