My First Pulse Width Modulator Mod

[CraigHB]

So I guess the secret here is deciding at what FREQ your device should operate at. The higher the FREQ, the smaller the droplets (keeping in mind that too high a FREQ will begin looking like DC to the atomizer).

The problem is the frequencies they require, in the MHz. Dealing with frequencies that high is involved, especially at the higher currents required to drive a nebulizer. Aside from the mechancial considerations, several electrical/electronic factors come into play that make it much more complicated (and expensive) than driving a heating coil.

In any case, I've seen discussions about ultra-sonic atomizers or nebulizers being employed in an e-cig. I believe the first e-cig prototype deisgned by Ruyan was ultra-sonic, but was never marketed, probably due to size and cost issues. Though I seem to recall seeing an ultra-sonic e-cigar sold by Ruyan, but I'm not positive about that.

I am wondering if the heater element in an atty is even capable of handling a freq that high, it needs a certain amount of time to heat and cool and reheat, etc. I am thinking that the wire itself may have an inherent frequency, above which it just constantly remains on (virtually).

Actually it doesn't. Apparent power is always going to be an integral function of applied voltage and resulting current regardles of frequency. The issue you point out actually stems from the impedance of the overall circuit. At very high frequencies, the reactive component of a circuit's impedance can cause currents to behave like DC. That's a specific example of the complications involved in designing high frequency circuits.

 

[bstedh]

I didn't find the info, sure it's here somewhere Those are 18350's??? I love how you modded the 2-C box. I would have never thought about that! I made a 5v mod with that box, using 18500's .. never thought of putting the batts in sideways .. ingenious!! I've got another of those boxes in my shop, this is giving me ideas!!!

my box with 18350's with the good old stack of nickels trick cause I didn't have any 18500's

By asnider123 at 2011-07-09

Yes, they are 18350's. =] I used the foam rubber out of a pack of ce2's as spacers as it is nice and dense.

 

[Uncle Screwtape]

Here's another thought .. as a casual observer I have been learning quite a bit about PWM theory by reading your great posts. I may have missed it, but have noticed that your circuit appears to be oscillating a nichrome heater element at about 400 hz. PWM lighting dimmers work great, but they are running at 60 hz. I am wondering if the heater element in an atty is even capable of handling a freq that high, it needs a certain amount of time to heat and cool and reheat, etc. I am thinking that the wire itself may have an inherent frequency, above which it just constantly remains on (virtually).

Perhaps lowering the FREQ would give better results across the duty cycle. I know, I'm no electronics tech, but I did stay at Holiday Inn express :

Just thinking out loud ...

Alan

I've been thinking the same thing. I laid this out on a breadboard (with some variations based on what I had laying around) and found that the output was not that variable, I was running it at about 225hz. The results were too hot on my dual coils no matter where I had the setting. I'll be trying some lower frequencies to see if I get better results.

I'd really like to thank everyone here. You all have inspired me to dust off some brain cells that I haven't used in a long time. It's been 30 years since EE classes, and I never used that knowledge professionally from a design standpoint, as I got right into the business/management end of things. When the summer comes to a close, I hope to find the time to get deeper into this and actually crank out a few nice mods. Thanks again everyone.

Vape on!

 

[bstedh]

I was thinking of playing with the frequency but the capacitor in this circuit is what sets the frequency so I have been looking at a couple of circuits that would have variable frequency.

 

[jrm850]

Just a guess here, but I would think that there is a certain amount of thermal capacitance that is related to mass/surface area* thermal conductance of the nichrome and the surounding medium. I'm thinking that because of this the PWM systems are going to be much more efficent when the frequencies equal or slightly exceed the total thermal capacitance. A loosely germane analogy would be some of these ultra bright PWM LED flashlights. The LEDS are not burning constantly but the percieved brightness is unchanged. The same kind of concept only we're swapping thermal capacitance for persistant vision.

If any of this is close to correct we can either drop the frequency low enough so that we can stick with the current "watts" paradigm, or we can forget what we know and just use empirical results as our guidelines. Better yet, lets just make an axial thermocouple and have a closed loop thermal control.

