Voltage reduction via resistors?
Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=34512
Printed Date: May 13, 2024 at 10:05 AM
Topic: Voltage reduction via resistors?
Posted By: GalpinAudio
Subject: Voltage reduction via resistors?
Date Posted: June 24, 2004 at 4:34 PM
I was wondering if anyone had a chart or formula for determining voltage reduction using resistors. I am trying to reduce my 12V + to 5.4V. Any help or suggestions anyone could provide would be greatly appreciated. Matthew ------------- Never Let Those Who Say It Cant Be Done Stand In The Way While You Do It!
Replies:
Posted By: sandt38
Date Posted: June 24, 2004 at 5:05 PM
That is totally current dependant Why would you want to do that? There are many 5 volt sources on a car, but my suggesting where to tap into it is dependant on why you choose to do it.
https://www.bcae1.com/ohmslaw.htm
Ohms law is your friend. Look at that link for the formula.
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Posted By: DYohn
Date Posted: June 24, 2004 at 5:12 PM
Here's a voltage divider calculator: https://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm Of course, another way to do it (depending on your current load) is using DC to DC converters.... ------------- Support the12volt.com
Posted By: xetmes
Date Posted: June 24, 2004 at 8:30 PM
I would highly recommend a voltage regulator for most cases, cheap and effective, resistors will always need a current requirments factored into the design..
Posted By: kgerry
Date Posted: June 25, 2004 at 10:42 AM
true...plus it's not a stable voltage value.... there are lots of good 5V regulators out there cheap.... go to any electronics supplier and ask for a 7805 regulator... probably cost you less than a buck
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Kevin Gerry
Certified Electronics Technician
MECP First Class Installer
Owner/Installer
Classic Car Audio
since 1979
Posted By: GalpinAudio
Date Posted: June 28, 2004 at 10:06 AM
Thanks everyone for the help. I am installing an aftermarket deck in a 2004 Ford F150 with factory subwoofer. The remote turn on voltage out of the factory deck is only 5V. If I hook the remote turn on directly off the aftermarket deck it creates a pretty loud turn on pop when initially turned on. I'm assuming that the reason is the higher voltage to the factory amp turn on. I'm hoping that if I reduce the turn on voltage from the aftermarket deck to 5V this will make my turn on pop dissapear. Any additional info or ideas is always appreciated. Thanks, Matthew ------------- Never Let Those Who Say It Cant Be Done Stand In The Way While You Do It!
Posted By: xetmes
Date Posted: June 28, 2004 at 1:03 PM
for that application i would probably just use a resistor voltage divider, Any idea what the current draw of the unit is? (at 5V)
Posted By: GalpinAudio
Date Posted: June 29, 2004 at 9:38 AM
I havnt actually measured the draw but i'd guess it's no more than 3 amps max. It's just a remote turn on for a very small factory amplifier powering a single 8" subwoofer .
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Never Let Those Who Say It Cant Be Done Stand In The Way While You Do It!
Posted By: kgerry
Date Posted: June 29, 2004 at 6:13 PM
if it's just a remote turn on circuit i'd be surprised if it was more than about 500 ma.... any 5V regulator will give you a stable regulated 5V output and takes all of 6 seconds to hook up. 12V in, ground, 5v out...what could be simpler....?? this way if your input supply voltage floats the output stays at a stable 5V
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Kevin Gerry
Certified Electronics Technician
MECP First Class Installer
Owner/Installer
Classic Car Audio
since 1979
Posted By: xetmes
Date Posted: June 29, 2004 at 8:30 PM
woah didnt think it was 3 A. You wouldnt want resistors for that. You could use a discrete regulator, use filter caps on the input and output for the best performance...
Posted By: GalpinAudio
Date Posted: June 30, 2004 at 9:52 AM
I went to Radio Shack and got one of the 7805 Regulators, looks like it will do the trick perfect. thanks for all the help.
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Never Let Those Who Say It Cant Be Done Stand In The Way While You Do It!
Posted By: GalpinAudio
Date Posted: July 09, 2004 at 5:16 PM
Just to give everyone an update... the 7805 regulator was a success! This will work for all Ford, Lincoln, Mercury vehicles with premium sound. If you want do change the head unit, but want to maintain the factory subwoofer and amp, you need to reduce the voltage on the remote turn on from the aftermarket head unit from 12VDC to 5 VDC. I found a great source for all you shop owners to purchase these regulators in bulk. The website is: WWW.Digikey.com Phone # is: 800-344-4539 Product model # is: LM7805CT-ND They have a $25.00 minimum order to avoid shipping surcharge, for an order of 60 pieces they were $0.456 each. My total was only like $27.00 for 60. If you go to a retail store to buy them (If you can find them) they are about $1.50 each. Hope this info is helpfull to everyone. ------------- Never Let Those Who Say It Cant Be Done Stand In The Way While You Do It!
Posted By: xetmes
Date Posted: July 09, 2004 at 6:42 PM
yea you *should* also use filtering capacitors on the input and output but it will probably work fine without. I do alot of ordering from digikey never had a problem...
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