12v filter for car electrical output

[Davidm]

I've bought a USB FM adaptor to enable me to play MP3 music files from a USB memory stick, via an FM signal to the car radio. It plugs in to a cigar lighter socket for it's power supply. I've tried it out and it seems to work reasonably well. I don't have any instructions but digging around on the web I found a few places where it advises not having the device plugged in while starting the engine - spikes on the supply might damage the unit, or corrupt it's settings.

My cigar lighter is not in a very convenient position, so I'm thinking of removing the plug bit from the device (there doesn't seem to be any electronics in that bit, just a fuse), and mounting the "head" onto a nearby panel, with it hard wired (via a fuse) to the cigar lighter supply. However I think I need to make or buy some sort of filter to remove/reduce any spikes on the dc supply. I know I could fit a switch in the supply, but I'm bound to forget to use it at some point!

Can anyone suggest a suitable circuit I can build, or filter I could just buy?

[Bill Wright]

I suppose you could simply fit a large value capacitor across the supply? It might blow a low value fuse though when you turn the power on . Or you could feed the device via a big choke in series with the + supply. Or both. Or power the device from a small nicad trickle charged via a high value resistor? Crude suggestions!

Bill

[The Natural Philosopher]

First of all check that the cigar lighter has power at all when starting the engine.

IME most if not all do not.

Power on almost everything is disconnected when cranking.

Frankly I think someone is talking bollocks, buy hey, this is the internet where 'one extraordinary case of use incompetence caused effect X simultaneous with event Y, therefore all instances of event Y are likely to 'cause' symptom X'

Yiou know the sort of thing''you shouldnt take your hoover apart without proper training because I did, dropped a glass of brandy in it, started it up and it caught fire, set fire to the piles of balsawood shavings on the floor, causing the whole house to burn down because I panicked and dropped my mobile phone and my wife had torn out the landline after an argument with her hairdresser the day before and I couldn't call the fire brigade'.

[Brian-Gaff]

Well its transients you ar after. I'd suggest something like a15 v high power zenner diode maybe. I also wonder, what is it using to transmit from, the car wiring or has it got a bit of wire on it? Brian

[Brian-Gaff]

Well as I said, a zener on the other side of the fuse should blow it when the zener conducts. At least if it never blows you would be fairly safe. Btrian

[jrwalliker]

It doesn't need much of an antenna to reach the legal limit of 50nW radiated power.

John

[tabbypurr]

Inductive generator plus load dumps is bound to equal spikes. The battery will swamp them for the most part, but alternator to battery does have inductance, so I'd expect rf spikes.

NT

[bert]

Not entirely. I find if I switch on and start in one quick motion the on-off-on of the supply confuses my JVC digital radio and I have to do a reset. Very annoying as several settings are lost.