I'm looking at using a 12V LDO with a 1-2V falloff, so if a vehicle is running the alternator overcomes the 1-2V falloff and the device circuit becomes powered, but I know that I don't know what then happens at 12.4V (nominal battery charge).

I don't know if it continues to supply some unsteady voltage and drain the battery or does it act as an immediate hard power kill.

Is there a better solution to the problem?

  • 2
    \$\begingroup\$ you can use op-amp as comparator. \$\endgroup\$
    – Mitu Raj
    Nov 10, 2017 at 16:04
  • \$\begingroup\$ 12.7 is considered "full" in a flooded lead-acid battery. I see your question but don't understand what kind of circuit you are trying to run with this. Can you please let us know what you are trying to accomplish? \$\endgroup\$
    – SDsolar
    Nov 10, 2017 at 17:40
  • \$\begingroup\$ I was just trying to get a little help with the first half the circuit (kill power to the peripheral completely when vehicle is not in the running state based on general electrical system voltage). The other parts I've seen a lot about, so I know what to read for those. This part of a circuit I saw a lot of one word two word answers or something that didn't quite fit - or I didn't know how to shoehorn it in to fitting as a "kill all power to A when B is crossed". My particular use case is >13V is on <13V is off. I think Mike answered it plenty well enough for me. \$\endgroup\$ Nov 10, 2017 at 18:01
  • \$\begingroup\$ @SDsolar - thanks for the interest btw. I wasn't looking for someone to lay out a circuit for me. I was looking for someone to tell me what to study. >13V pass it through <13V no current. I started by looking at zener diodes mounted backwards, but they didn't pass through enough amperage to be useful for me in this case, although I could have used a bunch of them in parallel I suppose. \$\endgroup\$ Nov 10, 2017 at 18:07

1 Answer 1


If your LDO IC has a shutdown / undervoltage input, you can use that with a resistive divider to enable your IC. What part number are you using?

If you don't have an input like that on your LDO, you could look into using a comparator circuit that looks at the input voltage and enables / disables your part through a logic enable line. Hopefully your part has one of those?

This part might be overkill, but it has what I'm talking about: http://cds.linear.com/docs/en/datasheet/3086fb.pdf

Also, if you're looking to power electronics from a vehicle, make sure you put some TVS diodes on the input - probably and SMA or SMC style package. This will protect you from transients on the power supply.

  • \$\begingroup\$ Thanks. Really new to this, so just doing a lot of reading and asking before I start trying and smoking things :) \$\endgroup\$ Nov 10, 2017 at 16:17

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