:huh: Why do car alarms go off in the presence of a thunder storm? Does the thunder cause it or the lightining and why?

 

Originally posted by nanobyte@Mar 18 2005, 05:34 PM
:huh: Why do car alarms go off in the presence of a thunder storm? Does the thunder cause it or the lightining and why?

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It's the strong and rapidly changing electric fields associated with lightning which can cause this. (Listen to an a.m. radio during an electrical sgtorm.

Many circuits, particularly those using c.m.o.s., are adversely affected.

I experienced the same problem whilst walking on a synthetic carpet in the vicinity of a device with a very high input impedance.

 

Specifically, why is the changing electrical fields of lightining causing this? How is it affecting electrical devices like car alarms?

 

Originally posted by nanobyte@Mar 24 2005, 09:40 PM
Specifically, why is the changing electrical fields of lightining causing this? How is it affecting electrical devices like car alarms?

[post=6359]Quoted post[/post]​

Associated with lightning discharges are brief but very intense electro-magnetic pulses.

You may have read somewhere about the concern over these "e.m.p.s" which can be used during warfare to disrupt an enemy's communications ability - both radio and computer networks.

The pulses are so intense that they can destroy the front end of receivers and any electronics which uses c.m.o.s. which, as you know, tends to be a bit neurotic in this respect - witness the warning labels on semiconductor packaging and the use of conductive foam into which chips have their pins stuck in for transport and storage.

A static discharge, whether from lightning or other sources, can trigger a c.m.o.s. chip. Car, and other, alarms use these extensively.

The fields are effectively short high-power bursts of r.f. energy which are picked up in any cables/wiring which may be in their vicinity.