I have an electromagnetic radiation detector that I've been playing with recently, going around the house and testing various appliances, and I noticed something weird.

When I charge my laptop (or phone or tablet, for that matter) they emit electromagnetic radiation. When I don't charge them, they don't emit anything. Even if I use them.

I would expect the laptop to emit something while on battery power, but it doesn't. And I don't understand why it emits only when I charge it since the charger is a few feet away from the laptop.

Is there something in the laptop battery that emits that electromagnetic field? Why doesn't it do the same when the laptop is not charging?

I'm playing with a cheap DT-1130 Electromagnetic Radiation Detector


closed as off-topic by brhans, Elliot Alderson, Finbarr, RoyC, PeterJ Dec 12 '18 at 0:24

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – brhans, Elliot Alderson, Finbarr, RoyC, PeterJ
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    \$\begingroup\$ Your battery charger is connecte to your AC network with 50Hz (US 60Hz). Even if the charger output is DC it can still have a frequency modulated onto it because of e.g. bad layout design. Any component in your Laptop can then start to emit this frequency. Does your detector tell you anything about the frequency? I think the laptop will always emit a field but the frequency (W-LAN, bluetooth, CPU Clock,...) is to high for your device to be measured. \$\endgroup\$ – A.R.C. Dec 5 '18 at 11:05
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    \$\begingroup\$ @A.R.C. "because of e.g. bad layout design". My phones and tablets do the same thing. I find it hard to believe there is something wrong with all of them \$\endgroup\$ – Pips Dec 5 '18 at 11:09
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    \$\begingroup\$ Your devices are emitting lots of EM all the time. Wifi, MHz and GHz frequencies from the digital signals inside, the light from the screen, all of these things are EM radiation. Apparently, your detector doesn't detect these. So unless we know what your detector does detect, it's hard to answer. Based on your description of when it lights up, my guess would be the detector is detecting 50/50Hz magnetic fields. But in which case it's not actually detecting EM radiation at all. does it have a datasheet? \$\endgroup\$ – Jack B Dec 5 '18 at 11:25
  • \$\begingroup\$ And if it didn't emit any electromagnetic radiation you wouldn't see anything on the screen. Light is EM radiation. \$\endgroup\$ – Transistor Dec 5 '18 at 13:22

You only detect it when charging because you are using a toy of a radiation detector.

Your laptop is generating electromagnetic energy at all times (when operating.)

It will probably generate more (at certain frequencies) when charging. Your detector can see them better - probably because the laptop begins emitting a frequency the detector can see, or because emits more of some frequency that was too weak before.

If your detector can't see it, that's more a problem with the detector.

The detectors you can buy on Amazon and Fleabay are more in the line of "separating worried but unknowledgeable people from their money" than "accurate measurement of electromagnetic intensity."

That thing definitely belongs in the class of "separating worried but unknowledgeable people from their money."

No mention of the units of measurement, the "specifications" don't match up in anyway, and there are more pictures of the box than the device.

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    \$\begingroup\$ You were 10 s faster to put what I wanted to say much more pointedly – so, fark it, have your well-deserved upvote :) \$\endgroup\$ – Marcus Müller Dec 5 '18 at 11:26
  • \$\begingroup\$ Busted :D. Yes, I'm using a crappy detector... Was reasonably cheap though... (see update above) \$\endgroup\$ – Pips Dec 5 '18 at 11:46

Is there something in the laptop battery that emits that electromagnetic field? Why doesn't it do the same when the laptop is not charging?

The laptop has to have some charging circuitry. That's basically an adjustable power supply "feeding" energy into the batteries, with an appropriate controller.

Efficient power supplies in that range of operation are switch-mode power supplies.

These switch some inductance on and off at high speeds to generate a controllable voltage or current. If not designed properly, that might lead to a lot of EM-leaked energy.

However, more likely than a badly designed SMPS (switch-mode power supply) is simply a selectively overly sensitive "electromagnetic radiation detector" (whatever that specifically is). Unless your device comes with a chart that defines sensitivity (e.g in dBm) over the frequency range it's supposed to cover, don't assume that two things giving the same alarm are even roughly emitting the same energy.

If your detector, however, is a calibrated spectrum analyzer with a matching wideband antenna, you could definitely tell what is emitting at which frequencies.

Edit: yes, since that doesn't come with any documented calibration, I'll go with "readings of that are about as helpful as reading the EM situation from tea leaves.

  • \$\begingroup\$ It did come with battery included though :-) \$\endgroup\$ – Pips Dec 5 '18 at 11:52

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