 

[bstedh]

Ok, now I'm thinking 3 batteries regulated to 7.5V and then PWM VV.... I seem to like the fresh battery PWM when it it above 7V and PWMed down to about 3 ~ 3.5Vrms. It just seems to vape better than 6 ~ 7V PWMed to 3 ~ 3.5Vrms

edit: May need to try 12V PWM

 

[bstedh]

Just a guess here, but I would think that there is a certain amount of thermal capacitance that is related to mass/surface area* thermal conductance of the nichrome and the surounding medium. I'm thinking that because of this the PWM systems are going to be much more efficent when the frequencies equal or slightly exceed the total thermal capacitance. A loosely germane analogy would be some of these ultra bright PWM LED flashlights. The LEDS are not burning constantly but the percieved brightness is unchanged. The same kind of concept only we're swapping thermal capacitance for persistant vision.

If any of this is close to correct we can either drop the frequency low enough so that we can stick with the current "watts" paradigm, or we can forget what we know and just use empirical results as our guidelines. Better yet, lets just make an axial thermocouple and have a closed loop thermal control.

Any idea what the thermal capacitance difference would be between Kanthal and nicrome??? Can't find my 510 to 808 adapter to try my kanthal and ceramic wick modded vortex.

 

[bstedh]

For those scratching their head... "How much the coil cools between Pulses"

 

[jrm850]

According to my material properties bible (matweb.com ) The specific heat capacity is nearly identical between the two, but the Kanthal has roughly 25%-30% greater thermal conductivity. There are several different versions of each material listed with slightly different properties for each. The Kanthal should react a little bit quicker in ideal situations, but my guess is that the juice is the big equalizer and that it would be hard to determine much of a real world difference between the two.

These are all just guesses on my part. You guys have a LOT more experience with these things than I do so take what I say with a grain of salt.

 

[Stosh]

Just a random thought, but if someone had an o-scope to put on an ego and see what sort of frequency they use... Some how the ego produces enough heat at 3.2v to be a workhorse, just enough heating time, cool off time. If you could mimic the frequency with some adjustment up and down you might just hit a sweet spot.

 

[Uncle Screwtape]

Just a random thought, but if someone had an o-scope to put on an ego and see what sort of frequency they use... Some how the ego produces enough heat at 3.2v to be a workhorse, just enough heating time, cool off time. If you could mimic the frequency with some adjustment up and down you might just hit a sweet spot.

Using my poor-man's Oscope (the audio input on my laptop) I took a look at a couple of batteries. A 510 manual batt showed a frequency of 50hz with very short pulses,less than a thousandth of a second. The ego I checked was a bit more complicated, there were 8 "bursts" per second, each burst comprised of three pulses, 1st pulse less than a thousandth, second pulse about 1/500th and the third being another short one.

My measurement method could be completely flawed, but assuming it has merit, I think we should be looking at freqs lower than 50hz with duty cycles of 10% or so.

My fuzzy, just before bed, aproximatin' math, assuming we want it hotter than an ego.

 

[jrm850]

Using my poor-man's Oscope (the audio input on my laptop) I took a look at a couple of batteries. A 510 manual batt showed a frequency of 50hz with very short pulses,less than a thousandth of a second. The ego I checked was a bit more complicated, there were 8 "bursts" per second, each burst comprised of three pulses, 1st pulse less than a thousandth, second pulse about 1/500th and the third being another short one.

My measurement method could be completely flawed, but assuming it has merit, I think we should be looking at freqs lower than 50hz with duty cycles of 10% or so.

My fuzzy, just before bed, aproximatin' math, assuming we want it hotter than an ego.

the 5% duty cycle is very interesting, but kind of lends support to some of the theories floating around in this thread. Has anyone ever compared battery life of a 510 to an unregulated 808? It would seem like the 510 would blow it away at 5% duty.

The strange pulse stream form the EGO could have several explanations. One guess would be that the short pulses are part of a current limited diagnostic script that does not add much to the heat generation. Did you put your probes across the coil or directly to the battery? The reason I ask is that one of my 808s will flash a 3 blip short code if I stick a low ohm carto on it. The short pulses could be checking for certain operating parameters before it enters into full pwm mode. I have never taken one apart so I have no idea how sophisticated the control circuitry is. I may be giving them way too much credit.

 

[CraigHB]

I have a bunch of eGo batts laying around my wife uses. I probed a fully charged one with a 3 ohm load using my regular analog scope. It output a clean PWM waveform. The duty cycle looks to be about 85% with a frequency around 50Hz. With a waveform peak of about 4.1V using a fully charged batt, that indicates an RMS voltage a little under 3.5V. I don't have an RMS voltmeter, but my DMM does measure duty cycle and frequency. I got similar numbers there.

With no load, the eGo batt doesn't generate a regular enough cycle for the analog scope to synch up on what's happening. Looks like it goes into some kind of idle mode with a series of short irregular blips. You could probably tell what it's doing with a digital storage scope, but it doesn't really matter since you normally wouldn't care what the output looks like unloaded.

