Help me choose a heating element...

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With another baby due in February, I'd like to make a blanket/towel warmer. Something along the lines of a small cabinet with the heater in the bottom drawer, and air vents to allow heat to rise to the upper drawers. Easy enough.

The concepts of control are, as well, easy enough, but I'm going in circles on choosing what to use as a heating element. Since this would be going in my home (in my baby's room, no less), I'd like it to be as safe as possible.

One of my thoughts is to buy a power resistor with a rating of 200-300 watts (they're cheap enough), and only run it at 100 watts or so. A fuse would, of course, be used, as would multiple temp sensors, and probably a thermal fuse as well.

It would not, of course, be run directly from the mains. I have enough PC power supplies around that I would probably use one of those, and control the 12V line via a FET.

Any thoughts or feedback on that plan?

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Not sure where you are located (or if that makes a difference in this case) but what about the products found here:

http://www.watlow.com/products/h...

Regards,
Steve

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Interesting stuff. It looks like they're meant for much higher temperature and power than I'm looking for, but I'll peruse their site and see what they have!

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Another possibility (maybe):

http://www.omega.com/heaters/hsc...

Specifically:

http://www.omega.com/toc_asp/sub...

Regards,
Steve

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be careful you dont build a baby cooker by accident.

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Yeah, that's why I'm trying to get lots of sanity-checks on it. :D

Since it would be heating a cabinet mounted on the wall, I don't think there's much chance of cooking the baby, but starting a fire, or even just a smolder, is the obvious worry.

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A question I might ask is... why? A search for 'towel warming cabinet' brings pushing 200,000 hits, starting from around $150. Let someone else take the development effort.

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You are going to tick off your woman if you start messing around with a cabinet and wires and electricity to put the baby into.

Maybe a halogen accent light would be safe. Those things heat up pretty good and wont start a fire.

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How about a 100 watt incandescent "Edison base" light bulb (normal A19, similar to a desk lamp). Or you could just buy an EASY BAKE OVEN and place towels in instead of little confections :lol: Seriously, put an incandescent lamp in a tin box with a stat! Or, just put "towel warmer" on your letter to Santa!!

John

By the by, thanks for the visual on the Logon!! Gluteal-cleft/ baby towel warmer Hhhhmmmm O....K....

Just kidding:-)

Just some guy

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outer_space wrote:
You are going to tick off your woman if you start messing around with a cabinet and wires and electricity to put the baby into.

Maybe a halogen accent light would be safe. Those things heat up pretty good and wont start a fire.

I don't know if I gave the right impression, the baby certainly won't go in the cabinet - it would just be used to store towels and blankets, and they'd be taken out to wrap or dry the new baby, my toddler, my wife, or myself.

If anything, my wife would be ECSTATIC to be able to use warm towels herself... she's a very petite lady, and gets cold pretty easily. With the provision that it's safe, she's pretty excited about it. Every time my wife gets out of the shower and wraps in a warm towel, I'll score points all anew. :D

I thought about a light bulb, but don't really care for the extra light. I have access to a ton of CPU heatsinks, and am currently thinking about some 50-watt TO220 resistors bolted to the bottom of a few of them. They're free, and are capable of dissipating around 100 watts and still keep the resistors 50 C under their rated spec.

Each resistor would have its own thermistor and thermal fuse, with a couple more thermistors and a thermal fuse for the air. To minimize the thermal cycling, I will probably PWM the power to the resistors. It'll be a good excuse to practice PID control. I'm sure that is overkill, but hey, it's easy and fun, so why not?

But, those are just current thoughts. I'll keep thinking about this for a while and see how things seem later.

As to the "Why?" question, there are several reasons. First, it will be fun. There's nothing I enjoy as much as engineering. Second, most of the cabinets out there aren't that great, and the better ones are rather expensive. $50 will buy me a build-it-yourself cabinet that is easily modified, and $30 will go a long way toward power resistors, thermistors, and thermal fuses. I have nearly everything else needed (PC board, micro, 4x16 LCD, buttons, etc.) just sitting around, I'll just need a few new values of resistors to round things out.

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I was just having fun! I do think it would be nice to have some toasty towels on hand. We (my gal and I) used to wrap her peewee up in warm clothes right out of the dryer. Good luck!
John

Just some guy

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Maybe you could use a TO220 regulator(s) the thermal cutoff feature. Run them hard and then they will turn off and on as the thermal cutoff feature comes into effect.

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That is an idea. I figured that as long as I had the AVR in the mix, I could do all kinds of fancy overkill-type stuff for fun. Things like PWM the fans on the heatsinks to keep the resistor at a fixed temperature, and then PWM the resistors to keep the chassis at a fixed temperature.

We'll see how much of that I actually do, if I follow through with the project at all.

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Make sure you have a smoke detector/fire alarm nearby the towel warmer, so worst case scenario is just a burned cabinet :)

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Gluteal-Cleft wrote:
Yeah, that's why I'm trying to get lots of sanity-checks on it. :D

Since it would be heating a cabinet mounted on the wall, I don't think there's much chance of cooking the baby, but starting a fire, or even just a smolder, is the obvious worry.

Well, if you put a thermal safety cut-off, commonly called a Klixon, you can automatically shut the thing down at a predetermined temperature. Klixon thermal cut-offs come in a variety of preset temperature settings.

In fact, taking a trip down Google and keying in on "Klixon " brings up the web-site:

http://www.sensata.com/products/...

Which references to the Klixon manufacturer.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston