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Best way to convert 0-14V (in vehicle) to 0-5V for A/D ?

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panalog9

Full Member level 2
voltage follower with 0-5v source

For a data-capture project I need to log several signals in a car. Some are 0-5V and some are 0-14V.

What is the best way to feed those signals into a 5V microcontroller A/D section ? Note, I need the values, not just on/off.

Simple voltage divider ?

btbass

14v to 5v dual power supply using z-diode

Yes, simple voltage divider, but buffer with voltage follower opamp for low output impedance.
:wink:

panalog9

Full Member level 2
diode 14v to 5v

Ah, OK.

Do you have any sample diagrams or hints ? I don't know analog/opamps Heh, this is how we learn ...

panalog9

Full Member level 2
diode input 14v output 5v

Hmmm - looking at opamps, it seems like they require both + and - voltage. That is, +12V, GND and -12V.

Do they have to have that -12V ? That would mean another voltage regulator to produce it in the circuit.

Member level 4
convert 14v to 5v

Hi
use the old tl084
it has 4 op amps and works rail to rail on 5 volts
For signaling i normaly use a uln2803 with a diode to
sense on/off inputs
Best

panalog9

Full Member level 2
convert 12v to 14v

Hmm. I've been told to stay away from voltage dividers as they will load the signal source. Just use opamps with a <1 gain to reduce the 0-12V to 0-5V.

Now, how do I do that ?

al_extreme

Member level 4
convert 0-5v

ya the best is to use a op-amp generic would be good look for just positive alimentation op-amp you will need only a positive supply voltage

panalog9

Full Member level 2
Does anyone know of a good link to see how to do this ?

That is, interface higher voltage ranges to 5V logic ?

secmig

Junior Member level 1
Texas Instruments has a really great E-Book about opamps called Op Amps For Everyone. It's filled with "for dummies" examples including the math calculations.

secmig

wkulee

Newbie level 6
Thanks. It is the most helpful for my project.

Phytex

Member level 4
Linear Technologies

You can also have a look at h**p://www.linear.com

algilsan

well, it depends on many factors:
The most importarnt is to know if the circuit will be powered with no key ignition on. In other way, if the circuit is to be powered with car battery directly. If this is correct then you most convenience aproach is to use a DC/DC down converter, due it high efficiency
if not, then you can use many types of circuits depending of the current you need and precision of your A/D, if you need low precision and low current you can use a simple zener diode. If you need much current and precission you will need linear regulators or specific regulator with more precission.

i'll hope you be usefull

panalog9

Full Member level 2
Very nice - thank you for the link.

The project will be powered from battery all the time. Ignition will be an input to tell system when to be "live". This is so when car is turned off system has a chance to to finish writing buffer out to flash memory.

It using a dual-level power supply. Switching regulator from +12V down to 5.5V, then LDO linear down to 5.0V for circuit, and another for 5.0V for A/D section of mcu.

Some signals to be logged in car are 0-12V though, hence need to scale them down. I imagine opamp powered by 12V with gain of 12/5 - reduce signal instead of typical amplify it.

I know using a 1k resistor in input to inverting input and 10k resistor in output feedback to inverting input will be 10x gain. I think then using 12k resistor in input and 5k in output feedback will be 12/5 (reduction) gain, right ? Or similar ratio resistors.

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