I have an older car that uses a key to start, (not a button). When the car is running and I’m listening to the radio, I can quickly turn the key all the way counterclockwise to shut the car off but leave the radio on.

There is a brief period when the key is being turned, (1/10 of a second), where power is cut from the radio. The radio however fortunately stays on during this 1/10 of a second when power is briefly lost and doesn’t reboot. This allows me to keep listening to the radio without being interrupted by a reboot.

I have added an external device to the radio (called a “CarPlay” box) that feeds the radio music. It generally draws about 500ma. When I quickly turn the car’s ignition key (as described above), the 1/10 second when power is lost results in the CarPlay being rebooted.

Does anyone know what the simplest way I can prevent the CarPlay box from losing power during this 1/10 of a second? (I of course don’t want to just connect it to battery power full time).

 

We need car make, model, year to see how what system it has for aux, ignition, start

 

You could try connecting a very large capacitor (Say 10000uF or more) inparallel with the power supply to the car play box. You would also need a diode in the positive feed to the capacitor and car play box. This it to prevent the capacitor from trying to power other things (Such as the radio itself.) that are on this 12 volt feed. I think you will just have to try increasing the capacitor value if the first one you try does not work. This is because we dont know at what voltage level the car play box considers that it has lost power. You would need a diode with a fairly high current rating as there will be a large current through the diode to charge the capacitor. An alternative is just use the capacitor to hold a relay closed and feed the car pay via the relay contacts from an unswitched power feed.

Les.

 

You could try connecting a very large capacitor (Say 10000uF or more) car play box…. also need a diode in the positive feed to the capacitor and car play box.…

Thanks, this sounds promising. The CarPlay most often draws 250-350ma (only maybe drawing 500ma on boot). [1] Should I try placing the capacitor inside the CarPlay box simply connecting + and - on the input to the CarPlay along with inserting the diode on the positive input of the capacitor?
[2] Is there a specific diode that may be appropriate?

 

I have never seen a car play box and have no idea what it's function is so I don't know if the capacitor will fit inside the device. I would try a diode with at least a 3 amp continous current rating. (It's peak current rating will be much higher) When testing the idea on the bench you would not need the diode. Just try the idea using capacitors that you have in stock.

Les.

 

OK, many thanks. I will try when I get a chance to dig up some old capacitors.

CarPlay is a device (about $100) with an LCD touchscreen screen that connects wirelessly to an iPhone and car radio. It runs streamlined versions of some of the iPhone’s apps such as Maps, music, backup camera, and the phone, playing the audio over the car radio. It mounts on the car’s dash. It is easier to use while driving and also takes voice commands. Mine is 9.5” x 4.5” x 1.25” and has a lot of empty space inside.

 

Certainly Les has described the simplest and most fail-safe scheme. I would add a 100 ohm series resistor to the diode to limit the charge current at first turn on. That will not effect how it works when the switch turns OFF. AND the resistor will also serve as a fuse if the capacitor fails shorted.

 

MisterBill 2, Adding the 100 ohm resistor would not work with the capacitor supplying enough current to power the car play device (Which takes 500mA.) It would work if an extra 2 diodes were added. One between the supply that drops out fot 100 mS and the car play device and one between the the capacitor and the car play device. The original diode which is now in series with the 100 ohm resistor would no longer need to supply the high initial charging current to the capacitor. All 3 diodes would only need to be about 1 amp rating such as the 1N400x series diodes.
It would work with my second suggestion using a relay if the relay drew a small enough current to work with the 100 ohm resistor in series with the coil. All these suggestions assume that the car play device will work with 0.7 volts less than the battery voltage.

Les.

 

MisterBill 2, Adding the 100 ohm resistor would not work with the capacitor supplying enough current to power the car play device (Which takes 500mA.) It would work if an extra 2 diodes were added. One between the supply that drops out fot 100 mS and the car play device and one between the the capacitor and the car play device. The original diode which is now in series with the 100 ohm resistor would no longer need to supply the high initial charging current to the capacitor. All 3 diodes would only need to be about 1 amp rating such as the 1N400x series diodes.
It would work with my second suggestion using a relay if the relay drew a small enough current to work with the 100 ohm resistor in series with the coil. All these suggestions assume that the car play device will work with 0.7 volts less than the battery voltage.

Les.

Given that the "car play device" is intended to work on an automotive electrical system, certainly it will work on 0.7 volts less that the 13 volts provided when the vehicle electrical system is charging.
And it does appear that I had neglected to consider the second diode to isolate that supply from the rest of the accessory loads. Sorry about that omission, good thing it was caught.