Controlling current for kanthal wire

djsfantasi

Joined Apr 11, 2010
9,156
Amazon specs are tricky at best, just plain wrong at worst! In your case, it's a single product listing for multiple wire sizes, and the specs should be different for each wire size, but they haven't given you that. Here's one example of info showing resistances (per inch) vs. wire gauge:

Kanthal Wire Gauge (AWG) in Resistance (68°F)
  • 24 Gauge (AWG) = 0.17 Ω / in.
  • 26 Gauge (AWG) = 0.28Ω / in.
  • 28 Gauge (AWG) = 0.44 Ω / in.
  • 30 Gauge (AWG) = 0.70 Ω / in.
  • 32 Gauge (AWG) = 1.09 Ω / in.
  • 34 Gauge (AWG) = 1.76 Ω / in.
You can see that the resistance gets lower with larger wire diameter. This list didn't go up to 22 gauge, but at 24 gauge it's already down to 2.04 ohm for 1 foot. So, your presumed resistance for the heater is probably off by a factor of 5 or more!

It's tricky to do any electronics project without a multi meter, but especially so when you're trying to handle any kind of real power. We might be able to walk you through this particular issue based on specs alone, but I'd STRONGLY urge you to get a meter and start checking your components before using them in the future.

The other lesson here is to be very careful when reading specs, and even more so if you're getting specs from anything other than the product datasheet. Reputable suppliers like digikey, mouser, allied, etc. usually (but not always) get specs right, but Amazon and eBay specs are nearly worthless, and I've heard that Ali Express and other similar sites are even worse. Trust nothing, and verify everything!
In the chart I posted above, 22g Kanthal wire is 1.31Ω per foot.
 

ebeowulf17

Joined Aug 12, 2014
3,307
In the chart I posted above, 22g Kanthal wire is 1.31Ω per foot.
Sorry if my comment came out wrong - didn't mean to disagree with, nor ignore, your post.

My point in that last post was mostly about how and why the Amazon numbers would be totally wrong, not so much me claiming to have the exact right numbers. My numbers came from a dubious website dedicated to vaping - just happened to be the first hit I got with actual numbers.

Anyway, I trust that your number is a good one. Regardless of the exact number, I'm confident that we've identified the problem. Now we just need a solution. If the TS has 10 feet of the Kanthal wire available, then it will be easy to arrive at the desired resistance. If not, this will be complicated, inefficient, or both!
 

TeeKay6

Joined Apr 20, 2019
573
Thanks for the revert djsfantasi :)

The wire I bought states Kanthal A1 Wrie 22g, 0.2 mm in width, Resistance (Ohm/FT @ room temp): 10.55. I bought from amazon.de, just search Kanthal A1 Wrie 22g (AAC doesn't allow me to post links).

Also, Both the resistors I am using in circuit with conductive stretch sensor are 10K 1%.

I checked my setup without the conductive stretch sensor and saw at this much voltage & current, I am able to get the smell of the scent (see attachment). As a change, I am thinking of following scenario:

Turn ON the heating to a default value when stretch sensor crosses some threshold value. Later based on the varying signals from smart phone change the current supplied to the kanthal wire so that when required it heats up more and when required less.

Can you please help me to recommend what should I do to be able to make this work, considering later I plan to control the frequency & intensity of the smell produce. Meaning, varying the amount of current Kanthal wire gets?

For this I am thinking to use a BLE Nano v2, consisting of a Nordic Semiconductor nRF51822 system on chip. The main role of the BLE Nano is to establish a Bluetooth connection to a smartphone in order to communicate control signals for current. It has an ultra-low power consumption,
integrated Bluetooth Low Energy (LE) capability and it runs at 16MHz. The BLE Nano accepts voltages from 3.3 V to 13 V through the VIN pin and regulates it to 3.3V via an integrated low-dropout regulator.

Also, from what I read from web, MOSFET only requires very little current of the BLE Nano to turn on and delivers a much higher current to the load. In order to save battery life, especially during the high drain phase of the scent release, a capacitor is ideal to be added upstream of the MOSFET.
Does not your image (post #14) show that driving the wire with 3.0V causes a current flow of 2.7A? Is that not surprising if the wire resistance is 10.5 ohms?
 

djsfantasi

Joined Apr 11, 2010
9,156
On your drawing the black wavy thing at the top right of the drawing looks more like a heating element than the piece or red wire.

Les.
I agree that’s confusing. But by closely reading the thread and looking at where it is connected, I came to the conclusion that the “black wavy thing” was indeed the sensor the TS referred to in his description. It took me a few posts to realize that the red line was the heating wire. Before that I was in the direct shirt camp.

I’m not sure about the two resistors in his Fritzing diagram. It doesn’t look to me that they’re doing what he wants them to do. Given that it’s a Fritzing diagram, I decided that it was too much work to analyze. Particularly after my difficulty in interpreting where the Kanthal wire was connected. Doesn’t Fritzing allow you to label components?
 

djsfantasi

Joined Apr 11, 2010
9,156
And another thing...

I would add a 10KΩ resistor from the MOSFET gate to ground. This helps when the MOSFET is switching on and off.

Which brings me to another point. There is no hysteresis in your code. When the sensor is returning a value of 250, it’s likely to oscillate around that data point.
250,248,250,249,250,249, etc...​
It’s better to have your code turn on at 250 and turn off at 245... or some lower value. That way when you just get to 250, it won’t immediately turn off and start to oscillate.

This oscillation around your set point can cause your MOSFET to heat up.
 

BobaMosfet

Joined Jul 1, 2009
2,110
I did that because I saw on web how to control a motor from arduino. I thought the same way I can cause kanthal wire to heat up. However, it didn't worked in my case :/
This is because you don't know enough about electronics to even be doing this, yet. Instead of looking on the web and getting incorrect information, you need to start with a book, learn enough about how the individual components work (like why and inductive motor won't behave like a resistive wire). If you insist on using a website, do so, but use one with structured information you can refer to. And get yourself a DMM (digital volt meter) so you can measure volts, ohms, and amps. Without this, you'll never get anywhere.

Here are info sources:

https://www.allaboutcircuits.com/textbook/direct-current/

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

Good luck to you.
 

TeeKay6

Joined Apr 20, 2019
573
This is because you don't know enough about electronics to even be doing this, yet. Instead of looking on the web and getting incorrect information, you need to start with a book, learn enough about how the individual components work (like why and inductive motor won't behave like a resistive wire). If you insist on using a website, do so, but use one with structured information you can refer to. And get yourself a DMM (digital volt meter) so you can measure volts, ohms, and amps. Without this, you'll never get anywhere.

Here are info sources:

https://www.allaboutcircuits.com/textbook/direct-current/

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

Good luck to you.
@Siddharth Mehrotra
Advice given by @BobaMosfet is correct. However, if every forum post had to pass the test of "don't know enough", most of AAC's forum posts could be deleted. Both Thread Starters and responders are rather often guilty of knowing less than they think they know. Keep learning!
 
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