My brother in law bought a Prius recently. He's experiencing something strange with his iPod. If he has his iPod plugged in via the USB cable to the Prius, everytime he starts up his car, it resets his iPod. If he unplugs his iPod first, starts the car and then plug in the iPod, the iPod is fine.

What can be the problem? Is there some sort of surge in the Prius circuitry that resets the iPod? Is there an electronic hack for the USB cable to fix this?


I would say it is very very dangerous for iPod. Eventually it might die instead of reset. This must be the transient spike going through USB.

So, I would take a look at it at oscilloscope. And before that I would switch to separate 12v->USB adaptor, not the bundled one.


How about decoupling capacitors for the iPod?

Capacitors basically resist voltage change, so you could design a device which will go in between the iPod and USB cable and have some decoupling capacitors which should be strong enough to power the iPod while the car is starting.

You could have one male and one female USB port on the device and put the capacitors between the GND and +5 V pins. The data pins should be directly connected between two plugs, because iPods may need some circuitry there and since the charger does work, it would be best to use its circuitry for that. Here's USB pinout.

From what I've read, if we have 3 V of ripple when the car is starting, 2200µF electrolytic capacitor would probably be enough for full USB 2.0 load, but I'm not too sure. For voltage rating, I'd take at least 25 V, but more is better in that area. You could also look for low ESR capacitors. They should work a bit better. Some people say that it's a good idea to put in parallel capacitors of several different values, because they all block noise at different frequencies.

Another option worth considering is to get Toyota's iPod connector, but it's going to be more expensive and defeat the purpose if having a hack.

  • 1
    \$\begingroup\$ I wonder if you could just take the one male and one female USB connector and take the power and ground, add the caps to smooth the power and connect the corresponding data lines together, and the circuitry that is needed for the charging is built into the cable. \$\endgroup\$ – jsolarski Feb 17 '11 at 13:44
  • \$\begingroup\$ @jsolarski That's what I meant. \$\endgroup\$ – AndrejaKo Feb 17 '11 at 14:59

I wouldn't be so sure that it is a voltage spike issue. In a Prius you don't have an engine starting when you first turn on the car so there isn't as high as noise as most standard engine cars. I am going to guess it is 1 of 2 things:

  1. I believe the Prius is one of the cars that will actually communicate with the iPod and pull off data over USB. If this is the case I could see the negotiation with the iPod getting interrupted on start up causing the iPod to think something funny was going on and resetting itself.
  2. There is a large amount of electronics getting turned on at the same time that are all pulling from a battery. This situation could cause a dip in power that could cause the iPod to go in a brownout state that it is smart enough to recover from.

If either of my guesses are true, then most of what has been mentioned before wont work. Ex:

voltage regulator - the car runs at higher then 5v so they already have to have a voltage regulator of some sort. I don't think regulating again will help at all

Switching to your own 12v to 5v regulator - If you do this then you will loose the ability to pull data over USB.

Decoupling Caps - This might help, the tricky part is figuring out where to put them. If it is a negotiation issue the cap will need to be placed on what ever component handles this negotiation. Probably will be hard to find and will most likely void a warranty.


When the car is started there is either a voltage spike or drop, as for a fix, Create your on DC-DC converter, 12v(car battery) to 5v(usb). you could easily use a simple 7805 regulator and a few caps to even out the signal, and prevent those spikes/drops.

  • \$\begingroup\$ Hmmm...I think I need to figure out what my brother-in-law is using to connect to the Prius. If he's just plugging in his USB cable directly into the Prius, then I think the 7805 is not going to work...don't they take a higher input voltage? USB is at 5V already no? \$\endgroup\$ – milesmeow Feb 17 '11 at 7:43
  • \$\begingroup\$ i was stating to bypass the prius usb and connect to the electrical system(12v) I really dont know of anyway to prevent it from browning out like that from the usb, unless you want to hack a Ipod Cable and put something to prevent that brownout/spike. \$\endgroup\$ – jsolarski Feb 17 '11 at 8:14
  • \$\begingroup\$ Oh I see. Ok...if we want to purely power up the iPod then this will work. We also want the iPod to communicate with the car's audio system...so your suggestion won't work in our situation. \$\endgroup\$ – milesmeow Feb 18 '11 at 8:03

Not sure about the Prius, but normal automotive power surges are a very nasty environment for electronics. You can have low dips and high voltage surges. Special care is required with caps as discussed above as well as diode protection at the input. The car lighter converter is probably a little light for the job. [update] An related article from a vendor http://www.maxim-ic.com/app-notes/index.mvp/id/760.

  • 1
    \$\begingroup\$ how are car lighter converts a little light for the job? \$\endgroup\$ – jsolarski Feb 17 '11 at 13:37
  • \$\begingroup\$ if the convertor is passing through a surge that is resetting the device, then it's too light/weak or not up for the job. \$\endgroup\$ – kenny Feb 17 '11 at 13:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.