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I have a Sony A7R camera and i want to make a power supply that runs of a 12v lead acid and has a output of 4.2v so i can take long time lapses, i don't know much about electronics so i came here to ask for some help.

I was thinking of using a 12v to 5v converter and a LT3083 to adjust the voltage from 5v to 4.2v, this would also mean i could use a 5v battery bank to power the voltage regulator, do you think this will work?

Because the camera is quite expensive i want to add over voltage protection and a display that shows the estimated battery life, for the voltage protection i could use a MCU and a voltage divider to measure the voltage on the LT3083 output, if the voltage is correct the MCU can turn on a transistor that goes from the voltage regulator to the camera, and i could use another voltage divider to measure the voltage of the 12v lead acid to give a estimated battery life.

If you could give me your thoughts and any improvements i would greatly appreciate it.

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  • \$\begingroup\$ You can get adjustable buck converter boards which are ideal for this: take the 12V directly down to 4.2V. \$\endgroup\$
    – pjc50
    Jul 9, 2015 at 20:21

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I was thinking of using a 12v to 5v converter and a LT3083 to adjust the voltage from 5v to 4.2v, this would also mean i could use a 5v battery bank to power the voltage regulator, do you think this will work?

Yes, but it isn't the best way. There will be losses with the LDO. Use an adjustable DC-DC converter to go from 12V directly to 4.2V. TI makes lots of parts that are easy to design around.

Because the camera is quite expensive i want to add over voltage protection and a display that shows the estimated battery life, for the voltage protection i could use a MCU and a voltage divider to measure the voltage on the LT3083 output, if the voltage is correct the MCU can turn on a transistor that goes from the voltage regulator to the camera,

This will be too slow. By the time the overvoltage is detected, it's likely your camera will be dead. There are several questions about overvoltage protection on here already, do a search.

...and i could use another voltage divider to measure the voltage of the 12v lead acid to give a estimated battery life.

That is reasonable. I would recommend a high side switch on the voltage divider so you can turn it off when not taking a measurement. Otherwise, it's a constant load on the battery that will slowly drain it.

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  • \$\begingroup\$ The drain of for example a 470K and a 47K resistor divider to the ADC will only drain ~23 microamps in order to detect battery level, surely for most situations this is not bad? Even a large filter capacitor array nearby would probably leak more than this continuously. \$\endgroup\$
    – KyranF
    Jul 9, 2015 at 20:28
  • \$\begingroup\$ @KyranF 12V SLA battery probably not, on the other hand on one of my current projects, the sleep current is less than 23uA. There is more to it than the current draw. The input resistance of the ADC will affect the resistor choices. \$\endgroup\$
    – Matt Young
    Jul 9, 2015 at 20:30
  • \$\begingroup\$ perhaps an ultra-low power op-amp buffer could act as an interface between the battery and the ADC input.. but yes I can see how with low capacity or very long operating durations, this would take a toll. For something as power hungry as a camera though, it would pale in comparison to the operating current/idle/leakage inside the device.. All good stuff! \$\endgroup\$
    – KyranF
    Jul 9, 2015 at 20:35

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