4 x AA/AAA powered (controlled) heater

Discussion in 'The Projects Forum' started by mstrdenz91, Oct 18, 2011.

  1. mstrdenz91

    Thread Starter New Member

    Oct 18, 2011
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    Hi,

    I am working on a project where I have to make a handheld device wich contains up to a maximum of 4 AAA/AA batteries.
    The device has to be heated to around 60-65°C and stay at this temp (great accuracy is not needed).
    The device contains a PIC controller which will be responsible for (if needed) PID controll software and PWM output. The temperature sensor is not a problem for now.

    My question is what is the best heating element I could use for this?
    At first I was thinking about a peltier element but they seem to consume to much power and I dont necessarily need the cooling so what are my alternatives?
    I have searched the web but it's hard to find AA/AAA battery powered solutions...

    Any further advice on this is welcome!

    Thanks in advance,

    Greetings, Dennis
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Simple resistors would suffice. However, unless the area being heated is very small those batteries are not going to last very long.

    A µC is major overkill for this, a simple op amp circuit using negative feedback one a sensor such as a thermistor would suffice. You can also get temperature controlling diodes that will conduct under the temperature and cut off at the temperature. I looked for references for this component (can't remember the name) but couldn't find it. You could also use an old bimetallic strip, there are lots of ways to do this project.

    What are you trying to heat?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Nichrome wire is commonly used as heating elements. It was the invention of Nichrome wire by Albert March in 1905 that lead to the development of the toaster.

    Without knowing what size of an object that you will need to heat, it's difficult to suggest what resistance wire to use; but it'll have to be pretty low. Four AA alkaline batteries in series gives ~6v when fresh, less if heavily loaded (which they will be).

    So, you decide how many Watts you need to heat up your object. Then P/E = I, or Power(Watts)/Voltage = Current(Amperes) required to produce the heat.
    Then, R=E/I; the resistance of your wire in Ohms = volts/current in Amperes.

    Nichrome wire has specifications of Ohms per length; where that length might be inches, feet, mm, meters, etc. If your resistance is too low, you'll kill your batteries very quickly. If resistance is too high, you won't be able to heat your item up enough.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    I know of several local sources of nichrome wire, but overall resistors are easier to get. Can you solder to nichrome wire Wook? Never tried, so I don't know. On my plastic impulse sealer they use crimp on lugs for the two nichrome strips.

    65°C is pretty low temperature, for folks in the USA it is 149°F.

    Myself, I would focus on a simple sensor scheme first. Is this going to be a small hot plate or an enclosed oven? It matters, a lot. I have designed and built a lot of hot plates, but never using batteries. I have also rebuilt a lot of bad oven designs, but all of them were pretty large overall.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Yep, resistors would be a good bit easier. You can wire-wrap Nichrome (if the diameter is small enough), or use crimp-on terminals. Soldering doesn't work easily.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    If you are concerned about battery life then you should consider a peltier cooler. It is actually more efficient in providing heat then a simple resistor since it takes heat from the ambient (hence the cool side of the element) to add to the heat produced by the resistance in the element (it's a type of heat pump). The down side is that peltiers tend to require high current.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    I've used a lot of peltier coolers, I don't think you will get as efficient as resistors. I could be wrong, been so many times. I would still like to hear what the OPs surface or chamber is going to be.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    As noted, a Peltier is always MORE efficient than a resistor because it can actually move some ambient heat to its hot side, in addition to all the resistive heat it makes (battery power consumed).

    They do require current, but they're actually more efficient at moving heat from one side to the other at current levels far below their max, so I think it's worth a serious look for this.

    A Peltier small enough to power with batteries will be physically small, even for a handheld device. A 10A max module is about 1 inch square, and operates great at 2A. For battery power, it may need to be, say, 1/4th that size, or half inch square. They're also fragile, so these properties might be a problem for this application.

    [EDIT] One huge point point though: For the Peltier to extract heat from the ambient air, its cool side obviously needs to be cooler than ambient so that heat flows into it. This defines a ∆T across the device, which might be 40°C in this application. That's could be a limiting factor.
     
  9. BMorse

    Senior Member

    Sep 26, 2009
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    If you use rechargeable NimH batteries, they will last longer, and output more current than alkaline or others....

    You can also use some really fine stainless steel mesh or screen as the heater element, use a solid copper wire (14 gauge) as the connection between the battery and "screen", or else you would just heat up the wire connecting them if you use small or stranded wire..... This actually works pretty well, enough to produce up to 280 F + within a few seconds, and only using 1 AA..... I had designed a small device for someone that uses this similar design.. :)
     
  10. someonesdad

    Senior Member

    Jul 7, 2009
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    My first engineering worry would be that batteries are not going to have enough stored energy for your task. If the volume is a few cubic mm, then they may be sufficient. But if the volume is more than, say, on the order of a cubic cm or so, you may have energy problems.

    Here's a back-of-the-envelope estimate. An alkaline AA battery has about 2500 mA*hr capacity. This is 9000 coulombs. If we assume the average voltage of the battery when delivering this charge is 1 volt, then that means 9 kJ of available energy (charge times potential equals energy). If you use 4 AA batteries in series to deliver around 4 volts (6 volts when the batteries are new), then you're talking around 36 kJ of energy. You want to maintain a temperature of about 65 deg C, so that means a 45 deg C differential from ambient. Assume an insulating material with thermal conductivity of 0.1 W/(m*K) (this is what paper is; foamed plastics can be a few times less). For simplicity, assume a spherical shell 1 cm inside diameter and 1 mm in thickness. The surface area is 314 sq mm. If we assume the interior surface is held at 65 deg C and the external surface is 20 deg C, then the heat flow will be, per Fourier's law (thermal conductivity times the temperature gradient), 4500 W per square meter. Over the given area, that's about 1.5 W. That's a lot of power for battery operation.

    Then, assuming 3/4 of the battery's available energy is available for heating, that means you could run the heater for (3/4)36 kJ/(1.5 W) or around 5 hours. The surface area varies as the square of the diameter and the heat flow decreases linearly with increasing insulation thickness, so you can see the benefits of reducing the size of the heated/insulated area and increasing the insulation thickness. You can fiddle with the area, insulation thickness, and thermal conductivity to see if you can get to some configuration that's acceptable. I suspect these basic physical considerations explain why you don't find many battery operated heaters like you're interested in.

    If such battery life is acceptable and you don't need precision, I'd first suggest taking a look at Roman Black's nice solution http://www.romanblack.com/xoven.htm to see if something like that might fit your needs.
     
  11. mstrdenz91

    Thread Starter New Member

    Oct 18, 2011
    2
    0
    Thanks for some great tips!

    To clarify some more:
    I need to heat up a biochip to speed up a reaction,
    this will be done inside the device.
    The chamber the biochip will be put in is around 2,5 * 3 * 3 = 22,5 cm3
    I thought since its a closed chamber it won't dissipate to much heat but maybe not...

    I'll try to crunch the numbers and look at the possibilities you presented.
    It might come down to a choice between shorter battery life or lower temperature (therefore longer reaction time).

    If i have more questions ill check back.
     
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