# How would I protect my hardware from unwanted magnetic induction?

To my understanding, magnetic induction is the process of varying magnetic fields producing a potential difference in a conductor, resulting in current.

How would I protect my hardware from strong, moving magnetic fields that could potentially produce a high voltage and damage my hardware? Do hardware manufacturers do anything to protect against the occurrence of unwanted magnetic induction?

What's to stop me from putting a large amount of magnet wire in my bag and connecting it to an alternating current supply (such as a car battery with a DC to AC inverter circuit), and then taking a trip down to the hardware store, causing damage to any hardware I walk past? (that's just an evil example - I doubt I would be able to produce a magnetic field strong enough to cause any damage, but my point still stands)

• The car battery is a DC supply – Scott Seidman Feb 19 '14 at 19:54
• @ScottSeidman that won't be a problem with some sort of DC to AC inverter circuit :) – user11047 Feb 19 '14 at 19:57
• If you really want to act maliciously like this, what you need is a nuclear reaction. – Phil Frost Feb 19 '14 at 21:36

If it were a TV or a radio then using a car battery and coil of wire would certainly produce interference - if you had (say) a thousand turns with a total resistance of say 1.2 ohms (probably using litz wire), you could generate a magneto motive force (MMF) of 10,000 ampere-turns each time you connected or disconnected from the battery.

How would this directly affect electronics that might be several cm inside a box? Say the most sensitive electronics were 5cm inside and say the coil were a couple of cm deep. The best round trip I can imagine for magnetic fields to make would be about 14cm but this will almost certainly render the effect nearly useless.

An air-cored coil projects flux all the way around its coils and maybe 1% of the flux would penetrate at this depth. This would lower your effective MMF to about 100 ampere-turns and, the length of the field (14cm) that this 1% can reach, implies a magnetic field strength (H) of 100/0.14 ampere-turns per metre = 714 At/m.

This level is about what you would see on a ferrite cored switch mode power supply and this doesn't appear to cause very local chips or components any trouble.

What if you used a ferrite core to "focus" the magnetism in order to damge an electronic component. This doesn't work - there is still the totally dominating air-gap from one end of the ferrite to the other which the magnetism has to flow - this air gap is the dominant factor - now matter how much ferrite you have - even a 1mm air gap can reduce the effective permeability of a ferrite core by over 50% on most reasonably sized ferrites. So what if your coil only had a 1mm air gap - it would be totally useless at being able to damage components more than a few mm from the air-gap - the field lines would fringe around the air gap but would naturally take the shortest path back through the ferrite - leakage flux would not be much.

If you wanted to improve the mechanism you could use a powerful (but simple) power oscillator and tune your coil with a capacitor to make it resonate. I've seen this done in pharmaceutical metal detectors tesing all manner of drugs for metal content BUT, does this affect the product being tested as it falls through the mouth of the detector and directly thru the plane of the transmit coil? No it doesn't - even foodstuffs like chicken and beaf are passed through resonant-coil metal detectors and there is no heating effect or bubbling even though probably 50% of the magnetic field actually passes through the salt-water saturated meat.

Try looking up equipement called "eddy-current heaters" to satisfy yourself on this.

• So what about wireless charging docks for smart phones? Those docks must produce a fairly strong magnetic field, right? What if someone was to accidentally put a non wireless charging compatible device on a dock? Surely that could induce a current in the device's circuitry? – user11047 Feb 19 '14 at 20:42
• The wireless charging docks that I'm aware of require a magnetic coil in the phone that is resonant tuned (with a capacitor) in order to get anything more than a few milliwatts from it. It's like some induction heater hobs - the pan has a coil in it that is tuned with a capacitor in order to get a large circulating current flowing in the inbuilt element. If it isn't tuned with the right capacitor then no-deal it bearly could warm a teaspoon of ice. – Andy aka Feb 19 '14 at 21:34

Lightning is an example of a current pulse that can cause strong enough magnetic fields to damage electronics. I've seen even relatively insensitive components such as resistors damaged (not a direct strike, but nearby so that current is induced in wires).

You can protect it by limiting the voltage and current. For example, a lightning arrestor and spark gap arrestor might limit the voltage to a thousand volts (the first by shunting massive current away, the second a much smaller current, through some series impedance), then some series resistance and a TVS might get you down to 50V, a voltage which your electronics could be designed to withstand.

If your equipment must withstand extreme EM fields (near a radio transmitter) or extreme magnetic fields (for example near an MRI machine) you do have to take precautions. Induced voltage and even the a high enough DC field can result in damage.

Magnetic shielding doesn't work all that well at room temperature, and shielding high intensity fields at low frequencies can be remarkably ineffective. For example, mu-metal has a nice high permeability for fields in the tens of $\mu T$ (like the weak field from the Earth), but will saturate and be little better than air if exposed to a significant magnetic field. Lower permeability materials (such as iron or ordinary steel) can be used to shield the mu-metal.

Generally, the kinds of magnetic fields that are found in ordinary circumstances don't induce a lot of voltage in random spots, and there is enough impedance present that damage is unlikely. You do occasionally see some disruption or degradation of operation, such hum in an audio amplifier or 60Hz pickup in a thermocouple signal conditioner.

For most ordinary electronic devices, you'd need a very extraordinary magnetic field to do them harm. Even your deliberate magnetic coil isn't likely to actually damage anything in a hardware store. The amount of power you'd have to put into it to have any chance of causing a observable effect would mean the device would get warm. You should be more concerned about the eddy currents caused in you than anything you walk by in a hardware store.

Inteference from magnetic fields can happen though, and the usual solution is magnetic shielding. This is often done with something called "mu metal", which refers to the fact that it has high magnetic conductivity.

• Right, so it's possible, but extremely hard/unlikely to be produce a field strong enough? – user11047 Feb 19 '14 at 20:39
• Or even just metal metal shielding, for time-varying magnetic fields. – Phil Frost Feb 19 '14 at 21:31

Strong alternating magnetic fields of a magnitude necessary to do that kind of damage aren't common, so most electronics aren't shielded. There are methods of shielding, which generally involve putting the electronics in a cage or box that directs the magnetic field away from the electronics.