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Topic: 12v to 5v - Voltage divider issues (Read 2 times) previous topic - next topic

Divinitous

Mar 28, 2013, 03:18 pm Last Edit: Mar 28, 2013, 05:17 pm by Divinitous Reason: 1
Gentlemen,
I'm working on a small project where I'm creating a box to help make things more efficient when servicing machines at work.  First a short bit about the machine I'm working with.  Basically the machine I service is used to find the viscosity of a liquid.  Inside a glass tube the liquid fills up a capillary and two sets of optics detect the amount of time it takes for the minuscus to travel from the upper optic to the lower optic.  Every few months or so the optic stands have to be taken out and the voltage adjusted for both optics to compensate for any wear the optics may have suffered.

The setup that I'm creating will do much more than just this, but this is the part that I'm having an issue with.  The stands will never have a voltage above 11.5v DC so as long as I can get the voltage divider operating properly I am not concerned about an over voltage situation.  My issues lies in the voltage input that I'm getting before the voltage divider.  When I connect the voltage divider circuit the Vin from the source the voltage will drop.  What would normally be a 9.6v drops down to 8.2v.  Connecting the arduino won't affect this, so i know it's not anything with a faulty arduino or bad programming.  I could compensate with some mathematics but the system needs to be operational while I have my setup connected.  Basically I need something to detect the voltage but not create any interference on the source circuit.  Below is my configuration for the voltage divider.  Overall, it works as in the 8.2v is read as 3.3v, but that voltage drop on Vin is gonna be an issue.

Vin -- R1 (220k) -- Vout -- R2 (150k) -- Gnd

fungus

Normally a voltage drop would mean too much current but 220k and 150k is plenty of resistance.

Double-check the resistors with a multimeter to make sure they really are that value (I know you're sure they are, but that's the symptoms...)
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Divinitous


Normally a voltage drop would mean too much current but 220k and 150k is plenty of resistance.

Double-check the resistors with a multimeter to make sure they really are that value (I know you're sure they are, but that's the symptoms...)



Yeah, that's something I checked previously.  The values are well within the 5% tolerance.

Any possible alternative to a voltage divider with resistance that i might be able to get away with?
  Originally I setup a simple LED voltage meter in a project box and it did a great job. The connector for these stands has 24v for me to work with.

1oldfox

Good morning Divinitus. First a question. Are there other items, circuits, instruments, etc. in your project that depend on the source (Vin) remaining stable and constant. From what you have described so far, voltage isn't the issue. You are wanting to measure *time*, the measure of which can be mathematically massaged to give you the viscosity measure you are looking for. Is my understanding correct? If so, you may want to investigate using OpAmps for the so called sensors rather than a voltage divider. This would transition you out of the *analog* world and take you into the *digital* world. Much easier to measure time.
-oldfox-

James C4S

Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

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