# Calculating magnetic field for DC current

Discussion in 'General Electronics Chat' started by radinc, Aug 3, 2009.

Aug 3, 2009
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I recently got a heated mattress pad that is powered by 16V DC current. My wife loves this thing because she is always cold and I always steal the blankets.

The problem is, she was talking to a friend who got her all freaked out about EMF's and magnetic fields. OK, so I understand it's DC current so it's a static magnetic field that's created, right? So it's not the terrible possibly leukemia causing EMF's. So that's good.

But my question is... what kind of magnetic field is this pad creating? Are we lying on like MRI level magnetic field or is it natural earth magnetic field level? Knowing the answer to this will determine the fate of this mattress pad.

Can anyone help with the math on this?

Thanks!
Steve

Last edited: Aug 3, 2009
2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Grab a magnetic compass and place it near the blanket when switched on - if there is a magnetic field greater than the Earth's you should see the compass needle swing off normal. The field will be orders of magnitude less than an MRI field. I don't believe static fields have been linked to cancers. You can buy "special" pillows with magnets inserted which are supposed to provide beneficial effects as you sleep ....... try cuddling up to your wife instead.

3. ### t06afre AAC Fanatic!

May 11, 2009
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I think you should keep the blanket. The magnetic field in a magnet bracelet as an example, will be higher. Some people do actually place magnets under the bed. They claim it is health-bringing. I am not in that group . But to answer your question i am quite sure the magnetic field is in natural earth magnetic field level magnitude

4. ### steveb Senior Member

Jul 3, 2008
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I don't believe there has ever been reliable proof that everyday magnetic field levels are helpful or harmful when they penetrate the human body. That doesn't mean that either is impossible, but so far no proper scientific study has shown a significant effect. However, keep in mind that the magnetic field from an electric blanket is much less than that from modern permanent magnets. The earths field is about half a Gauss, while a permanent magnet could be 10000 Gauss. That's a factor of 20000 times higher! Nobody has any qualms about letting a child play with magnets, so there should be no concern about common home & work appliances.

5. ### t06afre AAC Fanatic!

May 11, 2009
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Then I was about 7 years old, my older brother and I start arguing over a set of cool magnets taken from some large loudspeakers. It ended up with that my brother throw one at me. I manage to take cover. The magnet went through the window (the window was closed) and made a big dent in a car belonging to some friends of my parents

Last edited: Aug 3, 2009
6. ### steveb Senior Member

Jul 3, 2008
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I stand corrected. Kids can make just about anything dangerous!

As far as the electric blankets. I remember people used to worry about them catching the bed on fire, and I thought I remember hearing about a few accidents where people were injured in this way. However, I haven't heard this spoken about in a long time. Have the safety measures become so good that this is no longer an issue?

7. ### t06afre AAC Fanatic!

May 11, 2009
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I do not hope this is to much "off topic". But the rest of the story came to me. Well as a punishment the magnets was sent out of the house with the friends of my parents. Since theirs kids was somewhat older and more responsible. The sad thing was that they had a brand new(and very expensive) color-TV with something fancy as a remote. And they soon discovered that powerful magnets and cathode ray tube is not a good match. The TV did not ever quite recover. I wonder there those magnets ended up

Last edited: Aug 3, 2009
8. ### steveb Senior Member

Jul 3, 2008
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Now we're talking. That is an experiment worth a little damage!

Jul 7, 2009
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If you tell us what the DC current of the device is, we'd be able to tell you what the maximum magnitude of the magnetic field you'd be exposed to at a certain distance from the blanket. The voltage is irrelevant. It will likely be within an order of magnitude or so of the Earth's field's magnitude. Trying to give a better estimate without more data is like trying to answer "how long is a piece of rope?".

10. ### ELECTRONERD Senior Member

May 26, 2009
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I think that the blanket is harmless. Every wire, when current is passed through it, emits a EM wave that is very small. In fact, EM waves are constantly penetrating our bodies because of all the radio stations and such, and they would probably have a stronger EM wave than the blanket.. So I wouldn't worry about it.

Aug 3, 2009
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Thank you for the responses! To continue the electric blanket saga, I think it is fine too, but I thought it would be reassuring to get some hard numbers since I am an objective kind of guy. Nice idea for the compass, as soon as I can dig one up I will try that.
The mattress pad uses 1 watt. so I guess the current is 0.0625 Amp if it's 16V?

The only reason I would even think to worry about this issue is that we are in close and direct contact with the mattress pad for 8 hours a day, every day. So even if there was a very small effect, cumulatively over many years it may create a substantial effect.

Steveb, I looked into the fire thing and that is/was an issue with the AC powered blankets. If a wire breaks and a spark is created, you could have a fire. But I was told with the DC powered type it's not an issue, what do you think?

Thanks again!
Steve

12. ### steveb Senior Member

Jul 3, 2008
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It sounds reasonable. I would personally feel safer with low voltage DC, rather than 120 VAC. Burns are always a possibility and hopefully there are safety features such as thermal shutoff switches in the blanket, and current limiters in the power adapter.

My anecdotal observation is that I rarely hear anyone talk about accidents with electric blankets any more.

13. ### t06afre AAC Fanatic!

May 11, 2009
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If something goes wrong, the blanket could be so hot it starts smoldering. And if that happen you will probably never wake up again, even if it dies out and no fire is started. But if your blanket is UL approved this and the magnetic radiation should not be any problem. And I do mean no problem! We must not have a hysterical approach to electrical devices. I sleep well with several electrical devises in my bedroom. But anyway it is always a good practice to have a smoke detector in the bedroom. Electric blanket or not

Last edited: Aug 4, 2009

Jul 7, 2009
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1 watt! That's tiny! I'll assume it's a very small heating pad.

Edit: by the way, the reason I said that is because the human body radiates roughly 100 W to its environment.

15. ### steveb Senior Member

Jul 3, 2008
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Good point! A good insulating blanket is going to allow "human-power" to provide quite a bit of heat. The electic blanket could just be used before getting in bed to make the initial 1/2 hour more comfortable. That way there is no worry of EM-field effects or getting burned in bed. Not that those possibilities are signifant risks, but worries are not always rational, and if someone is worried about risks, they won't sleep very well.

Last edited: Aug 4, 2009
16. ### THE_RB AAC Fanatic!

Feb 11, 2008
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The blanket is almost certainly threaded in a zig-zag pattern, usually horizontally as it's easier for Chinese slave labour to sew. So each wire makes the opposite magnetic field to the wire next to it, causing an average field strength of zero to the person lying on it.

Now if it was threaded in a spiral you would have a + field on one side and - field on the other, but they would never thread as a spiral as the wire would have to cross itself when it finished in the centre. It will be a zig-zag with a return wire around one outside edge. Tell your wife to sleep without worry.

17. ### davebee Well-Known Member

Oct 22, 2008
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I can't believe anyone would bother making an electric blanket of only one watt. Something more like 100 watts would make more sense.

Are you sure about that power?