 

[Stosh]

I hate to ask, and appreciate your efforts but....... with an ego battery after a half days use, where the battery has dropped in output is there a change in duty cycle, frequency or both - neither? And the next question is can we replicate it easily? At a somewhat higher RMS voltage of course.

 

[Uncle Screwtape]

I have a bunch of eGo batts laying around my wife uses. I probed a fully charged one with a 3 ohm load using my regular analog scope. It output a clean PWM waveform. The duty cycle looks to be about 85% with a frequency around 50Hz. With a waveform peak of about 4.1V using a fully charged batt, that indicates an RMS voltage a little under 3.5V. I don't have an RMS voltmeter, but my DMM does measure duty cycle and frequency. I got similar numbers there.

With no load, the eGo batt doesn't generate a regular enough cycle for the analog scope to synch up on what's happening. Looks like it goes into some kind of idle mode with a series of short irregular blips. You could probably tell what it's doing with a digital storage scope, but it doesn't really matter since you normally wouldn't care what the output looks like unloaded.

Yeah, I did my little test with no load. I didn't really think that one through last night, it was late. I will have access to an oscilloscope next week. That will help me to test some theories in a much better way.

 

[wwturner]

asnider, couldn't you have mounted a voltage display in the middle slot sunk to the outside, and moved the circuit over it? lol

Holy Cow!!! Next time I do one of these, it's gonna be in a 4-AA box Too much stuff to cram in a single AA slot. Tried to take pic in progress .. 2 overlapping PCB, one with resistor/diode/cap matrix, the other with 555 and FET.

Breaktru ... I have a new found respect for you .. you are certainly better at cramming components than am I

 

[solarisx]

Good job all. I think the frequency is not at all important here. As long as it is above like 100Hz and less than the max frequency of the MOSFET, it should work fine. In PWM method, the pulse duty cycle determines the effective voltage applied to the load.
I don't think your ecig would taste different with a PWM controlled voltage or a simple voltage regulator as long as the effective voltage on both of them is the same. I guess the reason some people get different taste or huge vapor is the duty cycle. Play with it and you get different vaping experience.

 

[meatsneakers]

I am going to put in my Digikey order today, what do the gurus in this thread think of this circuit - taken from this thread. I changed the pot to 500 ohms, added an 'atty' and LED (with goofy values, but it should work in reality) and setup the switch so it can be a micro. Does anyone see any issues with the way it's laid out?

 

[jrm850]

...I think the frequency is not at all important here. As long as it is above like 100Hz and less than the max frequency of the MOSFET, it should work fine. In PWM method, the pulse duty cycle determines the effective voltage applied to the load.

I agree in part. I think that once the frequency exceeds the thermal cycle ability of the heating element then you won't be able to tell the difference between 60hz and 400khz. At lower frequencies I think there is the possibility of improving the efficiency ,especially if the duty cycle is on an inverse curve to the heat ramp.

Thinking out loud here...
The reason I say this is because of the delta T component of the heat energy equation. Greater change requires greater energy. In the basic pwm systems we are using here the heat curve in the functional range is probably almost linear. We are tuning it to produce a peak temp that will not burn the juice, but we waste a lot of energy getting to that point because we are spending energy over time outside of the functional heat range of the application. I think it would be much more efficent to pump the juice quickly and then back off on the duty cycle to maintain temps in the desired range. The inverse duty cycle should increase efficency somewhat at any frequency, but I think it would be even more effective if the frequency matched the thermal capcitance of the heating element. The element stores heat as kinetic energy in proportion to its mass. That energy is dissipated at a rate related to the mass, surface area, and thermal conductance. If we think of the heat/cool cycle as a sine, peak efficiency should occur at a frequency matched to the sine. In other words we just want the frequency to level the amplitude of the heat/cool sine to minimize delta T.
This is all just an uneducated theory of course.

 

[jrm850]

I am going to put in my Digikey order today, what do the gurus in this thread think of this circuit - taken from this thread. I changed the pot to 500 ohms, added an 'atty' and LED (with goofy values, but it should work in reality) and setup the switch so it can be a micro. Does anyone see any issues with the way it's laid out?

The pot in this circuit is just a voltage divider used to override the default threshold level. The value of the pot will not alter the PWM stream so you want a higher value to keep from dumping a lot of energy directly to ground. The 500ohm pot will waste 16ma. a 10k would probably be a better choice.