Electronic – Car bluetooth power delay- USB capacitor?


I just bought a bluetooth adapter for my car. It's input is 5V/0.3A and is powered via a 5V/1A USB power adaptor that is plugged into the 12V/10A/120W max. socket in the center console of my 2004 Honda Accord.

It turns on automatically when I turn my key halfway, but when I start my engine it shuts off, and I have to unplug it and plug it back in to get it to power back on.

I emailed the company, and this is what they said:

"We are sorry for all the inconvenience it caused to you. As you know, there is no internal battery for the SoundSync. The power is from the car or car charger. For some cars, when we turn the car ON, the SoundSync will power on at the same time. However, when we start the engine, the power shut off for a very short time. It shuts the SoundSync off and the SoundSync will not auto power on again. Therefore, we will change it to button control version in the short future."

I'm trying to figure out a way that I can fix this. Unfortunately, my knowledge of electronics is limited. If I had a capacitor between the device and my car, would it solve the problem? I.e. the capacitor takes 1-2 seconds to charge, which interrupts the on-off-on of the car starting, and provides continuous power to the bluetooth module?

I have a soldering iron and am pretty tech-savvy, I just don't know where to begin troubleshooting the issue. Any input would be greatly appreciated.


Edit: How would I go about adding a capacitor? Solder together a circuit board, capacitor, and male/female USB pieces? Cut an existing USB cable and use it instead of the USB components?

Best Answer

It looks like you bought a badly designed adapter, which is not tolerant to "brown-out" conditions. In cars, when starting an engine, the 12V outlet voltage drops or even toggles several times. All car-intended equipment are designed to withstand this power shortage.

However, adding a simple capacitor to USB cable might not be able to solve this problem. Since your power requirement is 0.3A at 5V, it means that the load is about 5/0.3=17 Ohms. To hold the VBUS power for, say, 1 second, on 17 Ohm load, you will need more than 60mF (milliFarad!!!) of capacitance, based on crude estimation from time-constant RC = 1s. This should be a super-capacitor to do the job.

ADDITION: You can try this 100mF super-cap/5.5V, it is just about $2.80. To avoid back discharge into the 12V - 5V converter, I would use a medium Schottky diode before the capacitor. enter image description here

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