[Power] [SOLVED] Measuring up to 25 V= without a power sensor ?

[Trixi]

Hoi friends,

i want to measure the current power on the rails of my model layout and read several postings etc. about that.

But i'm a little bit confused about how to do that: sometime is written, that i can connect something in relation with the AREF port, sometime is written i need a power sensor like a ACS709 or ACS712 or equal.

So, whats the truth or what is possible without a power sensor (or not)   ?

[dc42]

What is it you want to measure: the voltage across the track (which your title implies is up to 25V), the current drawn by the trains on the track, or both?

[Trixi]

only the voltage of the track - i won't measure the output of the trafo because that's always OK.
But on some places of the track i want to measure the current power which is available at that point

[dc42]

only the voltage of the track - i won't measure the output of the trafo because that's always OK.
But on some places of the track i want to measure the current power which is available at that point

Please explain what you mean by "the current power which is available at that point", it doesn't make sense to me.

[alnath]

Tell me if I'm wrong, for some reasons (bad rails connexions... ), there are some parts of the rails where you want to check that the voltage is equal to the transfo output ?

as I recall, the direction of the train is changed by reversing the "+" and "-"  on the rails, it will be a little tricky to measure it with an analog input. You'll need more than a resistor divider !!

[Trixi]

No, i won't compare the output of the trafo and the rail power 'cause it couldn't br equal.
Also not in case of bad conndctions (my rails are soldered)
Of course the direction could be changed, but as i have several parallel tracks the one i want to measure always points in the same direction (only "forward")

[polymorph]

You need to be precise about what you want to measure. Current, power, and voltage are all different things.

[Trixi]

its the Voltage 

[polymorph]

You can use a voltage divider to scale 25V max down to 5V max.

[Trixi]

So: the answer of the Topic is NO ? 

the function and use of a power sensor like the described ACS7xx is ok, but i only wanted to know if that is possible without any "hardware" (besides resistors and diodes) like these power sensors.

[alnath]

it is possible with a 2 resistors voltage divider .

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

here is a calculator, if you are lazy  
http://www.raltron.com/cust/tools/voltage_divider.asp

edit : about R1 et R2 , the values should be high enough so that they don't change the voltage you want to measure, and R2 low enough so that Analog Input  impedance has no impact on the measured voltage.

[polymorph]

So: the answer of the Topic is NO ?  smiley-sad-blue

the function and use of a power sensor like the described ACS7xx is ok, but i only wanted to know if that is possible without any "hardware" (besides resistors and diodes) like these power sensors.

You can use a voltage divider to scale 25V max down to 5V max.

Chopped liver?

[Trixi]

Thx to all who wanted to help - i bought a ACS712 

@Chopped liver:
you didn't wrote anything 

[alnath]

I certainly missed something somewhere 

You need to be precise about what you want to measure. Current, power, and voltage are all different things.

its the Voltage  smiley-cool

and you buy a....... current sensor ?

Thx to all who wanted to help - i bought a ACS712 

[Trixi]

and you buy a....... current sensor ?

Pffft, doesn't matter to me - current or voltage or simbalalla