I had the chance to check the output of my iStick today with the scope at work. Unfortunately, I don't know how to work the thing (apparently) and the extra info on the right side was replaced by the picture saving menu.
It was tested with a 1.6 ohm coil. The first trace (low duty cycle) is with the iStick set to 3 volts, and the second is with it set to 5.5 volts.It definate boosts to about 5.8 volts and then PWMs the output.
What you can't see is that the RMS voltage determined by the scope is NOT the same as the voltage it was set to. RMS was high on the 3 volt setting and low on the 5.5 volt setting. PWM frequency was about 81Hz.
View attachment 379676View attachment 379677
If I understood you properly, RMS voltages didn't match the readings on iStick's screen.... so that confirms 'PWM-gate', and it's consistent with the 'too hot' complaint and also checks up with my voltmeters....assuming they give averages....
After 30 seconds of bewilderment, I took charge on the maths (I can live with them....) and I quickly discovered that for any given squared wave always positive you can see that:
V
ave = V
max * X / T , where T is the period of the wave and X the fraction of time in which the wave is not zeroed (the duty cicle in unit fraction),
V
rms = SQRT ( V
ave * V
max) (SQRT means squared root),
and finally, if P
w is the power calculated using V
ave in the normal P = V
2 / R, then the actual power would be:
P = P
w * V
max / V
ave
You can easily guess what the 'W' subindex means.....
So, if the thing fires at V
max = 5,8 V with PWM, when we put 3,0 V on display (on VV or adjusting power to do so), the real power would be 193 % of the 'power' shown..... With a 1,5 ohm resistance, that means 6 W on display but 11,6 W applied to the atty.....
No surprise that you, carto guys, did find this so quickly. I was worried searching for my taste..... but it turns out that, as I'm almost always beyond the 10 W (on display) , i. e., beyond the 3,9 V, my actual power was about 50 % or less above the one on screen....
The funny thing is that even at 5,5 V the math tells us of 5 % extra power, so the iStick would have been a 21 W power-class device....but it also seems that at 'full voltage ahead' it goes behind the 5,8 V
max mark...so I'm starting to wonder if it really reaches the 20 W mark....
Nevermind, if you forget the damn numbers, you can surf along your taste to get (if you're lucky enough and you aren't using some very sensitive atty), the famous 'sweet spot'....
But the advertised power is not the actual one......What do you think of this, JoyetechUSA and company?
(They appear to be reading this thread, so ....)