A little heat in my intercom box

The intercom linked to my Samsung phone system starts giving false calls every winter.
I know that it must be a moisture/condensation related issue as it never happens in the summer.

I have a spare of wires going into the intercom box and I want to fit a small heat source to keep the electronics dry.
It never gets direct sunlight so over-heating should not be a problem.
Approx Dimensions Width:100mm   Height: 140mm   Depth: 70mm

Any suggestion on the heat source ?
Bulbs would be no good as they burn out .
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EirmanChief Operations ManagerAsked:
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mcsweenSr. Network AdministratorCommented:
Just use a plain resistor of the same value as the bulb which you were considering.
EirmanChief Operations ManagerAuthor Commented:
That looks great mcsween
Two cons ....
I might be difficult to obtain in Europe and I would prefer a low voltage solution (eg 12v)

Aburr, What kind of resistor / rating do you suggest
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EirmanChief Operations ManagerAuthor Commented:
Thanks nobus ... interesting device(s). I never heard of them before.

Anyhow I have decided to use the transformers I already have lying around with resistors (probably wire wound).
I have several 12 Volt, One Amp PSUs
I have a 30 Volt 800 mA PSU
I have a 12 Volt, Three Amp PSU.

I just need some help in choosing the correct resistor(s).
I know the basics of Ohm's law, but this is more complicated, because of the heat calculation.
2/3/4 resistors in series or in parallel might be better because of the more even heat distribution.
it depends on how much heat is needed in kcal - without that; it's just cut and try
EirmanChief Operations ManagerAuthor Commented:
Perhaps something like the heat from a 5 Watt Mains Voltage Tungsten Filament Bulb.
Or perhaps the sidelight of a car.

The Intercom box itself has a metal housing and there is a good bit free space in it.
I would like the intercom casing to feel slightly warm (not hot) to touch.

I haven't got a clue where start.
Gerwin Jansen, EE MVETopic Advisor Commented:
If you prefer 12V, I'd just get a couple of power resistors, if you want 5W, your power supply would deliver 2.4A.

Or get 2 1Ohm 11W restistors like these and put them in series:
11W power resistors(then you'd get 6W / 3A)

Available in Europe for about 2 euros each.
what's the lowest temperature you expect?
EirmanChief Operations ManagerAuthor Commented:
what's the lowest temperature you expect?
Typical winter temperatures at night: 2 - 5 Centigrade.
Lowest: -3 to - 5
Frequently humid & foggy .... Dank!
Gerwin Jansen .....
Power Resistors .... I think that might be the term I was looking for.

This calculator is telling me about 20 ohms
The current is low .... I wonder how hot 7 watts really is .... time to experiment.
in a closed box, you should not need much, so the resistor solution seems to be the best and lest costly solution
i would start with the lowest wattage solution - 12 V - 1A then you need a 12 ohm resistor
it would deliver 12 W

if you double the resistance, the power will drop to 6W, so you cna experiment

to get the power lowest, you best insulate the box you want to warm up !!
EirmanChief Operations ManagerAuthor Commented:
I going to try two or three of these in series ....

It depends on how hot they get
yeah yeah - but on what voltage????
Gerwin Jansen, EE MVETopic Advisor Commented:
Hmm, miscalculation on my side... 12V - 24 Ohm would mean 0.5A -> 6W

The 2 Ohm - 12V would be 72W - at bit too much I guess :)
EirmanChief Operations ManagerAuthor Commented:
12 Volts   10 Ohms

So two in series would be 7.2 Watts which is under the 9 Watts Specified for the resistors
Three in series would be 4.8 Watts
Gerwin Jansen, EE MVETopic Advisor Commented:
Correct, I'd start testing with 4.8 Watts - if possible I'd squeeze a wireless thermometer in there.
byundtMechanical EngineerCommented:
I suggest using a regulated heating element, such as the Omega solid state device in mcsween's first Comment. Doing so will minimize the risk of overheating the enclosure, and shortening the life of electronic components it contains. Virtually all electronic components suffer reduced life if operated at temperatures above their rated maximum. A common rated maximum temperature is 50C (122F).

The real concern is condensation, which you can avoid by keeping the inside temperature at least 15 F (8 C) hotter than the outside. This temperature differential would reduce 100% relative humidity to 50% RH, which you can confirm by looking at psychrometric charts.

