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Safe to use laptop PS for heated clothing?

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Deeg

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I'm working on a project to create a heated scarf for my sister. She gets cold when she's sitting at her computer (laptop) all day and I want to make something that she can drape over/around her neck and shoulders. For convenience I'm planning on using her laptop PS to provide the power; when she's cold can can unplug the laptop and fire up the heated scarf. To control the temperature I'll hook up a simple PWM in between the PS and the nichrome wire.

The PS is 19.5v and 6 amps. I will use nichrome resistance wire with a total resistance of 6 ohms; if I understand the calculations correctly this will result in ~65 watts of heat, well below the PS's rating. Do you guys have any thoughts on the feasibility/dangers of this project? Here are the concerns that I can think of:
  • Will this damage the PS at all? I can imagine that generating a PWM signal from a PS that is designed for steady current could be a problem.
  • I'd like insert a fuse between the PS and the PWM controller. Do you have any suggestions for what would work well? (Or even if it's necessary?)
  • My guess is that the biggest concern outside of the PS is making sure the nichrome wire doesn't short and allow a much bigger current draw.
  • Do you think a wool scarf is heat-resistant enough? I plan on folding over the scarf and sewing two channels for the wire to run through.
Thanks for any help!
 
65W sounds like quite a bit of power for a heated scarf, but I've never made / used one so I wouldn't really know if that's overkill. Presumably you've calculated how hot this thing is going to get?

An LC filter prior to the PWM switch would limit the ripple that your PSU would see but it would need to be designed appropriately.
Polly fuses are very common in low power consumer electronics, they reset once they've cooled down.
a varnish or similar coating stop the wire shorting - keeping in mind you don't want this to get to 200Deg, so most common varnished would be fine.
I would be very concerned that the wool could catch fire
 
I would guess 65W is far too much, but I haven't done any sums to check.
I share Misterbenn's concern re a fire risk. That amount of power is certainly enough to cause a fire if there is localised over-heating. I think "folding over the scarf and sewing two channels for the wire to run through." would not distribute the heat well enough, and would result in uncomfortable hot-spots.
My other concern would be that 65W = ~ half the available power from the supply. That would mean 1) the laptop gets only half its normal power or 2) the power supply will be overloaded. Neither seems a good outcome.
 
Here is what I would think about. There are countless 12 volt heating pads out there very reasonably priced. Since I don't know your location I can't suggest a source. You could even likely hack a seat warmer pad from an automotive junk yard as used in car / truck seats. As to a 12 VDC source? Find the laptop PSU and find a convenient 12 volts on it and add a jack for 12 volt power making sure the laptop can handle the added load. Option two is just make or buy a simple small 12 volt power supply powered off line voltage. These pads are machine washable and inexpensive. Many are even UL approved.

Ron
 
For convenience I'm planning on using her laptop PS to provide the power; when she's cold can can unplug the laptop and fire up the heated scarf.
I don't see how that is convenient.
While she is warming up the laptop is dying.
Just use a separate power supply.
 
Thanks for the comments. I'll look into lowering the wattage of the heater; I started with 65 watts because somewhere I saw someone use that value for heated clothing, but maybe that was for a (motorcycle?) jacket.

I would be very concerned that the wool could catch fire
According to this PDF wool has a ignition temperature of 570°C+. That seems high enough but I want to ask if anybody has had any experience using it.

Is this the kind of fuse you mean? http://www.digikey.com/product-detail/en/E53493-000/E53493-000-ND/1045805

I don't see how that is convenient.
While she is warming up the laptop is dying.
Just use a separate power supply.
She travels around with the laptop. If I set up the scarf to use the same PS she doesn't have to carry an additional PS around. Her laptop can last a few hours on its own battery power so she can be warm for at least that long. I have a 12v/3 amp PS from an old laptop; I'll look into that as well.
 
Her laptop can last a few hours on its own battery power so she can be warm for at least that long.

You do understand that any power you supply the warming device will be robbed from the computer and added to the total load of the power supply, so it'll no longer last as long as it did before, don't you?
 
wounder if you can get enough power out of the USB ports?
5 volts at .5 amp times 2.
 
You do understand that any power you supply the warming device will be robbed from the computer and added to the total load of the power supply, so it'll no longer last as long as it did before, don't you?
The PS won't be powering both at the same time.
 
Simple answer would be a flat out NO. Way too many variables to consider like material, forgetting to unplug it or turning it off, more excessive heat in your computer and what happens when it is folded and sat down that heat will build up. Give her a nice sweeter and some nice heavy socks.
 
also one of these, fused:
**broken link removed**

no worries yet with electrocutions, even when sox are wet,
the heat gear uses 9 watts max on just socks, i usually run at 45% , 100% when priming or in sub thermal temps.
my 90watt jacket usually runs at full tilt when in cold, 60% at the office ( 10 degrees C).
The jacket needs more power to stay warm due to the surface area and volume


5v 1a as 4pyros suggested could work for something small.

is this what you want:
https://creatter01.en.ec21.com/USB_Heating_Electric_Scarf_Wrap--6984823_6994232.html
just remember lower power means not as much warmth

ACTUALLY i just found this, i think it would be purfecto for her:
**broken link removed**
 
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