Why does the average voltage on a scope not match the set voltage on a PWM mod:

[iuseaspork]

I've seen this mentioned a lot in reviews, so here is why:

In a traditional mod lets say we set the voltage to 4 volts on a 3 ohm load. These are the results from OHM's law:
Current: 1.33333
Power: 5.33333


Now lets double the voltage to 8 volts:
Current: 2.66667
Power: 21.33333


You can see now that as we DOUBLE the VOLTAGE we end up QUADRUPLING the POWER


Now lets pulse the voltage at 50%. We end up with an average VOLTAGE of 4, but we also end up with an average POWER of over 10 which is double what we would expect from a more traditional mod.


This is the reason we see the average voltage on a scope differ from the voltage set on a PWM mod. If someone were expecting the same vape from the same setting on this type of mod compared to a more traditional mod, they would be very suprized that it would seem like almost double the power.

I want it understood that I am just trying to help, not stir up trouble.

 

[ndnshift]

so this is why the darwin at 4V (whatever the wattage) hits harder than anything else at straight 4V?

 

[Slurp812]

I see the same thing. 3.3 volts on the Provari hits harder than a 3.7 (fresh off the charger) volt mechanical mod on the same 1.7 ohm carto.

 

[ndnshift]

definitely makes sense... it would just seem that pulsed voltage would be harder on something that heats up (expands/contracts), but i imagine since it's so fast there's not much downside... certainly have not noticed any decreased life on anything using the darwin.

i am very curious from an EE perspective how the 1600 mah battery in the darwin can last so long... i even forget to charge it. even with PWM, it doesnt seem like there would be much power savings. is there another reason why it at least seems so much more efficient? and do people using the Kick notice the same longevity?

 

[HawkeyeCS]

Wattage is equals to your voltage squared divided by resistance. Hence why you see the exponential and not linear growth when increasing the voltage... As far as Pulse Width Modulation goes, the theory behind it from what I understand is that, by pulsing a higher voltage through the coil at 50% (or whatever it is) it should heat up the coil to the same temperature as a constant linear voltage. Trying to figure out pulse width modulation in terms of wattage and everything is a bit beyond my knowledge, so somebody else may be able to explain it better....

 

[milo hobo]

Mybe I'm a simpleton, but this is all mathemagics to me.

 

[iuseaspork]

What I'm trying to say is that we just keep camparing voltage. Maybe it's because they put that in as a display even on PWM mods. It really all comes down to watts, though. I believe to compare a PWM mod with a filtered or linear mod you have to solve for peak wattage then figure in the duty cycle.

Even then there is so much going on it would be super hard to compare. How perfect is the square wave? What frequency is it running at? The one I made runs at 2-3khz and I can hear a distinct tone with some atties so am I losing some energy to sound? Etc etc...

I'll use my original numbers in this.

8 volt peak voltage PWM mod at 100% duty cycle (no switching at all) on a 3ohm atty gives us 21.33 watts. To approximate (there are other variables such as not having a perfect square wave, etc.) the experience on the same atty on a 3.7 volt single battery mod (4.56 watts) we would need the duty cycle to be 21.38%

So lets do a more realistic scenario. I assume that most PWM mods regulate their peak voltage first then pulse it. So lets say that our shiny new PWM mod's peak voltage is 6 and we are using a 2 ohm boge carto that for some reason we want to duplicate the same experience we got on our silver bullet when the battery plateaued at 3.7 volts.

Peak power (watts) PWM mod:
(I'm old fasioned and do it in 2 steps)
Voltage-6 / resistance-2 = amps(current)-3
POWER-18 = Voltage-6 * Current-3

Power silver bullet:
(I'm old fasioned and do it in 2 steps)
Voltage-3.7 / resistance-2 = amps(current)-1.85
POWER-6.845 = Voltage-3.7 * Current-1.85

Solving for the desired Duty cycle:
Desired average power-6.845 / Peak output power 21.33 = .32 ...make it a percent and we have 32 as our duty cycle.

I would probably add a few percent to the duty cycle due all of the other dynamics involved and compare with a 35% duty cycle.

I wonder if someone can actually perform this experiment and tell us their findings. (I do not own an oscilloscope)

 

[cyberwolf]

I'm not nearly experienced enough with electronics to answer the questions that you posed, but have recently come across a good example of the difference found in PWM with the Smoke Tech VMax. I was initially disappointed to see the output voltage under load drop about half a volt from what the VMax was set at. Then I proceeded to fry a dual coil carto at the 4.5v setting, thinking that I was only getting around 4v.

It turns out that with PWM, this thing cranks out much more power than the average voltage that it is set at. PBusardo does a great review of the VMax and at about 23:00 hooks it up to an oscilloscope. What he finds is that with it set at 3v, the peak to peak voltage is actually over 9v even though the average is still 3. The vape is much hotter and I have to set the Provari at over .5v higher to get the same performance. As PBusardo puts it, the VMax is quite a performer, but it is "poorly tuned."

 

[carlton]

Without going through all of the math, power derived from a waveform is taken from the root mean square of the wave, or rms. The math is easy, if the wave is a perfectly formed pulse train. I believe that a lot of pwm mods set themselves for the average voltage, rather than the rms voltage.

There's something else going on, too, speaking about the VMax specifically. As the rms value still seems lower than the output of the vape at a similar dc voltage.

 

[milo hobo]

Wait, carlton, you just mentioned that a normal vape is dc, so a pwm changes it to ac? Or am I still misunderstanding.

 

[iuseaspork]

RMS calculators for different wave-forms:
Root mean square - Wikipedia, the free encyclopedia

 

[ndnshift]

this all makes good sense to me in terms of why PWM mods hit so much harder at lower voltages...

however, it still doesnt answer why my darwin lasts many days with only a 1600mah batt pack
is longer use something provari owners have experienced? kick? just curious if this is a darwin only benefit and how it might possibly be happening.