Maintaining power to a car FM transmitter

No, not that FM transmitter](

I own a Griffin RoadTrip transmitter for getting my iPod to play on my car stereo. I also try to keep my engine off when I know I’m going to be stopped for a couple of minutes. The problem comes when I restart my engine–the cigarette lighter (wisely) loses power when I turn the key to start, which causes the RoadTrip to shut off, which means I’m greeted each startup by gratuitous amounts of loud static from my stereo.

The slacker approach would be to find an FM transmitter that worked off of my iPod’s battery or an internal battery. Aside from being a nerd, I don’t see a good reason for tossing a perfectly good unit.

The nerdy solution, it seems, would be to find some way of storing enough energy to allow the RoadTrip to remain powered for two or three seconds when power from the cigarette lighter is lost. I’m not sure just how much juice the RoadTrip requires, though I imagine it varies according to the iPod’s charge state as well (as the RoadTrip can charge an iPod through the dock connector). Regardless, the cigarette lighter is fused for 10 amps at 12 volts. Anyone who’s been in or around my car will attest that there are no points for style.

What general approach would offer the best chance of success? What kind of precautions would need to be undertaken to protect the other things in the car beyond the obvious ones you learn from building FRC robots? Or am I just better off mashing the power button to turn it back on each time I restart my engine?

The cigarette lighter socket should not lose power when you start the engine…at least not on a “normal” car like a Chevy. Standard disclaimer for other brands applies.

Wife has a Griffin itrip and it’s a pain, it does turn itself off all by itself and needs to be unplugged and plugged in again sometimes to get it working after stopping the ipod.

The Monster transmitter she had always stayed set and working and never lost track of what channel it was set to, etc. kind of expensive, but it worked well, although eventually it lost a channel due to a flakey docking connector.

I have an older VR3 transmitter that does not power the player, it loses channel info when unplugged, but I leave it plugged in most of the time as it doesn’t use much power.

Not sure why you shut off the engine when stopped for a short time, as you most likely don’t drive a 55 chevy with a blown big block like I do :slight_smile:

One option I can think of right of hand would be to buy a decent sized capacitor and a diode with low voltage drop and wire them so that your car charges the cap and the cap is directly connected to the socket. Another option would be to add another socket to your car wired from the battery and fused live (on) all the time.

I have done both options but liked the later better. It is also convenient to have this other socket for running a charger for something while the car is off or running an inverter for someone playing on a game system in the back seat…

The reason for the socket losing power is from the contacts in the ignition switch opening momentarilly.


Odd, I’ve never seen a lighter socket on a switched circuit. But I don’t work on imports, either.

I would have to say nearly all cars now adays have the socket switched with the ignition. This prevents the battery from going dead if someone had something plugged in for a long period of time (trip) while the vehicle is off.


Interesting, must be a 21st century thing…both of my wife’s 1999 model cars have the lighter on full time.

Some cars have both ignition-on and always-on lighter sockets. They are usually distinguished by little pictograms (for example, a key for the ignition-on socket, and a battery for the always-on socket). Billfred, check if that’s the case, it could be an easy solution.

It may be easier to put a constant power socket in the car. I did that for my ham radio, it took about 30 mins to do.

i know that on our boat, the 12v sockets give out momentarily when the engine starts simply because the starter draws too much current causing the voltage to drop.

Actually, most lighter sockets cut off when the starter is engaged, to protect the equipment connected from the wild voltage transients that come from engaging a highly inductive kilowatt-plus load.

Anyway, to the problem: A simple solution is to make a fortifier. The cigar plug goes through a single diode (1N400x) in the + lead, then to a 12 volt rechargeable battery. A very small battery, smallest you can find. A large capacitor (Rated to 16 volts min) would work, too. The battery or cap is connected across the input, and also leads to your device.

Let me know if you need a wiring diagram or some suggestions for batteries. It should all fit inside a 2x3x5 Radio Shack project box, one end is a cigar plug, and a socket is mounted to the box, so you can use it for anything.