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Topic: Do breadboards contribute resistance to a circuit? (Read 714 times) previous topic - next topic

zcapozzi

Hi all,

Do breadboards typically introduce additional resistance to a circuit.  In my estimation, it would be about 300 ohms.

The situation is this...

1. I've got a multimeter that I can use to measure voltage coming out of my solar panel.
2. I've got an Arduino set up with a voltage divider circuit (on a breadboard) to read that same voltage.  The circuit is a 100K ohm resistor, then a 10K ohm resistor, with the Arduino input coming from between them. 
3. My Arduino recorder consistenly measures voltage 5% higher than what the multimeter reads. 

If I change the resistor values from 100K and 10K to 100K and 10300 in my calculations, the voltages match.  Is it possible that the breadboard is adding additional resistance that is causing my readings to be consistently.  My approach for now is to just adjust the voltage through the Arduino code, but I want to make sure I understand the problem before settling on that fix.

MAS3

Yes, any addition will also add resistance.
But not in that way.
Breadboards are not perfect but allow for quick(er) prototyping.
The connectors in the breadboard don't do a great job and soldering connections would be much better.

The extra resistance is more likely internal circuitry in your controller and/or board.
If the multimeter is the reference, try to see what happens if you vary the power supply slightly.
So what happens if the power supply isn't 5 Volts, but 4.9 Volts ?

Also, have a look at the reference (Aref) input of your controller and how to use that
Have a look at "blink without delay".
Did you connect the grounds ?
Je kunt hier ook in het Nederlands terecht: http://arduino.cc/forum/index.php/board,77.0.html

jack wp

A breadboard may not be the most reliable connection, but generally it does not add 300 ohms of resistance. In practice it should not add more than .01 ohm of resistance (my estimate only).

Use your volt meter, and read the voltage from your voltage divider (at the analog input pin). Then do the math.  What do you get?

Good luck, Jack

Grumpy_Mike

Quote
100K ohm resistor, then a 10K ohm resistor, with the Arduino input coming from between them.
3. My Arduino recorder consistenly measures voltage 5% higher than what the multimeter reads.

So what it the tolerance on those resistors?
Chances they are 5% resistors which would explain things neatly.
Have you measured them?
Measure each resistor and then do the calculations. Then use real values in the software to get real numbers out.

James C4S


it would be about 300 ohms

Remember in school when teachers said "check your work"?  This is a good example of why.  300 ohms is REALLY high for a device which is suppose to be a short.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

MarkT


A breadboard may not be the most reliable connection, but generally it does not add 300 ohms of resistance. In practice it should not add more than .01 ohm of resistance (my estimate only).

Use your volt meter, and read the voltage from your voltage divider (at the analog input pin). Then do the math.  What do you get?





No, don't measure the voltage like that, voltmeters have input resistance!  Measure the value of the resistors, find out what the true
voltage divide ratio is (remember its R1/(R1+R2), not R1/R2).  Then measure the 5V rail of the Arduino.  This gives the information
to calibrate the voltage measurement properly.   Resistors come in various tolerance ranges from 10% to 0.01% accuracym if you are
measuring you need to know the tolerance to see what your accuracy is.

An analog input has essentially infinite input resistance, unlike a multimeter.
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cjdelphi

The only major issues with breadboards are poorly made contacts on your jumper cables.

TomGeorge

Hi, I agree with Mike, your error looks like its in line with possible resistor tolerances.
In real life don't expect the calcs to be exact of what occurs in practice.
Usually I software calibrate, but if in field cals is required I put a trim pot in the divider.

(Don't take me back to HSC chemistry and all chem calculations had to have worked out error figures, working out the tolerances took longer than working out the actual value, using slide rule, isn't EXCELL great!!!!!!)

Tom... :)
Everything runs on smoke, let the smoke out, it stops running......VK3DMK

dc42

How are you powering the Arduino? If you are using USB power, chances are that it is a little lower than 5V. If it is as low as 4.76V then your voltage readings will be 5% high even if your resistor values are spot on.
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.

jackrae

No-one's mentioned the multimeter yet !
We tend to believe these things, even if we only paid a couple of pounds/euros/rupees/ etc for them.   Built to a price and generally very good value for the money but not precision instruments by a long way.

The simplest way to check your comparative readings is to measure the two conditions at the same time.  Hook up the circuit to the arduino input and connect the multimeter across the same input terminals at the same time.  Now both arduino and the meter are operating under identical conditions.  Do both readings tally ?   No doubt there will be a difference, it may be small it may be large.   If large you need to investigate further.

MarkT

Oh yes, good idea - and you can see if the input resistance of the meter affects the Arduino
reading.  Good meters tend to be 10Mohm or more, cheap ones might be 1M.  My
multimeter doesn't have its input resistance marked on it even, which is shocking
really....  The manual reveals 10M on all but 60mV and 600mV ranges
which are FET input direct (> 100M).

To explain a little about multimeters - to get the different ranges a network of resistive
dividers is used on the voltage inputs.  Thus when you connect this to another resistive
divider network to measure voltage you are changing the network you are measuring.
You have to hope the resistances used in the multmeter are much larger than in the
circuit you measure...

So beware when measure voltages of high impedance circuits with a multimeter...
[ I won't respond to messages, use the forum please ]

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