# Arduino Forum

## Using Arduino => General Electronics => Topic started by: Noraiz on Sep 14, 2017, 08:34 am

Title: Minimum value of resistance
Post by: Noraiz on Sep 14, 2017, 08:34 am
hello,
i want to know can an arduino sense resistor values in mili ohms? by voltage divider circuit?
Title: Re: Minimum value of resistance
Post by: jackrae on Sep 14, 2017, 08:51 am
Would that be 1milliohm or 999milliohms and to what resolution would you want to measure.
Title: Re: Minimum value of resistance
Post by: Noraiz on Sep 14, 2017, 08:57 am
less than 1 miliohm. until 0,5mohm
Title: Re: Minimum value of resistance
Post by: avr_fred on Sep 14, 2017, 12:17 pm
No, not by voltage divider. Why do you need to measure 0.001 ohms?
Title: Re: Minimum value of resistance
Post by: Wawa on 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..
Title: Re: Minimum value of resistance
Post by: allanhurst on 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
Title: Re: Minimum value of resistance
Post by: Noraiz on Sep 14, 2017, 01:24 pm
1000amp?? how it can be possible..
Title: Re: Minimum value of resistance
Post by: Wawa on Sep 14, 2017, 01:31 pm

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..
Title: Re: Minimum value of resistance
Post by: Noraiz on 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.
Title: Re: Minimum value of resistance
Post by: Wawa on Sep 14, 2017, 01:40 pm
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..
Title: Re: Minimum value of resistance
Post by: MarkT on 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.
Title: Re: Minimum value of resistance
Post by: Noraiz on 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?
Title: Re: Minimum value of resistance
Post by: kenwood120s on Sep 14, 2017, 03:54 pm
Very fine trances [sic]
I've had some of those, but the less said the better ;)

Title: Re: Minimum value of resistance
Post by: allanhurst on 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
Title: Re: Minimum value of resistance
Post by: Wawa on 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..
Title: Re: Minimum value of resistance
Post by: edgemoron on Sep 15, 2017, 07:46 am
Or read the V drop across the trace during a short high current pulse (not long enough to heat or magnetic field to jerk it loose). Risky but ...?  :o