What Resistor for Dropping 12V to 5V?

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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=84958
Printed Date: April 29, 2024 at 5:38 AM

Topic: What Resistor for Dropping 12V to 5V?

Posted By: Blackiewrx
Subject: What Resistor for Dropping 12V to 5V?
Date Posted: November 01, 2006 at 2:14 PM

i need to know how to covert 12v dc to 5 v dc using resistors or somthing? any ideas??

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03 wrx stage 4

Replies:

Posted By: SoundAudio
Date Posted: November 01, 2006 at 2:35 PM

Well the values depend on your current draw but a simple voltage divider could be built. PLace two resistors in series with the 2nd resistor value (5/7) of the first resistor value. Place the resistors between 12v and ground and then you will get 5v at the point between them. This is a very crude way of doing it. It is unregulated so the voltage out will be dependent of voltage in. You will also have to be concerned with power consumption. I believe there was a post earlier with a dedicated part form radio shack that is far better then a resistor voltage divider.

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Good Luck!
-Thad

Posted By: KPierson
Date Posted: November 01, 2006 at 3:03 PM

If you just want a 5vdc output and you will always have at least a 7vdc input use a 7805 from Radioshack. It will provide up to 1A of current (if used with a heat sink).

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Kevin Pierson

Posted By: i am an idiot
Date Posted: November 01, 2006 at 3:17 PM

If you are trying to put a 5 volt LED on a 12 volt source like the others said it is dependent on the current draw, but you will be safe with a 1K Ohm resistor.

If you are needing it for something other than an LED as KPierson said the 7805 is the way to go and the tab of the 7805 is the ground connection so if you need 1 amp of current just mount it to the chassis of the vehicle and it will use the car as a heat sink.

If you need several amps of current you can parallel the regulators.

Posted By: Blackiewrx
Date Posted: November 02, 2006 at 10:13 AM

its just for a little portable xm sattlite radio thingy. im sure one amp will be enough. i will look into this radio shack 7805 thing. thanks.

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03 wrx stage 4

Posted By: soultinter
Date Posted: November 02, 2006 at 11:06 AM

isn't this just like using a resistor to cut the voltage for a ford factory amp turn on? not sure if it was 1k or 1.5k

Posted By: KPierson
Date Posted: November 02, 2006 at 11:31 AM

No, it's not the same. The Ford amp uses a low current input to monitor when it should and shouldn't be on.

The OP is trying to drive a load with a certain voltage. In a voltage divider circuit the more you load the circuit the more the voltage supplied will drive. Resistors should only be used to create reference voltages, and should never be used to create a voltage based power supply

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Kevin Pierson