Hi,
I control a load (R2) with PWM from my Arduino Uno, using a IRLB3034PBF-ND MOSFET.
The load (R2) have variable resistance 0.10 - 4.00 ohm.
I would like to be able to measure the resistance with an analog pin of my Arduino.
But I don't have any idea about how to connect a voltage divider without inflict the PWM circuit or the current being to big for the analog pin.
Any help would be appreciated,
Thanks,
Lars
Resistance is determined either by measuring the voltage drop across a device while a known current is passing through it, or measuring the current when a known voltage is applied, or measure both voltage and current.
Measuring voltage is easy, just use a voltage divider and the analog input of the Arduino.
However, since the current is very large, to measure it you would need a 100 amp current sensor, like the ACS756 series. http://www.allegromicro.com/en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs.aspx
It's not possible to measure the resistance while the MOSFET is off? Do I need to decouple the positive side then?
It's not possible to measure the resistance while the MOSFET is off?
Yes, possible, but quite complicated. You would use one of the three methods already outlined, with a circuit that is in addition isolated from the 8.4 V power source.
If load heats up, then the resistance almost certainly changes with temperature.
Might be strange question:
Is the intent of the OP to make some "sub-ohm" vaping device?
Yes it's my currently box mod (vaping device) project 
I need to be able to measure the resistance to be able to set the right power (duty cycle).
Right now I can set it with percentage (duty cycle), but I would like to be able to choose the correct wattage instead.
A little piece of advice, up to you if you take it.
Don't do it.
I have been on those vaping fora, and they are full of idiots, encouraging others to follow their "example".
80Amp lithium-battery device placed in your mouth... start smoking cigs, they are considerably safer.
There have been "vapers" had their teeth blown out and jaw bone shattered doing this, someone will die doing this.
What is a vaping device?
Thanks
An electronic cigarette.
Yes with a mech mod vaping crazy sub-ohm builds I understand it's dangerous.
But with a regulated box mod vaping at max 30A, I consider it quite safe.
Considered that you use IMR batteries that can handle the current.
I'm going to use 30A fuses and measure battery temperature as well.
A current limiter would be better. 30 amp fuse might not blow till 60 amps 
Yes that's true - but it's just for if a short should occur - amplimit I measure battery voltage and calculate the maximum PWM duty cycle, I think that's safe enough.
But then I need to know the resistance of the coil/load.
Unregulated doses of nicotine vapour can kill - liquid nicotine can kill by skin absorption.
It is so clear from the aims of this group of people that they do not understand electrical principles, and that they will ignore advice from those that do.
This is a Darwin award in the making.
Oh, if any of these halfwit subohm vaping nutjobs understood, they would realise that a load of 100mOhm within a stainless steel case&conduction path, most of the energy is being dissipated across anything but the load.
The cells themselves have an internal resistance of around 100mOhms.
Two in series (without any sign of balancing or knowledge thereof) has resistance of 200mOhms, so twice the power is being dissipated within the battery itself!
I've just lowered my nicotine usage to 3mg nicotine per ml.
So I don't think there is any risk
Balancing is already in order - but when I lab in multisim I don't set everything up.
I'm using a BQ29209 for balancing.
Personaly I use a coil at 0.50ohm at max 60 watt, and I don't think that going to harm a 40amp continious discharge battery.
An electronic cigarette.
Thanks. (Do people keep on that?)
vffgaston:
Thanks. (Do people keep on that?)
Yes, they start spending huge amounts of cash on fancy steel tubes.
Then some compete for the highest current device.
Crazy isn't it.
Which 18650 is rated for 40amp continuous?
I don't use 18650 batteries, I use 26650 batteries.
Currently using - http://www.kidneypuncher.com/vappower-imr-26650-4200-mah/
But consider buying - Vape Batteries & Chargers | Shop Vape Mod Batteries | UK ECIG STORE
I agree with you that it's dangerous and accidents is going to happen, and I would never use a mech mod that is totaly unregulated without any safety at all.
And I think it is stupid and ridiculous with all the highest current competitions.
And some people using dangerous ICR batteries or Lipo packs or sub-ohm a lot over the rated battery limits.
I try to do it as safe as possible, but there is still risks.
We also try to inform each other about the risks and how to vape safer.
But I understand your concern, thanks.
Well I have said my bit, Legolas and I think I edged on being impolite already.
That is not my intention.
Take note, although IMR is considered safer, it will still suffer thermal runaway, burn and release toxic gases.
I hope that some reading will take heed...
A couple of watt is all you need to vape, this 160watt is analogous to smoking ten foot long potentially explosive cigarettes, then claiming it is for health benefits.
Play safe folks!
I'm leaving the thread alone.