# How do you regulate a car battery down to exactly / not more than 12V?

My problem:
I want to light up the inside of transporters with a 12V LED strip. But I don't think powering it with the 12V car battery directly is a good idea. Although the name implies 12V, a car battery does not deliver exactly 12V, but between 13.5V and 11.5V depending on the level of charge and rate of (dis)charging.
So with that, I fear overvolting the LEDs. The forward voltage of LEDs grow very little with the current passing through them, so overvolting can easily increase current by a lot and therefor reduce their live time massively.

My solution so far:
Can't be that hard to regulate the voltage down to 12V, right? ... wrong.

First I tried a KA7812 12V Voltage Regulator [PDF] that I had laying around. But turns out these types of regulators only work for input voltages higher (at least 2V) than the regulated output voltage. (so ≥14.5V→12V, but 13V→11.1V, 12V→10.2V, ...)

Then I tried to design my own voltage regulator that can regulate voltages down to 12V, but lets voltages lower than 12V pass (more or less untouched). I started with the basic op-amp design:

But I soon discovered that this design has also a minimum voltage drop of at least 0.7V because the base-emitter junction not allowing the op-amp to regulate Vo=Vi. When Vo is the emitter of the transistor and the op amp can only output it's supply voltage (=Vi) as a maximum to the base pin, a reduction of 0.7V is given to satisfy the BE forward voltage and CE current can flow.
And it's getting worse: Assuming Vi is near 12V, then Vo will be even lower than Vi-0.7V. An op amp provides an output voltage floating between it supply potentials with a padding of ruffly ±2V. So 12V as Vi makes 10V as a maximum op amp output, that with the 0.7V forward on the BE ... so Vo can't be more than 9.3V!

So after more hours of trial and error than I am willing to admit, I ended up with his design:
It works in simulation: The voltage drop on Vi≤12V is minimal (on a weak load) and at no point strip± gets bigger than 12V.

But I have never seen such a design before. At this point I'm not sure if this circuit works well in practice or if there are better ways of doing this. So my question is: Should this for me be the way to go?

• Your LED strip will have some form of current limiting - they aren't just bare LEDs - so they will probably work fine on the automotive "12 volts". Apr 5 at 18:36
• I think you need a buck-boost converter to drive the LEDs at constant 12V regardless of the input voltage. Do bear in mind that LEDs that are not driven with constant voltage but with constant current would anyway need a converter which might take in variable voltages, so having a converter and then 12V constant voltage LEDs that have series resistors is less efficient. Apr 5 at 18:48
• One option is just to use automotive rated lighting strips. The voltage range of a car battery gets even wider during charging. I've never compared resistor values, but otherwise they appear near identical to the indoor ones. They don't appear to burn out particularly fast but I haven't had the chance to monitor one for more than 2 years.
– K H
Apr 6 at 2:19
• I would say you are over-thinking this. The strips have LEDs and resistors making up the full voltage drop. Measure the current at 11, 12, 14 V and then decide if it's worth the trouble. A large diode dropping 0.7 V, or two, might be all you need; the strips produce plenty of light well below rated voltage. If you're using resistor-based led strips and linear droppers then efficiency is not your primary concern anyway. Apr 9 at 18:47
• @tomnexus I think I'll actually go with a big diode in series. The thing's not worth the trouble. I already own diodes that are chunky enough for the job and when I tested the LED strip with 11V, that worked very well. Apr 9 at 20:20