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Topic: 0-5 v into 0-2.5 volts ? (Read 4342 times) previous topic - next topic

db2db

#15
May 03, 2012, 08:16 am Last Edit: May 03, 2012, 08:27 am by db2db Reason: 1
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..

CrossRoads

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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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 ?

Jassper


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?


Gadget999

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 ?

AWOL

Quote
how do i find the input impedence ?

It'll be in the datasheet.
Which device are you using?
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

db2db


Which takes me right back to my original reply:

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

A micro input sure seems like it'd be high impedance.
Two 10k resistors.  This will scale 0 to 5v to be 0 to 2.5v.

Jassper



Which takes me right back to my original reply:

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

A micro input sure seems like it'd be high impedance.
Two 10k resistors.  This will scale 0 to 5v to be 0 to 2.5v.



What he said.

If your not "powering" something then 2 equal value resistors should do it.

db2db

I should clarify:

Two 10k resistors *wired as a voltage divider*, as mentioned earlier in this thread.

Gadget999

hi guys i tried a test today with two 22 ohm resistors

i wired up a 5v voltage regulator and measure the power out

just over 5 volts

then i added the two 22 ohm resistors and the voltage went down to 2.75 volts

not quite 2.5 volts but good enough

now i rewired the circuit

this time a pressure sensor was added and powered from the 5v regulator

the output with room pressure was 1.22 volts

then i added the two 22 ohm resistors

I expected to see approx 0.6 volts

but i saw 0.18 volts ?

the voltage was not powering anything just going to my multimeter

Is the problem caused by using low value resistors, so the resistance in the wires of the multimeter could upset the reading ?

confused ;-S

AWOL

Quote
Is the problem caused by using low value resistors
Yes
Quote
so the resistance in the wires of the multimeter could upset the reading ?
No.

5 volts into 44 ohms is over 100mA
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

Jassper

The output of the pressure sensor isn't designed to sink power and should be connected to a high impedance input, like an Analog read pin. I'll read back, but what is it you are trying to do? why do you need to divide the sensor output? If you must, then use two 1M resistors.

Gadget999

i am using the output of the pressure sensor into an analogue input on a board

db2db


hi guys i tried a test today with two 22 ohm resistors


Two 10k resistors *wired as a voltage divider*, as mentioned earlier in this thread.

Gadget999

why would it work with two 10k resistors and not with two 22 ohm resistors ?

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