I am looking to get a constant, clean 12V from my cars 12V line. I am wondering if all I am in need of to pull this off would be a simple 12V fixed regulator?

The 12V would be hooked up to a pico motherboard (19v max input), 7" LCD monitor and an Arduino.

I did find this schematic here:

enter image description here

So would a simple regulator be good or not? Will it need a heat sink?

  • 3
    \$\begingroup\$ As others note - car 12V rail drops to << 12 on starting. BUT auto 12V line is noisy and spikey and electronically very dangerous, A purpose built linear regulator will be MUCH better than your transistor version. LM2940 series is one such. See my longish answer here re jump starting for relevant comment. LM2940 datasheet here Note dropout is typically 0.5V at 1A so will fail at about 12.5V in. \$\endgroup\$
    – Russell McMahon
    Commented Jun 24, 2013 at 17:07
  • \$\begingroup\$ For heatsink comment see My linear voltage regulator is overheating \$\endgroup\$
    – Russell McMahon
    Commented Jun 24, 2013 at 17:08
  • \$\begingroup\$ Use of an adequately large filter cap MAY save you from cranking voltage drop. Or a small battery floated on the output. Note that small 12V lead acid batteries floated 1 diode drop below car battery will die young as even a diode drop gives too low a float voltage [Ask me how I know :-) ]. \$\endgroup\$
    – Russell McMahon
    Commented Jun 24, 2013 at 17:10
  • \$\begingroup\$ @RussellMcMahon babies need more food don't they LOL \$\endgroup\$
    – Andy aka
    Commented Jun 24, 2013 at 17:43
  • \$\begingroup\$ In addition, a one-transistor regulator will not give you good regulation. It's only a single stage of gain with only local negative feedback. Regulators need to have a large open-loop gain in order to have good PSR. \$\endgroup\$
    – Kaz
    Commented Jun 24, 2013 at 17:45

1 Answer 1


The 12V on your vehicle will probably drop below 12V while you start the engine. That, and the noise/spikes on top of the 12V means a simple linear regulator like the one you have in your diagram won't do what you want or expect.

Your circuit is for an input of 12.8V to 30V and it produces an output which is 12V and a little less noisy. If the input voltage (specified at 12.8V) fell a few hundred milli-volts the output would do the same and probably more so, for a 12V input you might only be getting 11V out and, the circuit is no longer regulating. It is providing some input noise rejection but probably not enough as you might expect.

The next question is how low can you tolerate the supply to fall below 12V before the pico motherboard, 7" lcd monitor and and arduino don't work. If it's 10V then you might be OK but not during the period when you start the engine because the input voltage could drop to below 8V - then you'll only be getting about 6 or 7 volts from the circuit.

Ideally, and without fully understanding what you really need I'd go for a buck boost regulator and there are plenty to choose from Linear Technology amongst others: -

enter image description here

  • \$\begingroup\$ would it be possible to produce a schematic that i would need? \$\endgroup\$
    – StealthRT
    Commented Jun 24, 2013 at 23:52
  • \$\begingroup\$ I've attached a schematic from the data sheet - if you check the data sheet it has more information about components. \$\endgroup\$
    – Andy aka
    Commented Jun 25, 2013 at 6:59

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