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Topic: Voltage divider problem (Read 6 times) previous topic - next topic

tack

#50
Jan 10, 2013, 05:42 pm Last Edit: Jan 10, 2013, 05:46 pm by tack Reason: 1
Also, don't forget that the act of taking a measurement actually affects the circuit.

A DC Ammeter should have a low resistance, close to zero, but crucially NOT zero. Same, but opposite, for a DC Voltmeter; it should have high resistance, theoretically infinite, but it will just be very high.

This means that you never read the true value. Adding an ammeter will put a little more resistance in series, meaning a little less current flowing. Adding a voltmeter will add a very large resistance in parallel, meaning a little more current flowing, so more voltdrop in the series part of the circuit, than without it.

The effect will be dependant on the existing conditions of the circuit, in relation to the connection point, and value of resistance , of your test instrument. You can't take a measurement without affecting what you are measuring.

Biggest errors will be measuring current when you already have a low resistance circuit, or measuring voltage when you have a very high resistance circuit.

Using a resistor divider network and then measuring voltage at a midpoint will put the voltmeter in parallel with one of the resistors and mean the combined resistance value is now lower; 1/(1/R1+1/Rmeter). This will change your divider ratio and the voltage at that point to a lower volt drop than would be present without the meter connected. Therefore, the value with and without your meter connected WILL be different anyway, dependent on the values in the circuit and that of your meter.

Add on the errors mentioned previously and you will NEVER measure exactly the same with and without the meter connected, unless you actually had an infinite impedance voltmeters.

You could well be chasing ghosts here, by trying to get a value that is the same (or as close as you seem to want it), with all the factors that are affecting the measurement of that value via different methods.

tack

#51
Jan 10, 2013, 05:54 pm Last Edit: Jan 10, 2013, 06:01 pm by tack Reason: 1
As an example, imagine:-

Your voltmeter has a resistance of 10 Megohm

You have a 12V supply with 2 x 10 Megohm resistors across it (R1 and R2), making a 1:1 divider. Calculations tell you that there will be a 6v volt drop across each one, so you'll measure 6v at the mid point with reference to Gnd, or -6v with reference to +12v.

Now, you take your voltmeter and connect it across R2 and Gnd to measure the output voltage of your divider. This puts a 10Megohm resistance in parallel with R2, meaning you now have an effective series resistance in your circuit of 10 Megohm (R1) and 5 Megohm (R2 + Rmeter paralleled), totalling 15 Megohm. This is now a 2:1 divider.

So, you expect to measure 6v at the midpoint, but you actually measure 4v, an 'error' of -33% ;-)

This is without considering the resolution and accuracy of the instrument itself, which will mean that measured 4v could be up or down by a few percent too.

When testing and measuring it is important to understand the effect your instruments can have on what you are measuring. I regularly test for both extremely low and extremely high resistances in high voltage circuits. This is why I have specific instruments for each application. A micro-ohmeter for injecting 100A to measure micro-ohms and 40kV+ test sets for measuring Gigohms and Terraohms and leakage currents in micro-amps at, say, 25kV.

Boopidoo

#52
Jan 10, 2013, 06:47 pm Last Edit: Jan 10, 2013, 06:49 pm by Boopidoo Reason: 1
Well maybe I'm just lucky but I'm actually very happy with the measurements which are within 0.01V from what my volt-meter measures. This has now been tested between 4-12V and between -5 to +25 degrees C. If this is consistent I don't need the results to be any better in this project.

The measured voltage on my Arduino doesn't seem to change noticably when I add a volt-meter to the circuit and vice versa.

Now my problem is making the PCB which was harder then I imagined... a real puzzle. :)

retrolefty


Well maybe I'm just lucky but I'm actually very happy with the measurements which are within 0.01V from what my volt-meter measures. This has now been tested between 4-12V and between -5 to +25 degrees C. If this is consistent I don't need the results to be any better in this project.

The measured voltage on my Arduino doesn't seem to change noticably when I add a volt-meter to the circuit and vice versa.

It's all about the output impedance of whatever you are wiring to the analog input if the 10/11 megohm input impedance of your DMM will effect the reading or not. As the analogRead() command is optimized (and recommended) for reading a voltage source with an output impedance of 10K ohms or less, a DMM reading at the same time will have no effect noticeable.

Lefty


Now my problem is making the PCB which was harder then I imagined... a real puzzle. :)

Boopidoo

#54
Jan 11, 2013, 10:58 am Last Edit: Jan 11, 2013, 11:00 am by Boopidoo Reason: 1
What if I removed the caps on RX1 & RX2-inputs and placed resistors in series with the ground (to eliminate ground loops) or eliminated the ground for RX1&2 inputs altogether.

The ground on the RX1&2 is from two devices that is powered from the 12V-source which I am monitoring so that ground is already connected.

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