Why EMP pulse destroy electronics?

[yungman]

Any "conductive" material will block EM wave, not ferromagnetic material. Its because the electric field cannot sustain inside the conductor. Thus EM wave decays to 1/e at the skin depth inside conductor. Ferromagnetic material will just block magnetic field.

In RF sense, in a particular frequency a material can either be dielectric or conductive and it depends on the EM wave's frequency. Thus mica a dielectric for low freq EM wave becomes conductor for high freq EM waves. Look at the link below for seawater and metallic mirror reference.
http://farside.ph.utexas.edu/teaching/315/Waves/node49.html

I guess the frequency of EMP was so high that the skin depth of those metals were not sufficient to block it.

The higher the frequency, the thinner the conductor metal needed to shield the EM wave as the skin depth is very small. Steel skin of cars are both ferromagnetic and good conductor. The skin depth is very small meaning the EM wave will be attenuated and not passing through. I understand the exposed harness will be affected, but if the military machines are designed to have a total shield, then it should protect the electronics inside. Transorbs can be very very fast, the SMD packaging enable the design to rid of most of the parasitic inductance.

Also if the communication link is designed to be slower speed, you can afford to put bigger transorbs or more in parallel. Seems like there is always a way to protect electronics from EMP.

My understanding is cars use the popular CAN bus which is slow like snail, you can triple transorbs and won't affect the communications.

face it, majority of the pcbs are badly design with bad EMC practice. They pass emission test by back door approach of some copper tape etc. They don't stand a chance on any significant EM susceptibility. If boards are well designed and with transorbs protection, there got to be a way to protect the circuit.

 

[jim hardy]

Will the fast (E1 and E2) part of EMP have a direction to it, like a flux field? maybe generally down, perpendicular to Earth's magnetic field?

so if a fellow wanted to EMP-harden his personal vehicle a good start might be to shield the wiring harness? Especially around the back of the hood where there's a space for windshield wipers to sleep, and a big harness just beneath?

i think i'll keep my '83 Ford Ranger diesel - only electronics in it is the radio.

 

[yungman]

i think i'll keep my '83 Ford Ranger diesel - only electronics in it is the radio.

 

[cmb]

i think i'll keep my '83 Ford Ranger diesel - only electronics in it is the radio.

Presuming you're saying it is a fully mechanical system, I suspect you'll still find there are several weak components in there, such as alternator electronics and fuel lifter pump and control. Does it have a manual knob you pull and hold, to turn the engine off? If not, there is an electronic solenoid too. How much of the instrument panel is electronic?

I've had a few ancient Merc diesels that mostly fitted the bill of 'EMP proof', even with pure mechanical dashboard parts (ever had an oil leak from your dashboard? The oil pressure gauge had a direct capillary from the engine that could, therefore, leak oil!) but still a few circuits. Actually, I think the most EMP proof was an ageing American Plymouth with manual carbs, spark gap distributor (nothing to go wrong there) fully hydraulic gearbox [your Ranger probably has an electronic auto box?] and 'best' (read; most ancient) of all - a solenoid based voltage regulator, from the days when you needed a set of feeler gauges to set the points in the voltage regulator! I don't suppose there's many folks left that remember setting their regulator points!

 

[jim hardy]

I don't suppose there's many folks left that remember setting their regulator points!

you and i are among the few I'm sure... airgap and spring tension too.

re Ranger:
instruments are the old Ford heated wire type from 1930's, they'll be okay.

but you're right there's an electric valve that must close to run, it dumps fuel pressure.
but injector pump is an old fashioned mechanical one. Should stand tall after "The Big One".

that particular model is still sought after, for the little engine is an industrial Perkins design that's known for lasting nearly forever. there's an active community of enthusiasts still driving them.

ahh nostalgia...

 

[Antiphon]

A gap is as good as a wire antenna or coupling. A car has so many gaps it might as well not have the hood on at all for EMP purposes.

As for how it blows up electronics, it's mostly overvoltage on semiconductor devices.

The EMP fields my devices are tested to are 50kV/meter. If you have any kind of antenna (deliberate or incidental) in a field like that, you need special circuits to protect the ICs inside. It's exactly the same damage mechanism as a carpet (static) shock only much larger in area.

Vacuum tube electronics are the most robust without special protection. I have a collection of around 150 triodes and pentodes "just in case."

 

[gorgon53]

50kV/meter? Will there be anyone left to care about what what happened to electronics?

 

[Antiphon]

I don't thinks that's a lethal field- unless maybe you're perched on the edge of the grand canyon in your Hoveround electric wheelchair. The EMP might fry just the joystick controls but not the motor drive...