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### Topic: Voltage Divider & Voltmeter help (Read 21009 times)previous topic - next topic

#### scottydb4b

#15
##### Oct 29, 2011, 08:13 amLast Edit: Oct 29, 2011, 08:18 am by scottydb4b Reason: 1
The 4.555 is the voltage I got when I measured the analogue input variable power supply and multimeter, so 1023 is reached at roughly 4.555 volts not 5 volts exactly, unless its the multimeter telling me different.

and I think i just added the *2 in there just to make the amps close to what it is ment to be. But yes i got a 0.5 ohm shunt on the ground rail.

Im no coding expert and just improvising where i can.

thanks for the help though.

#### Grumpy_Mike

#16
##### Oct 29, 2011, 11:06 am
What does the +5V pin measure? The 4.555V is a bit low, maybe your meter is off? All measurements are relitave to the voltage on the +5V pin.

#### dc42

#17
##### Oct 29, 2011, 11:36 am

So I will change out the resistors to be 4.7k & 10k respectively.. and lets me keep the draw form the 'meter' low...

but why @ a stable 7.4v from the PSU am I getting readings of 1023 from +5v - +7.4v??

My guess is that either the resistors you are using are not really 470 ohms and 1K (e.g. the one you think is 1K is actually 10k, or the one you think is 470 ohms is actually 47 ohms), or you are using one of those breadboards that has a break in the middle of the 5v and gnd rails and you haven't taken account of that break (so the 1k resistor is not connected to ground).
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### scottydb4b

#18
##### Oct 29, 2011, 02:42 pm
Yeh the 4.55 might be a bit low but at the end of the day i have to calibrate it to something and thats the reading i get @ 1023, its really no big deal i can change it to 5 but is less accurate, according to my meter. But its doesn't have anything to do with the voltage drop issue, or lack of it should i say.

#### xl97

#19
##### Oct 29, 2011, 02:52 pm

anyways..

@ CR-

5V = (8.4V x 10000)/(x + 10000)

should be:

5v = (8.4v x 10000) / (6800 + 10000)

right?

Im still a bit unclear that even with the other resistors in place for a the 7.4v setting.. 'why' it didnt represent a 7.4v 'value' in the serial print?

I mean it was steady 1023 from +5v up to 7.4v..??

I understand why change the resistors.. (since in the end I'll be using 2 x 3.7v li-ions)..

but what was wrong with it currently.. at the 7.4v??

In my mind.. changing out these resistors will do NOTHING for me?  (but are only to be put in place because in the end I'll be using a 2 x li-ions)

the question (for me) still remains and is unanswered..

WHY with the current set-up of resistors/voltage divder.. and a 7.4v source am I NOT getting any changes in my analogRea() from 5v to 7.4v?

#### xl97

#20
##### Oct 29, 2011, 02:54 pm

So I will change out the resistors to be 4.7k & 10k respectively.. and lets me keep the draw form the 'meter' low...

but why @ a stable 7.4v from the PSU am I getting readings of 1023 from +5v - +7.4v??

My guess is that either the resistors you are using are not really 470 ohms and 1K (e.g. the one you think is 1K is actually 10k, or the one you think is 470 ohms is actually 47 ohms), or you are using one of those breadboards that has a break in the middle of the 5v and gnd rails and you haven't taken account of that break (so the 1k resistor is not connected to ground).

sorry I missed this post.. (DOH)..

is there a way I can meter my resistors? to accurately find out the values?

or I suppose I could also try to look up the color bands?  (always hate that)..

any better way?

#### xl97

#21
##### Oct 29, 2011, 03:22 pm
ok..

I took BRAND NEW 4.7k Ohm and a 10k Ohm resistor (fresh out of the pack)...  and put them in place

4.7k in between the +Vin and +Vout

the 10k between + and the -   (same as illustrated in the picks.. (but the values changed)

and I 'again' get the same results?  once I turn the PSU up.. and I get to +5v.. Im already capped at 1023 analog value??

what the 'heck' am I missing here? lol..

I dont even have the LEDS in the mix or part of the project until I can fix the analog read values to be 'somewhat' near my ranges..??

I mean damn.. I only have 2 wires going to the Arduino!!!  LOL  GND and A5

some resistors..and two leads going to my variable PSU..

#### dc42

#22
##### Oct 29, 2011, 04:14 pm
It sounds to me that you do not have the 10K resistor connected as you intend. Can you post a photo of your wiring?

If you disconnect the 4k7 resistor, do you get a stable reading of zero from the analog pin?
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### runaway_pancake

#23
##### Oct 29, 2011, 04:19 pmLast Edit: Oct 29, 2011, 04:28 pm by runaway_pancake Reason: 1
Since you're just experiencing, using a pot will make things a little easier.

Edit -- corrected drawing
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### xl97

#24
##### Oct 29, 2011, 05:01 pm

It sounds to me that you do not have the 10K resistor connected as you intend. Can you post a photo of your wiring?

If you disconnect the 4k7 resistor, do you get a stable reading of zero from the analog pin?

hi-

I'll see if the camera is charged (usually isnt)...  and post one up of a 'real pic'..if it'll help.

@runaway_pancake-

I'll grab another 10k post I have laying around to use..

thanks

#### xl97

#25
##### Oct 29, 2011, 05:26 pm
@runaway_pancake-

I did as your diagram outlined..and it works 'perfect'...

I even left the meter included when I hooked up the Arduino just to keep an eye-on readings.

SO goes back to..  I must be doing something wrong on the breadboard set-up?

here is a pic:

thanks for the help guys!.

#### dc42

#26
##### Oct 29, 2011, 05:59 pm
Your 4k7 resistor is shorted out because you have both ends in the same row of the breadboard.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

#### xl97

#27
##### Oct 29, 2011, 07:24 pm
got ya!.
thanks.
I have also corrected my diagram to show my error/fix

I am now going to go back and change out the resistors to match for 8.4v input.

Thanks to everyone for your help.

#28
##### Oct 29, 2011, 09:38 pm
So you've had 7.4V going into your analog pin all this time?  That's not good.
Does the chip feel warm when powered up now? If so, you likely blew the pin.
And if not, it may be partly damaged and fail on you eventually.

"is there a way I can meter my resistors? to accurately find out the values?"
Yes, just measure them with your multimeter. You have gold bands on the resistors? I think that's 10% tolerance (maybe 5%, I'm  not sure which).
If you know the actual values, you can use that in your math for more accurate results.
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.

#29