What techniques can you use to kill voltage spikes on car power outlets.

I mindful of the fact that older (and imagine newer cars) can throw a hefty decay voltage into the electrical circuits. I am dubious about using the simple cigarette lighter sockets with the cheap 5volt adaptors available for sale. I had thought of putting a meaty tenner diode of 20 volts across the lighter socket?Any other ideas?

Hi Crusty-I haven't had any problems with failure of those 5 volt lighter adapters, but that doesn't mean there won't be problems in the future. I haven't measured it, but I hear that it's not uncommon to have spikes upward of 90 volts on the 12 volt line. I think the zener diode idea is fine, but if you search for a TVS, or "Transient Voltage Suppression" diode you might be better off.  The TVS diodes are really just zener diodes, but they generally have a bigger slab of silicon for better surge suppression, and the voltage tolerance is often sloppier than a traditional zener diode.Metal Oxide Varistors would be worth considering too.Some of the cigarette lighter adapters don't specify an input voltage range.  If you stick with the ones that claim to be good up to 24 volts or greater that might help stack the odds in your favor.-Rick  

Hi Rick_Thanks for your thoughts on this. My sons iPhone died at the end of a long run plugged into one of these cigarette lighter adaptors and it made me think of the specs we used to write  for  London Double Decker buses (Yes the 97 horse power types) where ancillary electronic equipment was concerned. We had measured decay voltage spikes of 1000 volts.  I think it was mainly an awful lot of lighting inverters switching off that generated these spikes.I am just thinking that putting a passive spike protection in the plugin adaptor will just stop any future problem.Crusty

Crusty,
When I was doing some work on forklifts, I found  that the TVS would die after a while as a result of one too many hits. I would recommend adding a resistor in series in order to limit the current. The bigger the better, but you of course have to worry about volt drops and power dissipation. If I remember correctly we worked with something around 5 ohms, but we were drawing some current so it was one of those 10W (I am guessing now) aluminium finned devices. Since they were easily mountable with screws it made the retrofit much easier.

Crusty,
I should mention that Linear Tech also make a sophisticated  "surge stoppers". Check out the different products herehttp://www.linear.com/products/surge_stopper,_overvoltage_*_overcurrent_protection
I should add that I have not tried these, but it's from Linear so it should work as advertised.

Crusty.... modern cars are full of electronics so there would be a bit of protection there already hopefully.  It would be interesting to see just what you do get there.  With my newly acquired oscilloscope I could do that for you, but I wouldn't get back here in time for tea.....  I think your idea of a Zener (hope it won't cost you a tenner is good , I think an 18V one would be adequate (your battery voltage shouldn't get to much above 14.  I'd have a look at the P6KE series - they are rated at 600W (pulse) and are bidirectional.  They are tough as old boots and not much gets past them.   A P6KE18 should do what you want.  Google it for more info.

David,Yes I totally agree , Its the way I have done things in the past, and it seems to be the way forward still, and it is easy to put in the adaptor (probably)Have a nice tea while I use my scope on the battery.