Thanks Bill
I actually felt lower resistance coils provided a higher perceived wattage at any set watts for me. Your explanation provided a foundation for me to do some research, and if my math is correct, if you set the Wattage at 10 on the iStick, with a 2 ohm coils the rms wattage ( perceived ) is 12.25, with a 1.5 ohm coil it's 14.1 and with a 1 ohm coil 17.22. Apparently because of the lower limit Vavg of 3.0, you won't be able to get a lower than a 16.64 Wrms (perceived)on the iStick with a 1 ohm coil.
This is all theoretical obviously.
Yes, I knew that my response to your previous question would seem counter-intuitive, but that was because of the particular way the question was phrased. When you're comparing Vavg to Vrms by keeping the wattage constant while increasing the resistance, you're also raising the voltage. As the voltage increases toward the iStick's max of 5.5 volts, the perceived difference between Vavg and Vrms decreases.
What I mean by this is that
Pulse Width Modulation (PWM) works by "pulsing" the voltage on and off. If the "duty cycle" is 48 hertz on the iStick (which is what someone measured with a scope), then the voltage turns on and off 48 times per second. VAMOs use a duty cycle of 33 hertz, which is why the "rattlesnake effect" happens---at 33 hertz, the on-off pulses can actually be heard/felt. Anyway, to achieve a certain user-selected voltage (or wattage), the PWM chip varies the ratio between the
on-part and the
off-part of each pulse. It
always fires at maximum voltage (5.5 volts for the iStick), and the voltage is either full-on or full-off. To get an 'average' voltage of 2.75, the PWM chip sets the pulse to equal amounts of time at 5.5 volts (on) and 0 volts (off), resulting in an average voltage of 2.75 (Vavg). However, the coil is getting hit with 5.5 volts 48 times each second, so even though it's getting no voltage at all for half of that second in the 48 pulses where the voltage is 0, the coil doesn't have time to cool down and stays much hotter than if it were getting a constant 2.75 volts (rms) from a battery.
In other words, 2.75 volts in Vavg with the iStick's implementation of PWM feels
much higher to the vaper.
No matter what resistance coil you use on the iStick, it's always hitting the coil with 5.5 volts. What varies is how long in each of the 48 pulses/second the voltage is turned off. 50/50 gives a Vavg of 2.75 volts. 60/40 raises the Vavg to 3.3 volts, 70/30 to 3.85 volts, and so on.
All right. That's a lot of explanation to get to the point: If you set the iStick's voltage display to deliver maximum voltage (5.5 volts), then the duty cycle ratio becomes 100/0, meaning that the iStick simply cranks out 5.5 volts and doesn't pulse at all. That's when Vavg and Vrms become identical---at maximum volts/watts. Conversely, the perceived difference between Vavg and Vrms is greatest at the
lowest voltage setting.
Now, on to your example of coils with different resistances. Take that 1.0 ohm coil. You put it on the iStick and set the wattage, but wherever you set it, you're hitting that coil with 5.5 volts every time you press the fire button. Unless the coil is completely wet, it's going to get
REALLY hot very fast. Most clearos/tanks/RDAs can't wick fast enough to keep a 1.0 ohm coil wet when it's being pounded by 5.5 volts. The result? Burned juice. Even if you set the Vavg to be 3.0 volts (as low as the iStick will go), you're still using a duty cycle of 55/45 (55% of each pulse ON, and 45% of each pulse OFF), so out of each second you're firing the device, the coil is getting hit with 5.5 volts for more than half of that second with enough current (amps) to heat it to red hot almost instantly.
Basically, PWM mods aren't designed for sub-ohm coils because the mods always fire at their maximum boosted voltage. That's why sub-ohm cloud chasers use mechanical mods with batteries that provide 4.2 volts or less. Even then, sub-ohming requires massive air flow and very efficient wicking to keep the coil cool enough to prevent burning the juice. That's also why sub-ohmers do direct-lung inhaling, because that increases the air flow over the coil, keeping it cooler. Those huge, thick clouds of vapor that sub-ohmers love happen because the coil is so hot that it's vaporizing the juice like crazy, and the vaper is drawing it all into his/her lungs.
By the way, hollow vertical coils can handle higher voltage/watts/current because the wicking material is wrapped around the outside of the coil, keeping the coil wetter than if it were wicked with a standard "rope" wick threaded through the center of the coil.
My apologies for hijacking the thread. Back to our regularly scheduled programming...
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