Power MOSFET - easy VV?

[AttyPops]

OK guys... I'm a noob, and will probably just do a BigBlue evercool type of thing in the future. Nice fairly turn-key solution. So, please bear with me, but I'm having trouble absorbing power MOSFET tech. So, for theory's sake as well as my understanding:

MOSFET is good technology for power switching. OK. It can handle PWM if the gate is logic-level. Atties don't care about ripple. VV can thus be obtained with PWM using a mosfet? Now, the module that BB used in the evercool is basically doing that (and some filtering with caps... and maybe a lot of other under-hood magic too).

Why, for an e-cig, can't we just use a POT and proper capacitor to make a simple oscillator and drive a power MOSFET? (I could use a low-pin-count microcontroller here too with PWM, and even drive an LCD, or USB port....).

+ --->current limiting resistor (may not be needed with RC Osc)-> Power Button ----> Some RC Oscillator ------->Gate PIN of Power MOSFET

Source and drain hooked to 2 3.7 volt 14500's (4 example) through atty.

I know this is probably a dumb question, but heck... It will help me understand why I need an entire module for VV. And remember, be kind......

 

[Scottbee]

Just running the resistor and cap into the gate won't give you an oscillator. But.... with just a little more gimmickry it can be done. And the PWM concept is sound.

 

[AttyPops]

yeah, my bad. I'll edit......

 

[Java_Az]

The proveri uses a microcontroller to adjust the voltage. Only mod out right now i know of that uses one. I guess it uses a digital pot by way of push buttons. But thats just a guess waiting to see some reviews on it. Too bad it is ugly as hell ( in my opinion ).

Not sure i can answer the why you need a module for variable voltage. You can for use a pot to adjust one if you can fit a decent one in a mod, damn things are big at least the good ones. But the module used in the evercool i dont think would have been used if it wasnt for free samples. Linear regs are way cheaper but also are not as good , they produce heat and also have voltage drop puts They also are not as efficient as switching regs. Price wise though 2 bucks and change for a high amp linear compared to almost 20 bucks for Ti ptn08100w switching reg. We might not have seen that reg if it wasn't for free samples.

 

[NuMoNiC]

WERD Java..

 

[AttyPops]

Thanks for the input guys..... Yeah.... I use LDO linear regs for my 5v mod but, of course, it isn't vv. I was just looking for a simple adjustable PWM/oscillator that would drive the regulator 3.7 -> 5.0 volts. IDK if doing that is real hard on the regulator and would fry it, or make it fail after, say, a week. I'm sure you're correct, Java, about the efficiency (and other points).

At $15.00 - $20.00 for a module, I can get like 3-4 new 14500s. OTOH, I'd rather put the $$ into the MCU and other components. They have built-in PWM hardware (and it's easy to implement in software too) and are generally < $5.00. They need some supporting hardware, and can be flaky (resets) if not buffered from brown-outs. I'm thinking about a nifty display used to display current voltage, maybe usage stats. Possibly USB data port for programming and statistics.

Won't be nearly as efficient as BB's model though. I have other ideas about using it to read the atty ohms and adjust to watts rather than volts.... (they also have A/D converters on-chip).

 

[warbdan]

I saw one mod that was VV with only a pot and ucc383-adj in a 2AA box. No caps or anything. I won't post the link here, it was on another forum. Feel free to pm

 

[Scottbee]

Another point.... putting together an oscillator with variable frequency isn't really going to get your the desired results. You really want to vary the duty cycle, not the frequency.

 

[duby]

We might not have seen that reg if it wasn't for free samples.

I completely disagree. Prior to BigBlue's design people were using dimension engineering SWADJ3 switching regulators.
3A Adjustable step down switching voltage regulator
At $25. People were already willing to pay extra for a decent VV reg. Atleast people who understood the difference between Switching and linear regulators.
People have paid $80 for a pencil sharpener which you couldn't vape.

 

[WillyB]

I saw one mod that was VV with only a pot and ucc383-adj in a 2AA box. No caps or anything. I won't post the link here, it was on another forum. Feel free to pm

What's the big deal.

Here's a diagram I posted on these forums for a member a while back (using Mamu's pic as a base). He was looking for a small footprint.

It's for the TI UCC383 or 283-ADJ.

 

[warbdan]

Yeah, that's it! Thanks WillyB

 

[5cardstud]

For $25.00 I just use this 3A Adjustable step down switching voltage regulator and it's been a great little regulator. Small and I attached a knob for controlling the voltage.

 

[warbdan]

Ooo, that's a neat little package, but samples from TI are free.

 

[AttyPops]

Scottbee....ur correct. 50% is 50% (or whatever) regardless of frequency (doh). Although...<thinking> a low enough frequency may lower the effective wattage delivered to the atty..... like if you manually "pulsed" the button yourself. You know...like averaging it out over time using the cooling factor of the coil as a parameter. The MCU would have a duty cycle tho.......and be more effective as a voltage regulator PWM source, since you are correct about PWM usually being fairly high frequency and needing a duty cycle to pull off a "faked" voltage.

thanks warbdan and WillyB for the ucc383 tip. So it already adjusts based on a pot..... hmmmm.... better.

5card... yeah, another good 20/25 dollar module. But the reason for this post was to discuss alternatives to that type of thing for fun/education. The ucc283-adj, for example, looks to be about $2.50 in a real small package. Its a Linear LDO reg, adjustable, rather than a more efficient switching power regulator, but .... 1/10th the cost and small package.

I'd highly recommend that people check out those turn-key solutions... (I probably will too someday) this is just a "for fun" discussion on power regulation.

I guess "easy vv" was a misnomer in the OP - I should have said, simpler/lower cost (but not necessarily as efficient). The turn-key modules are easy, to be sure.