# 0-5 v into 0-2.5 volts ?

i have a board with a 0-2.5 v input (not arduino)

i have a sensor with a 0-5v output

is it possible to use a resistor to reduce the voltage ?

what sort of resisitor should i use ?

you need to build a voltage divider.

http://en.wikipedia.org/wiki/Voltage_divider

Look at the first picture of the page (at the right). Thats how you need to set it up. Use the first formula on the page to calculate your resistor values.

Cheers

p.

Thank you ;-)

p.

In this case, you'd use two of the same value resistors, so you dont have to do much calculating.

thats neat - 2 resistor values the same, reduce the value by half ;)

It's like splitting dinner with a friend.

As long as your friend is a light eater. Otherwise the current into your load overcomes the current into the bottom resistor and throws the voltage off.

With a lightload, say 2 5K resistors, Vin to R1 to R2 to ground, Vout from the R1/R2 junction:

Vout = Vin x R2/(R1+R2) = 5V x 5000/(5000+5000) = 2.5V

However if Vout was connected the equivalent of a 1K resistor, the 5K and the 1K in parrallel act like an 833 ohm resistor (2 resistors in parallel = R1*R2(R1+R2) ) Then Vout = Vin x 833/(833+5000) = 0.71V

That's why voltage dividers are okay for signals, but not as power sources.

I would use a pot of say 50K ohms. The run the voltage through the pot and the wiper will be the voltage out. It is the same as a voltage divider circuit but uses the wiper to make two resistors, one on each side of the wiper.

Something like this:
-0 | 0-

the circles are the terminals to the pot and the | is the wiper. This is a general solution that can be used whenever you need to get less voltage then you have.

Use your VOM to see what the voltage is on the wiper and you are done. If you want to check it you can then test the resistance between each terminal and the wiper. And, with nothing up my sleeve, I predict that you will find that the ratio is the same as what the voltage divider formula predicted.

You need to know the input impedance of the 2.5V circuit really - if it has a very large impedance the simple R - R divider will work, if it has a low impedance then it will become part of the divider and the voltage will drop by more than half...

so you are saying a simple resisistor will do the same trick reducing a voltage from 0-5 v to 0 to 2.5 volts

how do i calculate the correct resistor - it is a signal wire

on another thought - the board is -2.5 v to +2.5 v - is it easy to convert 0-5v into -2.5 to +2.5 ?

thanks for the replies

so you are saying a simple resisistor will do the same trick reducing a voltage from 0-5 v to 0 to 2.5 volts

Only if you know the impedance of the input.

how do i calculate the correct resistor - it is a signal wire

What sort of "signal wire"?

Simple way is take your 0-5V signal and run it thru a capacitor. Like 10uF.
If that doesn’t give enough drive, then buffer the output of the cap with an op-amp with +/- power supply.

If it's from a sensor to a input on a micro,a voltage divider seems better (assuming he just wants to scale 0-5v to 0-2.5v)

How does the output of a cap give you a negative voltage?

The cap lets the signal swing back & forth across the level on the other side of the cap - in this case, the virtual ground of the op amp.

http://en.wikipedia.org/wiki/AC_Coupling

"on another thought - the board is -2.5 v to +2.5 v - is it easy to convert 0-5v into -2.5 to +2.5 ?"

But only if the op amp is there. I read your post as "use a cap, but add a buffer IC if you need more drive".

You have to use the IC or this isn't going to work, wouldn't you agree?

Also, I think he's not going below 0 from the sensor. I think he is asking to map 0v to -2.5, and 5v to +2.5, Not sure why though..

If no buffer, then a simple resistor to gnd. Just like an amp driving a speaker, the cap keeps the DC out while letting the AC swing back & forth.

hi guys - thanks for the replies

if i wanted to use a single resistor, do i need to know the impedance in order to be able to calulate it ?

how do i find out the impedence ?

Gadget999: hi guys - thanks for the replies

if i wanted to use a single resistor, do i need to know the impedance in order to be able to calulate it ?

how do i find out the impedence ?

You will need 2, unless you use a POT (which is essentially 2 resistors).

You can calculate your resistor values here if you know the input impedance. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

What is it you are powering with the 0 - 2.5 volts? is it an input to a micro?

Hi Guys

yes i am taking a 0-5v signal from a sensor and reading it using a 0-2.5 volt microprcessor

how do i find the input impedence ?