device to limit voltage?

Printed From: the12volt.com
Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=93629
Printed Date: May 15, 2025 at 2:25 PM

Topic: device to limit voltage?

Posted By: frozenus
Subject: device to limit voltage?
Date Posted: May 04, 2007 at 7:06 PM

Hey all, this is my first post, hope its in the right spot.

I have a carpc setup, with a laptop and a dc-dc power supply. The problem is that while the PS is regulated to 19v, it's not precise enough and sometimes goes as high as 20v (typically right after engine cranking), which is too much for the laptop, and afterwards i cant start the laptop unless i remove the plug, or restart the power supply. It normally supplies 19.3v which is perfect. Is there a device I can use to limit the voltage to 19.5v max? Not sure what I'm looking for, just hoping you can point me in the right direction. Is this do-able with a zener diode?

Thanks.

Replies:

Posted By: i am an idiot
Date Posted: May 04, 2007 at 8:19 PM

Other than charging the battery your laptop should run fine with 12 volts input. Just remove the battery from your laptop when using it in the car

Posted By: frozenus
Date Posted: May 05, 2007 at 12:36 AM

sorry i guess i wansnt very clear.

i dont have a battery in it. the PS turns on when i turn the ignition on, and after a couple seconds the laptop starts automatically. i just want to limit the voltage for the first couple of seconds the ignition is turned on because it's too high. (laptop requires a 19v input, and the PS is delivering 19.8 at its high point).

Posted By: i am an idiot
Date Posted: May 05, 2007 at 6:27 PM

No, you were very clear.

   I was the one that must not have been clear.

   Your laptop will run with 12 volt dc input "meaning you do not need the DC to DC power supply".

    The reason the laptop requires 19 volt input is to charge the battery.

    Yes you can use a zener diode to build a regulator circuit. Depending on the current draw of your laptop you will need an 18 volt 1 watt zener a couple 1n400X diodes, a resistor and a couple transistors.   Let me know if you need a digram to build it.