Need for heat

Hi Everyone,
I'm doing a experiment were I need to get a ¼" x 8" x 12"
piece of aluminum to 150-200 degrees F. Outside temperature would be
around 70 degree F. Whatever I use needs to run off of 6-12vdc. In the
past I used a light bulb in a metal can (120vac), but it is too hard to
regulate the temperature that way for me.
I was thinking of using a Peltier type chip for a heat source. I find
a lot of info about its cooling affect (or lack of) but not much about
the heat effect. Could this be done effectively on battery power
(6-12vac)?
The other way I was thinking about is to use resistors. But I have no
clue on the size of the resistors or how many I would need to get to
the metal too that temperature. Anyone have a formula or past
experience about this?
Or is there something a lot easier that I'm not thinking of?
Thanks Jeff

 

Hi Everyone,
I'm doing a experiment were I need to get a ¼" x 8" x 12"
piece of aluminum to 150-200 degrees F. Outside temperature would be
around 70 degree F. Whatever I use needs to run off of 6-12vdc. In the
past I used a light bulb in a metal can (120vac), but it is too hard to
regulate the temperature that way for me.
I was thinking of using a Peltier type chip for a heat source. I find
a lot of info about its cooling affect (or lack of) but not much about
the heat effect. Could this be done effectively on battery power
(6-12vac)?
The other way I was thinking about is to use resistors. But I have no
clue on the size of the resistors or how many I would need to get to
the metal too that temperature. Anyone have a formula or past
experience about this?
Or is there something a lot easier that I'm not thinking of?
Thanks Jeff

Howbout a light dimmer with the globe setup?

 

For a ball-park, back of the envelope estimation, your plate should run
around 2 to 3 C/W. You're going from about 20 C to 80 C, so you'll need
to dissipate something on the order of 150 W to maintain that delta-T.

Far too many unknown parameters. How rapidly does it need to come up to
temperature? How closely should temperature be maintained? Any issues
with water or critters getting into the works? Will it always be
observed by an operator or does it need to run for long periods (days,
weeks, more) unattended? Etc.

A simple bang-bang thermostat controller could be put together with a
bimetallic strip; or a thermistor, a comparator, and a few resistors; or
go all out and drop in a small microcontroller for a PWM controller.

 

Hi Rheilly and Rich,
If I stay with using a light, a dimmer would be the ticket. I don't
know why I didn't think of that before, Great idea, Thanks.

Rich. I would not need it to heat up right away. It could take as
long as an hour or so to get to temperature. It will be out all night
or longer unattened. I was thinking about using a Pic16F627a or the
16F628a and a thermistor to control the temperature. Also, it would
have a roof over it and a weather proof box for the electronics.

So you think resistors could heat up the aluminum? If so how, what
type of battery voltage am I looking at and how many mah (or amps)
would I need? How long would it last?

Thanks Jeff

 

You can get metal-cased power resistors that are intended to be bolted
to a heatsink - these would be most suitable for your application,
because they are designed to make good thermal contact to the mounting
surface (your aluminium plate). Rich calculated that you'd need 150
watts - if you're powering this from a 12 volt battery, it would draw
about 12.5 amps. A Group 24 (traditional car battery size) deep cycle
battery is rated at about 60 AH, if I recall correctly - this would
theoretically power the heater for under four hours. However, for
best battery life, it is recommended that you don't discharge below
50%, so you shouldn't run it for more than a couple of hours.
(actually, the AH rating of a battery is based on a 20 hour discharge.
Discharging in 4 hours will give you much less than the advertised
AH.)

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

Hi, Jeff. If you're considering sticking with line voltage for power
(a good idea -- 200 watts is something like leaving the car headlights
on, your 12V battery won't last long), you also might want to consider
just cannibalizing a drip coffeemaker hotplate. The heater element can
be affixed to the aluminum without too much difficulty. The built-in
bimetallic switch will take care of the temp control.

If you're a little crafty in how you mount the heater, you can achieve
pretty much the right temperature, and it will have the ability to
compensate for changes in line voltage, or an occasional breeze cooling
the surface of the aluminum. A lamp dimmer doesn't have this
capability. Also, you'll have the capability to come up to temp
quickly.

On the other hand, the coffeepot hotplate temp controller will have
some hysteresis, and the aluminum plate temp will go up and down a bit.
However, from your original description, it doesn't seem that really
precise control is much of an issue for your experiment, just that it
be within the range.

And yes, if you aren't familiar with using line voltage safely, this
solution is not for you.

Good luck
Chris

 

Hi Peter,
Thanks for the info. But I think Rich maybe wrong about the 150 watts
needed. I only say that because when I was experimenting last year I
was using a (110vac) 40 watt oven bulb in a 6" diameter metal duct
pipe. It would reach 195 degrees F. in about 30 minutes or so.

The only reason I was thinking of using a battery is so that it would
make the unit portable, but unless I can figure out a way to get the
amps down I don't it is going to work. Draging out a battery the size
of a car battery and than having to charge it is to much work. I would
be better off going with 110vac.

If I used the 40 watt bulb again what type of switches would I need
to use with the microchip? The chip is only good for 25ma dc. So what
type of switch(s) does a person use to turn on and off 110vac?