Assuming that one side of your enclosure is mounted on a wall or other substrate not subject to heat loss, you have (2*100+2*140)*70 + 100*140 = 47600 mm2 surface area, or 0.51 ft2. Assuming an overall heat transfer coefficient of 1 BTU/hr-ft2-F, the heat loss would be 1.0*0.51*15 = 7.7 BTU/hr (2.25 W). You should therefore be looking for a 2.25 W heat source.

If using resistors, it is necessary to pay attention to both the power rating and resistance value. The power consumed by a resistor is V^2/R. Common precision resistors are rated at 0.25 W. Power resistors are rated at 1 W or higher. I suggest operating the resistor at half its power rating or less to avoid burning it out.

In my opinion, you don't need to worry about distributing the heat in the enclosure. Convection currents will do that for you. You do need to worry, however, about burning wire insulation should anything accidentally bear against the resistor.

If you use a light bulb as the heat source, its life will be considerably extended by operation at a lower voltage than nameplate. Wikipedia suggests the life may increase as (V nameplate/V actual)^16 so using two 12 V lamps in series with a 12 V voltage source would give a 10 year or more life.

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byundtMechanical EngineerCommented:
Hoffman, a major manufacturer of electronic enclosures, has a pdf describing how you would calculate the heat gain in an enclosure.  http://www.hoffmanonline.com/stream_document.aspx?rRID=233309&pRID=162533 Using the chart on page 1 of that pdf, you would see that 4.5 W/ft2 of exposed surface would raise the internal temperature by 20 F (the calcs in my previous Comment had estimated only 15 F temperature increase).

I had also made reference to a psychrometric chart, such as provided here: http://www.coolerado.com/pdfs/Psychrmtrcs/0000Psych11x17US_SI.pdf   You would use such a chart to determine the amount of temperature rise needed to move from 100% relative humidity to one that allows more reliable operations of your intercom equipment. You do so by moving horizontally on the chart from a point on the top edge of the chart (100% relative humidity at varying temperatures) to the desired relative humidity (50% in my previous Comment). Once you hit the desired relative humidity curve, estimate the internal wet bulb temperature by interpolating between the isotemp contours (wet bulb temperatures) that slope downward to the right. Moving horizontally means that the ratio of moisture to dry air remains constant. As an example, you might move horizontally from 35 F and 100% RH to just under 45 F and 50% RH--so 10F increase in temperature would reduce the RH from 100% to 50%.

Using both the Hoffman chart and psychrometric chart, you would only need 1.12 W heat input to drop from 100% RH to 50% RH at an outside temperature of 35 F (2 C). That's half the heat input I had estimated using my rules of thumb.
EirmanChief Operations ManagerAuthor Commented:
This topic is getting more interesting as time goes on!
Great posts byundt & byundt

The housing is all metal with some plastic at the front.
There is loads of room in the housing. If the depth was reduced by 30mm there would still be enough room.
As an aside ...... I think all scientific mention of temperature should be confined to Centigrade.
It's so much more logical and intuitive. (Except @ -40 -:)

I watched an edition of "The Universe" last week (on H2 (history2)) and they were discussing the coldest places in the universe. They used Fahrenheit and Kelvin simultaneously.
I think National Geographic use Centigrade most of the time.
Hi nobus,  I'll give eenookami the benefit of the doubt for removing the posts because they were "off-topic".
(For anyone new to the thread, my comment was critical of some over-zealous, 'by the book' moderation).
They are now gone to private messaging between me and the mods, although an open discussion in 'Expert Input' would be better. The private message to me started ...
"I have contacted you directly as to not divert the conversation in your thread as well as to explain ...."

At least my appeal, not to delete mcsween's post was heeded.
a good insulation in the box could help also; maybe no heater needed then?
>>  They are now gone to private messaging between me and the mods, although an open discussion in 'Expert Input' would be better  <<  i think that too; it looks to me that EE (with good intentions) is restricting too much, without discussion possible - that's how i see it
EirmanChief Operations ManagerAuthor Commented:
Great comments all around with some interesting side issues.

I'll post back here in the spring and let you know how I got on.
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