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Need a small 12v heat source for simulated HVAC project Answered

Okay, so for our year end micro controller project we need to control a temperature inside two 2L pop bottles connnected together. We have a computer fan mounted at the end of the "heating bottle" which needs a heat source in it. The fan will blow the hot air from "heating bottle" into "temperature bottle" which contains an analog temperature sensor. We have to control a set reference temperature point in "temperature bottle" using a PIC16f690 microcontroller by manipulating the fan speed and heat ON/OFF. Problem: I need a decent heating source inside "heating bottle" . Ive been using 12v automotive lightbulbs which work great for a minute or two but then die out. My crappy power supply can only pump out 1 amp, could this be why? Any suggestions on what to use as a heat source? I was thinking some 5w resistors but classmates have said our power supplys cant handle them either. Thanks in advance!

22 Replies

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tech-king (author)2008-04-21

solid state relay.... do you mean scr? or triac? and good for you.

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Korey (author)tech-king2008-04-23

Nope, i mean solid state relay.. It says right on the relay

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Korey (author)2008-04-21

I ended up getting a 5v solid state relay that works amazing. I was then able to hook up a 60w lightbulb and it works perfect! Valuable learning experience none the less...

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westfw (author)2008-04-13

I don't understand why your 12V bulbs are burning out. NM implies that you broke the bulb and let air in, but you don't say that.... 12V at 1amp is 12W, so in theory, all you need is a 12ohm power resistor (say, 20W) which will make a fine heat source. (or you can use a combination of smaller resistors that work out to the same values, like a series chain of 12 2W 1ohm resistors.)

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NachoMahma (author)westfw2008-04-14

> NM implies that you broke the bulb and let air in, but you don't say that... . Not sure how I came to that conclusion. On re-reading the post, it certainly doesn't say, or even imply, that. Oops.

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guyfrom7up (author)westfw2008-04-13

that'd work You wouldn't get any hotter using any other source inless you increase your wattage.

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NachoMahma (author)2008-04-12

. 12V filaments will always burn up quickly in air (when run at 12V). You can extend the life by lowering the voltage, but this will decrease the heat output. . A 120V filament will probably last long enough for your project. Or steal the heating element for a small space heater - you'll probably need to shorten it a bit. . If your PS is limited to 1A then the resistance of the heating element needs to be 12 ohms or greater.

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Korey (author)NachoMahma2008-04-12

I would like to use a 120v filament, but i need a way for control ON/OFF with a 5v output from the PIC microcontroller...and dont know how. The bulbs all have a resistance of 2 ohms, so that explains that problem

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tech-king (author)Korey2008-04-13

a transistor acting as switch! energize the base through a 1k resistor to turn power from the collector to the emitter on. ground it (de-energize) to turn it off.

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Korey (author)tech-king2008-04-13

Thats how im running the 12v fan..But i can switch 120v with a transistor???

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tech-king (author)Korey2008-04-14

yes. but a power transistor, like a 2n3055 (is that the right number?), not a 2n2222 or some other wussy thing like that.

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guyfrom7up (author)tech-king2008-04-13

doesn't resistor size depend on the hfe of the transistor?

I always use:
1.3(A/hfe)/E=R
Where A is in mA and it's how much current you want through the transistor
hfe = hfe of transistor
E = voltage used (with a uC it's usually 3.3 or 5)
R = Resistance in ohms needed.

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tech-king (author)guyfrom7up2008-04-13

yeah. you can get by with between 1k and 10k, but its better to calculate the true value needed to prevent overheating.

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NachoMahma (author)Korey2008-04-12

. BTW, 12V x 1A = 12W. Your not going to get a lot of heat out of that. Probably enough for your project, though.

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NachoMahma (author)Korey2008-04-12

. Put 6-8 of 'em in series.

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NachoMahma (author)Korey2008-04-12

. Either use a power transistor/triac/&c; or an interposing relay (5VDC coil with contacts rated for your load)

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guyfrom7up (author)2008-04-12

use the elements in toasters

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tech-king (author)Korey2008-04-13

yes. and considerable amps too.

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guyfrom7up (author)tech-king2008-04-13

you could reduce the voltage by cutting it into strips so that each strip has less resistance.

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tech-king (author)guyfrom7up2008-04-13

if by that you mean run them in parallel, that would use even more amps

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