Testing an Electric Blanket

Use an AC ammeter to check the current draw, and compare it to the specifications on the controller.

Nelson-

Of even more concern than fire and shock, is that a temperature she likes may actually be too hot and cause harm.

You should ask a doctor about this. We "do gooders" may be giving you advice that would roast your wife!

The hot water bottle(s) sounds like a good idea. If she were sufficiently comfortable to fall asleep, would she remain asleep after it cooled?

Fred

On Wed, 26 Jul 2006 15:33:26 -0400, Fred McKenzie wrote (in article ):

I assume by "cause harm" you are referring to possible burns. I don't know about that but you reminded me of an interesting point which I hadn't considered and that is that MS patients are frequently negatively affected by heat... they get noticibly weaker (Uhthoff's Phenomenon).

I think I am going to try the "Space Blanket"/Hot Water Bottle combination. I appreciate all the helpful suggestions and the group's patience with this "Off Topic" subject.

On Wed, 26 Jul 2006 07:54:51 +0100, "n cook" put finger to keyboard and composed:

I lift the blanket and warm the bed with the heater while holding both in my hands. Sorry if I implied otherwise. I find that one minute is more than enough.

- Franc Zabkar

likes

sufficiently

If anyone needs convincing about space blanket effectiveness, borrow one or buy a cheap thin emergency one and try this. Cover your hand with a pillow case or T shirt or something to avoid direct skin contact (then has opposite effect of conducting heat out) and place inside a folded section of the blanket - a few seconds and your hand will begin feeling warmer. Such an effect and a blanket top and bottom would probably be too effective

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Most electric blankets use diodes in the power selector switch - this might confuse an AC ammeter!

I've never seen diodes in any I've worked on. They just had a mechanical thermostat that switched the element on and off.

Were they made in the 30's?

No, in the '70s and '80s, here in the US.

I suppose there's some merit to using a simmerstat since ambient temp influences on time and therefore heating level, but the constant clicking does tend to keep some people awake!

All the modern ones I've seen used very thin element wire, and presumably a large PTC for self regulation with switch selected diodes for power control. The last one I opened the switch unit had a lot of diodes the element was sort of co-axial, the centre wire was thinner than a human hair and the second element was spiral wound thicker wire round the inner insulation, I think the switch se;selected various combinations of one, other or both elements with or without series diodes to give about 5 different power levels.

Put a GFCI receptacle in where you plug this blanket in.

I'm not sure a GFI would do any good. All electric blankets I have ever seen are two-wire devices, and there is no ground in or around the blanket. No ground = no ground fault = happy GFI.

I did see a blanket fail once, out in the middle of the blanket (i.e., not near the connector). I THINK the failure mechanism was a heating wire that had been flexed enough that it was ready to break - but hadn't quite broken yet. The fracturing wire got thinner, which increased the resistance at that point, which overheated the wire at the incipient break, which caused a hot spot that was hot enough to scorch the blanket material (the material does not support flame, by the way).

This was NOT a short circuit. The heating wires are well separated, and anchored in place quite well.

Bill

Of course it would, provided the victim had a path to ground, ie through the bed itself..

I can't speak for the rest of the world, but AIUI here in the UK such safety devices work by detecting an inconsistency between live current and neutral current, which would indicate some current is flowing to earth, either through a human being or other fault condition. It works just as well for two wire as three wire devices, and in fact the device doesn't care or know one way or the other.

Dave

Right, but still not much of a help with an electric blanket, since there is unlikely to be another path to ground. The hazards are: (1) a fire, or (2) person in bed making electrical contact with the heating element at 2 points that are at substantially different voltages.

Well I have a smoke alarm on the ceiling over my bed, as everyone should. The blanket provides comfort that I'm willing to take a small risk to enjoy.

Maybe they're just not built like they used to be. My grandma has one that's at least 40 years old, it's on the bed in a spare room now and when I've stayed there I still never turn it up past 3 or so, set it to

On Sat, 29 Jul 2006 20:45:03 -0400, Bob wrote (in article ):

Bob,

I was unaware that they existed until you pointed it out. Most seem to operate on low voltage DC which is, of course, inherently safer. And I notice that there are even some blankets that do also. I think I will definitely try one if the "space blanket" solution doesn't pan out.

I think I can swing one if necessary, but I appreciate your generous offer. I am frequently amazed at the kindness of strangers on usenet.

Ground fault current interrupters do not sense ground current directly - they sense any difference between the current in the line and return wires, if the line current exceeds the return current then it must be going somewhere it shouldn't!!!