My scenario is simple. I have a 12V to 5V inline USB convertor powering a dashcam/dvr. However, the circuit of the car I'm using to power it isn't clean enough, especially with the car's stop-start function (the cig lighter is fine but gives me unsightly cables everywhere).

Looking at this How do I create a USB cable with a capacitor in-line?

Someone there suggested the formula for calculating the size of capacitor needed.

So assuming the car has 12V feeding it, I could hopefully handle a few volts drop (say 2V) for ~ 1 second. Assuming the DVR consumes 500ma.

This gives me a fairly large 0.25F capacitor. Can someone please correct my maths.

Also when I check the 12V to 5V USB, they've provided solder points on both sides of the circuit board. So I could put the capacitor on the 12V or the 5V side - or both. Can someone clue me in what's the problem with each approach? Will the discharge rush cause my DVR to become unhappy on the 5V side? Will the capacitor upset the car's internal gadgets if I put it on the 12V side (fault codes and what not).


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    \$\begingroup\$ Clarify specs: Input V conditions, requirements. Does it go open circuit during start or just Vdrop 12 to 10 yet converts to 5V so why does this matter. \$\endgroup\$ Aug 13 '17 at 9:56
  • \$\begingroup\$ To clarify, when the car stop-starts, the DVR/dashcam shuts down and uses it's inbuilt capacitor to write back. When connected to the cig lighter, this does not occur. \$\endgroup\$
    – brett-au
    Aug 13 '17 at 10:13
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    \$\begingroup\$ Your math is probably correct. A battery backup is better. \$\endgroup\$ Aug 13 '17 at 10:28
  • \$\begingroup\$ Sounds like you could tap in to the circuit that powers the cig lighter and bypass the problem. \$\endgroup\$ Aug 13 '17 at 19:00

When cranking a car engine, the battery voltage sags significantly. Real automotive electronics is designed and tested against these special brown-out conditions. One solution to this problem (except it was a bluetooth device) is given here.

The solution includes a super capacitor (100 mF) and a Schottky diode, to prevent possible back flow discharge.

While technically the diode-capactior network should be placed at 12V side (it would require less capacitance to hold the same amount of charge), the cost of 16-V supercaps is much higher (`20X) than 5-V supercups.

The other solution would be a 5-V circuit with battery backup with hold time of 20-30 seconds. It will have smaller size (a 50 - 100 mAh Li-ion battery would be enough), but electronics will be much more sophisticated and expensive.


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