Heated/Electrical Blanket Causing Odd Sensation

[SeerWS]

Hi all,

Maybe you inquisitive folks can help me figure this out. Let's just say I don't have a lot of inventory in the physics department.

The other night I was laying in bed with my girlfriend. We were using a king-sized electric blanket, which has 2 separate (as far as I can tell) heating circuits - 1 for her side and 1 for mine. I assume it's 2 because we each have a power cable to plug into the wall.

My hand brushed against her back and I felt a subtle sensation, like a vibration, or when you rub your finger over rubber. There was no shock, static, or anything unpleasant.

At first I thought there was friction because she had just rubbed lotion into her skin - maybe my finger rubbing across was resonating with the natural frequency of her flesh or something.

No matter where I touched her - arm, neck, etc - it created the exact same sensation.

By this time she had woken up and was getting annoyed at me (it was late and she thought I was trying to wake her up for something else :D), but I was beginning to worry that this was being caused by the electric blanket, and that perhaps it was faulty.

I told her to hang on so I could experiment more. I discovered that whenever I placed both of my hands on her, the sensation disappeared. When I only touched her with one hand, the sensation returned.

The sensation also occurred only when my hand moved against her skin. When my hand was at rest on her back, I could feel nothing different.

I removed the blanket from the bed and the sensations disappeared.

Anyone here know what's going on?

And should I be terrified of getting electrocuted by this blanket? We haven't used it since.

 

[marcusl]

It's hard to know from what you describe, but it doesn't sound very good to me. I once took out our waffle iron to make waffles, and each time I touched it I got a little prickly feeling. Had I been standing on a wet floor, who knows what might have happened? I threw it out promptly even though we had only used it a few times.

 

[SredniVashtar]

I can tell you that what happened to you with your girlfriend happened to me when I moved my hand over the metallic case of my computer. But I guess this just says more about me than about the electrical problem :-).

Looks like a dispersion current effect to me.
This would sound crazy, but, try again after unplugging (not only turn off, unplug them) all the devices in your apartment: computers, tvs, washing machines, refrigerators...
does that happen again?

If it does not, I would have the Earth connection checked.

It would be also be nice to know how these electric blanket are wired. Have you tried to see if you get the same effect with only one part of the blanket plugged in?

 

[SredniVashtar]

Sorry, something misfired and a double post resulted.

 

[NascentOxygen]

I removed the blanket from the bed and the sensations disappeared.

I would have suggested that you simply unplug it at the power socket, to see. But now that you have removed it, I guess it's for the best that you discard it. Cut the lead off so no one tries to reuse it.

 

[SredniVashtar]

I would have suggested that you simply unplug it at the power socket, to see. But now that you have removed it, I guess it's for the best that you discard it. Cut the lead off so no one tries to reuse it.

But this is a physics forum, right? Can't we devise some sort of safe test to figure out what is going on? I understand we should not ask the OP to open up the appliance and stick his fingers in the mains plug, but - the problem could reside in his ground connection and not in the blanket itself (provided it is grounded at all...).
At least, even after the OP has disposed of the blanket, locking it in a wooden chest full of heavy rocks and has tossed it at the bottom of the sea, the problem could still be analyzed in order to try to understand what might be going on.

How are these blankets usually wired, to begin with?
Any takers?

 

[NascentOxygen]

Mains power electric blankets have 2-pin plugs.

 

[jim hardy]

i'd suggest you get an inexpensive tester like this

and check the wall receptacle. It is a handy tool to have anyway, and only around $5 in US Walmarts.

Old fashioned electric blankets had a simple mechanical thermostat that worked quite well.
Newfangled ones have a computer controlled thyristor control. My experience with them is thay aren't worth a tinker's d**n, if they ever heat at all it is only for a few nights.
Some of them are so "smart" they conduct only over a narrow part of the line cycle, when sinewave voltage is near zero, in order to cut down on capacitive coupled voltage. It is very easy for me to imagine one of those contraptions misfiring.

Do you have an AC voltmeter?
If so , cut about a 1 foot square of aluminum foil and lay it on the blanket. Connect your meter, one wire to that foil and other to the ground prong in your outlet (AFTER you have checked outlet with that tester above for proper wiring).
If you read more than about twenty volts you probably have your answer. Turn blanket on and off a few times, then unplug it, to verify .

There exist 'gourmet' electric blankets that have a large AC/DC power supply, like in a computer, which makes low voltage DC for the heating elements in the blanket. You might try one of those. DC can't capacitively couple.

Myself i found a old, old electric blanket in a thrift shop that still had mechanical thermostats. I brought it home and discared three almost new but "DEAD" computerized Sunbeams.
IMHO America has lost the ability to manfacture a functional electric blanket.

old jim, a grumpy old man

 

[Graniar]

You should definitely to try only unplug the blanket.
There big probability of kinestetic illusion.
When you moved your hand, back side sensed blanket texture.
But, your attention focused mainly on your girlfriend skin.
Thus, regarding a bit altered state of consciousness (half-sleep, girlfriend...), your brain could mixed things up.

 

[sophiecentaur]

This phenomenon is not uncommon with electrical equipment. It's the result of a very high resistance path from the mains and your nerves are very sensitive to the tiny resulting current (which is harmless). It happens when the insulation is not perfect and the effect / sensation is very subtle. I have felt it on the casing of an old Electric Kettle, on a modern touch-switched lamp and on an electric guitar pickup. Remember, nerves work on microVolts / microAmps and it's not too surprising that, when presented with very low level inputs, they will detect them sometimes. You normally only feel it when brushing lightly against the 'live' object (girlfriend / kettle) and the sensation goes away when you grasp it firmly (simmer down now!). I guess that's because the area of contact varies and the current (from what is, esssentially, a current source) has a different density.
I guess you should not use the blanket unless you used an isolating transformer with it. Take it back to the shop and complain.
It might be interesting to leave it on for a long time in an empty bed so that any dampness has gone away and see if the effect is till there.
It's years since I switched to a Duvet from sheets and blankets and have not felt the need for an electric blanket since then. I remember my parents would remind each other, in the late winter evening to "go up and plug in the blanket" - happy days. They (along with most UK citizens) didn't have Central Heating in those days, so they had an excuse, perhaps.

 

[vk6kro]

Electric blankets used to be made by threading resistance wire through hundreds of cylindrical ceramic beads. So, the insulation was impeccable, yet this tingling shock effect was common.

It may be a capacitive effect. A human body laying on a bed forms one plate of a capacitor with an AC signal at the other plate.

If two of these capacitors are placed in series as when the two human bodies touch, and if the AC signals are different, then a small current will flow.
If one of the human bodies is large and the other small the part of the mains voltage that each is coupled to will be different, so there will be a current flowing. Also, the way that the blankets are wired will contribute to this asymmetry.

Electric blankets with a wool overlay are more expensive, but don't seem to have this problem.
This may be because the distance between the heating element and the body is greater and hence the capacitance is lower.