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### Topic: Minimum value of resistance (Read 774 times)previous topic - next topic

#### Noraiz

##### Sep 14, 2017, 08:34 am
hello,
i want to know can an arduino sense resistor values in mili ohms? by voltage divider circuit?

#### jackrae

#1
##### Sep 14, 2017, 08:51 am
Would that be 1milliohm or 999milliohms and to what resolution would you want to measure.

#### Noraiz

#2
##### Sep 14, 2017, 08:57 am
less than 1 miliohm. until 0,5mohm

#### avr_fred

#3
##### Sep 14, 2017, 12:17 pmLast Edit: Sep 14, 2017, 12:17 pm by avr_fred
No, not by voltage divider. Why do you need to measure 0.001 ohms?

#### Wawa

#4
##### Sep 14, 2017, 12:38 pm
i want to know can an arduino sense resistor values in mili ohms?

by voltage divider circuit?
Sure.

No.

Just send 1000Amp through your resistor.
That will result in 1volt per milliohm
The Arduino can measure that.
Leo..

#### allanhurst

#5
##### Sep 14, 2017, 12:41 pm
'Just send 1000Amp through your resistor.'

I hope your resistor is a big one, or it'll get quite hot.

Allan

#### Noraiz

#6
##### Sep 14, 2017, 01:24 pm
1000amp?? how it can be possible..

#### Wawa

#7
##### Sep 14, 2017, 01:31 pmLast Edit: Sep 14, 2017, 01:32 pm by Wawa

0.001 ohm is a solid rod of copper as long and thick as a pencil.
Not easy to measure without some serious current and a kelvin connection.
Is this a current shunt?
Leo..

#### Noraiz

#8
##### Sep 14, 2017, 01:38 pm
i want to calculte a resistance of copper traces on a PCB. which i expect to be in mili ohms.

#### Wawa

#9
##### Sep 14, 2017, 01:40 pmLast Edit: Sep 14, 2017, 01:45 pm by Wawa
There are lots of online calculators for circuit board traces.
http://circuitcalculator.com/wordpress/2006/01/24/trace-resistance-calculator
http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
Leo..

#### MarkT

#10
##### Sep 14, 2017, 02:19 pm
Pass 1A along the trace, measure the voltage difference with a multimeter (it will be microvolts or
millivolts), compute the resistance.

Thin PCB traces will warm up with 1A flowing, increasing the resistance.  Very fine trances might
overheat, try 100mA if unsure.

Having a current limited power supply makes this sort of thing easy (setting a constant current).

Alternative is use something like a 10 ohm power resistor in series with a low voltage supply to
produce a suitable current, but you'll have to explicitly measure the current (measure the voltage
across the 10 ohm resistor for this).  In effect you have a voltage divider with 10 ohms and
the pcb trace.

Tables of pcb trace resistance for standard plating thicknesses are available online.  Copper
always has a resistivity close to 1.7e-8 ohm-m, so you can calculate the approx resistance from
the dimensions easily enough.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### Noraiz

#11
##### Sep 14, 2017, 03:48 pm
and thats the question how can i give 100mA from arduino
or if use separate power supply how it can be connected to the arduino to pass 100mA current?

#### kenwood120s

#12
##### Sep 14, 2017, 03:54 pm
Very fine trances [sic]
I've had some of those, but the less said the better

[gumby]I've got my head stuck in the cupboard[/gumby]

#### allanhurst

#13
##### Sep 14, 2017, 03:57 pm
Why would you need to know the resistance of a particular track to accuracy better than the calculations recommended above would give you?

And if you need to pass a high current, there are industry standards for track width and thickness
- track spacing as well for high voltages.

Allan

#### Wawa

#14
##### Sep 14, 2017, 11:46 pm
and thats the question how can i give 100mA from arduino
or if use separate power supply how it can be connected to the arduino to pass 100mA current?
100mA will drop 100uV across a 0.001ohm trace.
Way too small for Arduino's 10bit A/D to detect.
You need at least 10x that current to even see one A/D step.
Leo..

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