the12volt.com spacer
the12volt.com spacer
the12volt.com spacer
the12volt.com spacer
icon

How to drop a 12v signal to 5v


Post ReplyPost New Topic
< Prev Topic Next Topic >
jimxms 
Member - Posts: 2
Member spacespace
Joined: June 16, 2017
Posted: June 16, 2017 at 3:39 AM / IP Logged Link to Post Post Reply Quote jimxms
Hi all, first post here from a complete electronics novice.
I have a 0-12v signal that I would like to monitor in my car using a datalogger that I have installed. However the datalogger inputs are only 0-5v.
So I think what I need to do is find a resistor that will drop the voltage like so: 12.0v would be 5.0v, 10v would be something like 4.1v, 6v would be something like 2.4v.....etc
stuc 
Member - Posts: 9
Member spacespace
Joined: September 12, 2008
Location: Massachusetts, United States
Posted: June 16, 2017 at 12:16 PM / IP Logged Link to Post Post Reply Quote stuc
A resistor won't do the trick. You want to use a voltage regulator
https://electronics.stackexchange.com/questions/109039/reducing-12v-to-5v
If you have a newer car you can probably pull it off the USB port. If you want to be lazy you can use the electronics in a car phone charger to get the 5V
jimxms 
Member - Posts: 2
Member spacespace
Joined: June 16, 2017
Posted: June 16, 2017 at 12:22 PM / IP Logged Link to Post Post Reply Quote jimxms
The top answer on that link refers to using a LM7805 voltage regulator. However the specs sheet states: "7V To 25V In, 5V Out". This sounds like no matter what boltage between 7-25v you put in, you will always get 5v out? If so, this is not what I want.
stuc 
Member - Posts: 9
Member spacespace
Joined: September 12, 2008
Location: Massachusetts, United States
Posted: June 16, 2017 at 2:29 PM / IP Logged Link to Post Post Reply Quote stuc
My bad I can't read sometimes
Assuming signal integrity isn't critical you can use a voltage divider (two resistors). You'll want to throw a opamp/voltage follower on there so you don't have to mess with tuning the resistors based on the dataloggers properties
http://www.learningaboutelectronics.com/Articles/Voltage-follower
Theoretically you'd choose resistors with values R1 = (7/5)*R2 but you may want to round down a bit so you don't put the signal in the datalogger over 5V
You could go with a 3.3k and a 2.2k
http://tinyurl.com/y794e5ea
(the bottom resistor is where you'd put the input to the datalogger)
You can do it without the op amp but you'll want to measure the datalogger channel resistance and account for that
i am an idiot 
Platinum - Posts: 13,666
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: June 16, 2017 at 9:01 PM / IP Logged Link to Post Post Reply Quote i am an idiot
A regulator will not work for what you are trying to do. A regulator will put out 5 volts any time the input voltage is over 6 volts or so. You need to build a voltage divider. 1.2 K resistor and a 1K resistor. Connect the 1.2 K to to 12 volt wire. Connect the 1K to ground. Connect the 2 resistors to each other. Connect your device to the junction where you connected the 2 resistors together. I did not do the math on this, but with 12 volts across the 2 resistors there should be around 5 on the junction. 6 volts across the resistors should yield around 2,5 at the junction.
No idea what the input resistance of your device, you may have to adjust values if the voltage drops when you connect the device.
I did not read the above post till after I made this post. I too so not think you will need an op amp. Build either of the dividers, check voltages after you connect your device. Let us know what the junction voltage is and which resistors you used and we can guide you on necessary adjustments.

If you wish to post a reply to this topic, you must first login.
If you are not already registered, you must first register.

  Printable version Printable version Post ReplyPost New Topic
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot delete your posts in this forum
You cannot edit your posts in this forum
You cannot create polls in this forum
You cannot vote in polls in this forum

  •  
Search the12volt.com
Follow the12volt.com Follow the12volt.com on Facebook
Thursday, March 28, 2024 • Copyright © 1999-2024 the12volt.com, All Rights Reserved Privacy Policy & Use of Cookies
Disclaimer: *All information on this site ( the12volt.com ) is provided "as is" without any warranty of any kind, either expressed or implied, including but not limited to fitness for a particular use. Any user assumes the entire risk as to the accuracy and use of this information. Please verify all wire colors and diagrams before applying any information.

Secured by Sectigo
the12volt.com spacer
the12volt.com spacer
the12volt.com spacer
Support the12volt.com
Top
the12volt.com spacer
the12volt.com spacer