Electrowarmth 12 volt mattress heater

[daveb]

Being AC watts and DC watts are basically a single unit of measurement the difference between the two only hinges on a few things. One issue with a AC electric blanket or bed warmer is they do not do well on a modified sine wave converter from what I've read. Secondly, the fact it costs you battery power just to invert DC, the power draw can stack up a bit on an AC blanket compared to a DC version. Not like it's a huge load by any means but if I were to use one I'd get a pure sine wave converter that is as close to the draw of the blankets high setting as possible and use it for that only. Lastly is the fear of AC current and how it effects the body but hell as a lineman, I've been handling high voltage for ears sand mothing isn rong wit mee.

[ctb]

Dave...you funny sob (respect)!!!

[E350]

Can I ask a stupid question?* Are you guys sure that there is any wiring difference whatsoever between a DC and an AC electric blanket? In other words, JWA why don't you try hooking your AC electric blanket to DC current and see whether and how well it works or not? Or pick up a used electric blanket at Salvation Army or Deseret Industries for the experiment if you don't want to risk experimenting with your existing blanket?

I mean isn't it just a resistance wire coil with a rheostat wheel to control the heat (i.e., a continuously variable electrical resistor used to regulate current)? Does it really care whether or not it is DC or AC current flowing through the resistance wire? And daveb knows way more about electrical stuff than me, but why would a resistance wire care if the AC current wave is pure sine or modified (square) sine?

Edit: All of my questions are answered here:

http://www.homesteadingtoday.com/specia ... anket.html

* That is itself a stupid question, since I am only here to ask stupid questions...

P.S. I got my Pryopen today but it looks so delicate I don't think I could pry open anything with it. So instead I am going to try it outside this weekend to solder an extension of the light wiring for the tongue which I extended on one of the trailers.

[JWA]

E350 the only real issue that might arise from using the AC powered bunk warmer (home version aka "mattress heating pad") with an inverter is whether the AC controls would be compatible with the inverter's output. If the more particular or delicate electronic devices don't fare well with certain types of synthesized sine wave outputs that might be an issue.

I have a watt meter I'll use with my bed's heater pad, see what AC wattage it pulls on a comfortable setting----reports to follow.

Of course the only reason I mentioned this is the limited sizes available for the DC powered versions----just don't seem compatible if the SO is along for the overnight stays.

[boywonder]

As daveB mentioned.....Watts is Watts, AC, DC, power to the rear wheels, whatever your want....

...and yes the blankets are simpler resistive heaters......so heat produced is I squared times R per Ohms's law.

An AC electric blanket rated for 110V is going to have way higher resistance than a 12V heater so you'll get some heat out of running one at 12V, just very very little. Notice the I (current) term above is squared.......when this gets reduced due to increased resistance you wont get much power out.

You would get similar results trying to run a 110V water heater element at 12V.....not much hot water; the resistance of the element is way too high.

A Watt is a unit of power, which is energy divided by time. There are 746 watts in one horsepower.

[E350]

boywonder and JWA: So even though Watts is Watts regardless of AC vs DC, the resistance wire for a 120v electric blanket will be larger than in a 12v electric blanket, such that the former when powered by 12v will throw off less heat - presumably because the larger reistance 120v wire creates less resistance for the 12v current? Got it. Thank you.

[JWA]

Originally Posted by E350

boywonder and JWA: So even though Watts is Watts regardless of AC vs DC, the resistance wire for a 120v electric blanket will be larger than in a 12v electric blanket, such that the former when powered by 12v will throw off less heat - presumably because the larger reistance 120v wire creates less resistance for the 12v current? Got it. Thank you.

Errr ummm ahhhhh---I'm not trying to or suggesting we attempt to power a 120 VAC electric heater with 12 VDC, rather I was asking if one could be effectively powered through an inverter, would the load on the batteries be more than or equal to an acceptable overnight draw?

What brought me to ask that rhetorical question was the size of the dedicated bunk warmer not being sufficient to accommodate two people laying side-by-side. My WAG of using a heater intended mostly for a home application was/is pure speculation.

As for modified sine wave vs pure sine wave inverters my question there was could the microprocessor for the home style heating pad deal with either of those waves or would it require something more closely mimicking the 120 VAC 60Hz input it would normally see?

And FWIW overnight my heating pad was set to "7" which is more than comfortable albeit arbitrary as it doesn't correspond or relate to any known metric. Regardless amp draw was just at 1.7, fluctuating from .99 to 1.7. I believe this was due this particular heater's ability to "sense" a body on it and adjust its output across its surface to keep said body warm. When the body is removed that portion of the heater "idles" until the body returns.

All that being said any discussion of DC vs AC and wire size and resistance therein is moot--or at least it seems to me to be. We're not powering a 120 VAC 60 Hz appliance directly with 12 VDC or vice versa--power to my suggested heater would be 120 VAC supplied from an on-board inverter, sine wave profile TBD.

[boywonder]

Originally Posted by E350

presumably because the larger reistance 120v wire creates less resistance for the 12v current? Got it. Thank you.

......the wire resistance is fixed, it is what it is......flowing 12V through a lot of resistance doesn't make for much current (I).....resistive heating power is Isquared x R....so not much heat......the current (I) is squared....that's the main driver of heating power.

[E350]

JWA I know you weren't talking about what would happen experimenting with running 12v through a 120v resistance wire (you are too smart for that), but I was. boywonder had it right, that was my question. Thanks for the answer.

[Today I will continue to weld an awning frame. Miled steel .065 square tubing. I don't like how MIG welding is treating my eyes and I can't get my auto darkening variable shade Miller 9000i Titanium helmet to work right for me, so I am experimenting with 2.25" x 4" fixed shade gold welding lenses with a magenta filter behind (all from Phillips Safety) to see if welding can be gentler on my eyes.]

We have a limited window for working outside today before the rain, probably you will be getting some too boywonder. All of you have a great weekend! Thanks.

[daveb]

Originally Posted by JWA

As for modified sine wave vs pure sine wave inverters my question there was could the microprocessor for the home style heating pad deal with either of those waves or would it require something more closely mimicking the 120 VAC 60Hz input it would normally see?

This company is (from testing reports) the only modern blanket that will work normally with a MSW inverter available to US customers.
SoftHeat Low Voltage™ line of heated products from Perfect Fit Industries

There is a lot of info from the boating world that verifies MSW inverters fry the controllers in many cases. Not like I've ever confirmed this myself and I don't want to.