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I have a USB car charger plugged in the accessory port of my car and I also have a simple bluetooth USB to aux adapter plugged into it. As I start the car, the power drops to the accessory port and the adapter's bluetooth connection disconnects from my phone. It's annoying having to wait for it to reconnect and start my music every time, plus its illegal to operate a phone here in the UK.

I want to know if I could create a simple circuit to hold enough power to keep the adapter running while the power drops. I think the power drop lasts roughly 5-10 seconds and the bluetooth adapter is only 5v 0.1a. Could a capacitor help in this situation?

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2 Answers 2

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Assuming the input current to your charger is (say) 50% of the output current i.e. 50 mA, you can calculate the capacitance to hold up a voltage (in conjunction with a diode) using this formula: -

$$I = C\dfrac{dv}{dt}$$

"dV" is the droop voltage that you might reasonable expect when supplying 50 mA from the capacitor and, I'd go with a figure of 3 volts for this. "dt" is 10 seconds and C is the capacitance.

So, plugging those numbers into the formula and solving for capacitance gives you a value of 167,000 uF. That is a big capacitor. This is the sort of circuit I'm talking about: -

enter image description here

Alternatively you could buy a buck-boost charger circuit that might work down to 3 or 4 volts and stay working at this level when the car is being started. If you were to design one I'd recommend this sort of circuit: -

enter image description here

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  • \$\begingroup\$ Would something like this work? I can connect it to either the USB charger or a ignition live fuse. \$\endgroup\$
    – rainbow6-for-life
    Commented Jul 30, 2018 at 15:34
  • \$\begingroup\$ It will start to fail when the 12 volts drops to 5 volts or lower so I'd go with a buck-boost regulator rather than just a buck regulator. Given that we don't know how low the voltage from the battery might drop to during brown out, it's probably best to think "buck-boost". \$\endgroup\$
    – Andy aka
    Commented Jul 30, 2018 at 15:37
  • \$\begingroup\$ What about these products, sorry for not fully understanding if they'd work. 1 2 \$\endgroup\$
    – rainbow6-for-life
    Commented Jul 30, 2018 at 15:53
  • \$\begingroup\$ (1) has the right sort of spec - stepup-step down but (2) looks the more robust but I can't recommend anything not bought from a reputable supplier (ebay/amazon/aliexpress don't count as reputable in my book) plus the device clearly has to be stated as being suitable for automotive applications because the surges and transisents can easily destroy most seemingly suitable electronics. Even the LTC3115 circuit above would need a small series power limiting resistor and a 20 volt transzorb to make it safe to use. \$\endgroup\$
    – Andy aka
    Commented Jul 30, 2018 at 16:31
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The way to address this is to charge a large value capacitor up to 12V through a diode. The diode is there to prevent discharging the capacitor when the 12V source drops out. Then select a 12V to 5V switching regulator module that is powered off the 12V stored in the capacitor. Make sure to select a switching module (buck regulator) that is specified to work in a wide range of input voltages (for example 7.5V to 18V). Such regulator should be able to supply the output 5V as long as the capacitor voltage is greater than the minimum input voltage of the switching regulator.

So then the source brownout occurs the capacitor will be able to supply the regulator till it has discharged the capacitor too much. Select the capacitor value (or add additional capacitors in parallel) till your load is maintained through the brownout.

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