Thanks Jeff

 

Hi Chris,
I can't believe all that great ideas I'm getting and how come I
didn't think about them. Anyway the coffeepot heating element is a
great idea. I happen to have a hot plate that I've been dragging around
for 20 some odd years. I see now that I should be able to use that to
get the heat I need.

I have been using this to attract mosquitoes. It is a fun experiment
to play around with. I'm very good at working with line current, it is
the small electric circuits that I'm just begining to learn. Do you
have any idea of what type of switches to use with the pic16f627 that
only puts out 25ma. I'm thinking the switch would need a rating of 15
amps @ 110vac.

Thanks Jeff

 

I would use a solid state relay that is drivable directly with the
logic output. A mechanical relay is a bit cheaper, but takes more
interface circuitry. At 15 amps load, the solid state relay will need
a bit of a heat sink, since it wasts about 1.5 watts for each ampere
it passes.
Something like:
http://rocky.digikey.com/WebLib/Crydom/Web Data/EZ Series.pdf

 

Well, there was a fair amount of hand-waving there. Some effects, also,
for having the heat source enclosed in a duct as compared to being
mounted on one side of a metal plate.

You could use a relay, typically driven by a switching transistor that
turns on (sinks to ground) to give a path for the coil current. Do some
reading at Omron's web site for good info on the how-tos. For example:
<http://oeiwcsnts1.omron.com/ocb_pdf...56AF80060A947/$File/M20Z54E11.pdf?OpenElement>

You could also go with a solid state relay, or a diac/triac pair, or a
MOC3011 or MOC3031 plus a triac, or ...

 

As I understand it, they are attracted by heat and CO2, so maybe a
small propane catalytic heater would be more appropriate. That is what
the mosquito magnet(tm) and it's imitators use.


--Dale

 

Hi John,
Thanks for the link. I'm thinking I need the EZ240A12 120vac @ 12
amps. Do you hook up pin 1 and 2 like a simple switch, Black wire
(power-in) Black wire (power-out)? I know there are other ways to wire
a simple switch but is this the way EZ240A12 does it, meaning does it
let the power go through Pin 1 to Pin 2 when Pin 3 goes over 3vdc? If
so then all I have to do is have a input/output pin go from low to high
to turn it on? Also do I have to protect the I/O with a resistor or
diode? If it is this easy it is going to help me out with another
project. I'm just finishing a air-hockey table for the kids. Actualy it
has been in use for quite awhile but I am now finishing up on the
controls. Scoring and being able to turn the fan on/off automaticaly. 2
problems solved in one night, how lucky is that.

Rich I hope I didn't make you mad it was not my intention. I know my
description was n't the best, I guess I was hoping that I there might
be a way to generate heat without burning up alot of amps. Like maybe
charging up a capacitor and then have it go through a resistor so that
I could still use a small battery power pack.

Dale thery are attracted 4 different ways, heat, co2, moisture and
smell. Actually I think there is a five and that is motion. My idea is
to keep it simple and inexpensive, that way I can put more lures/traps
up to get better results. Heat and co2 are the best attraction but I
haven't gotten around to the co2 yet (except to run through a long
tube, it works).

Thanks again Jeff

 

John,
After looking at the spec sheet I was wondering what rms stands for?
It has Arms which I figure A stands for amps. mArms which is mill-amps
and then Vrms v for volts. Is this correct? But what is rms?
Thanks Jeff

 

(snip)

There are two pairs of terminals on these things. DC (3-32 volts)
applied across one pair turns on the power path through the other
pair. The two pairs of terminals (control and output) are isolated
from each other. The DC control current turns on an internal LED,
which triggers the circuits in the power switch. I can't find the
data sheet for the variation you list (and Digikey has none in stock)
so you need to look for a unit that has the 3-32 volt DC input and a
120 or 240 volt AC output with a current rating high enough for your
load. A little margin is a good idea on that last spec. I would also
look for one that specifies zero voltage turn on, so it doesn't make
any radio frequency interference.

 

RMS stands for square Root of the Mean of the Square of the
instantaneous value. This is a mathematical way to measure an AC
current as a number that represents its heating effect on a resistor.
In other words 10 amps AC RMS heats a heating element the same
amount as 10 amps DC would. But 10 amps AC RMS hits peaks of plus and
minus 14 amps each cycle and also is zero twice a cycle.

Same goes for volts AC RMS or Vrms. As long as you are dealing with
resistors and sine wave AC, Vrms and Arms is the normal way to measure
volts and amps, and is what a multimeter will show.

 

Grrrrr...

(Just kidding, of course.) Actually, for a back of the envelope swag I
think I got pretty close!

Chris's idea of using a coffee warmer sounds like the best so far for a
one-off project.

 

Hi John and Rich,
Thanks for all the info. I think I now have a better understanding on
how to go about phase two of my project. The heating element out of a
coffee unit sounds like the best way to go. I'll be ordering some
relays this week and I'm also going to go around the thrift and
recycling centers to see if I can get some coffee pots cheap.
Thanks Again Jeff

 

Didn't we answer this same homework quesiton just last semester?

Look up "thermal resistance" and stuff.

Good Luck!
Rich

 

If you get a hefty enough dimmer, you could use it on an ordinary toaster
oven. That'd get you to temp in minutes - but with a real toaster oven, it
might have a built-in thermostat that'd get you between 175-200 with a
twist of the knob.

Good Luck!
Rich