My First Pulse Width Modulator Mod

[CraigHB]

I haven't worked with the 555 at all so I can't tell you for sure on those caps, but a lot of times, they're simply used for decoupling (filtering out noise) and you can use any ceramic capacitor in the .01uf to 1uF range for that. It looks like the bigger one is required for timing, but the two samller ones are just decoupling capacitors. I typically use a µcontroller for that sort of thing since they always have PWM generators built-in. I usually want to do more than that as well so I've never actully needed to use a 555 on it's own. I've seen it pop up in a lot of circuits though.

If I was going to do this task, I would use a µcontroller. There's another thread in this section here where a guy is looking to do the same thing (PWM VV) with an ATtiny25. However, that's a much more complicated way to go since the chip requires coding. Though physically, it uses less components on the whole and is easier to build.

As far as size, don't be intimidated by surface mount components, they're actually easier to work with. It's not hard to etch your own PCBs. For soldering, you just need a good magnifier and a fine .01" conical tip on your iron. I use a 10x stereo microscope myself, but any magnification method shoud work. With SMD, you can also do your own reflow soldering using a toaster oven.

 

[asnider123]

As far as size, don't be intimidated by surface mount components, they're actually easier to work with. It's not hard to etch your own PCBs. For soldering, you just need a good magnifier and a fine .01" conical tip on your iron. I use a 10x stereo microscope myself, but any magnification method shoud work. With SMD, you can also do your own reflow soldering using a toaster oven.

Funny you mention SMD .. I ordered some 555's and didn't realize they were SMDs .. man those boogers are small .. kinda put them in a drawer until I could figure out how to deal with them. LOL About my iron tip, you're right, I think mine is too fat to do that kind of precise work (not to mention my eyes and shaky hands) ... since I already have them, nothing to lose trying them out.

 

[CraigHB]

You'll open up a whole new world of applications when you swtich to doing your projects in SMD. I switched about 15 years ago when SMD started to really take over the product lines so it was a matter of necessity. I was really discouraged about it at first, but it didn't take long for me to adapt. Now, I would never consider not using SMD.

I'm no spring chicken either, turned 51 a couple months ago. My hands are not the steadiest either and my close vision is pretty much shot. Though, it's not too much of a handicap. With the microscope and a little support on the soldering fingers, I usually don't have a problem, though sometimes I slip a little. I've done DFN and .5mm pin pitches without too much trouble. The DC-DC controller I'm currently using for my mods (shown in my avatar) is .65mm pitch which is relatively easy. The .5mm pitch stuff can be a littel rough. The FET I linked to before is 1.27mm pitch, piece of cake. I'm using that one in my current mod as my energizing FET (single battery boost).

The trick is to use a very fine tip on your iron, ideally a digital temperature controller one. I currently use a Weller WESD51, a soldering station under $100. I use the ETS .01" conical tips with it. You also need a fine wire solder. I use .015" wire solder, 63/37 with a mildly activated core (Kester 24-6337-9703), but you can also use a more active solder. I use a flux pen to glue down my parts before soldering and that's an active rosin flux (Kester #186). For fixing mistakes, I use a desoldering braid (Pro-Wick sizes #1, 2, & 3). You invariably solder pins together at times and the braid is a necessity for sopping up overspill.

Optionally, you can use a toaster oven to reflow solder your parts, but the iron is more precise and doesn't heat things up as much. A lot slower though. I think doing your own reflow is more popular, but the problem is you can overheat your parts pretty easily. Without an expensive oven made for that, reflow can be kind of a ......

 

[CapeCAD]

The cap from pin 6/2 to GND is used to set the frequency range. The chart shown in this simulation is what I'm using to test new stepper motor drivers I'm designing:


C1 sets the frequency range.
R1 sets the pulse width.
R2 sets the frequency.
D1 allows duty cycle to go below 50%.

I prefer the toaster oven, but a digital controlled iron is needed to touch up or change components.

 

[bstedh]

What would be a preferred frequency to efficiently drive an atty with. I am assuming a lower frequency would be less efficient and higher frequency would give less adjustment.

Would driving the atty at Microwave frequencies cause cancer? =]

I'm starting to envision the wifi Vape. =]

 

[asnider123]

Thanks Craig .. is it possible to breadboard SMD components? I may need to learn a new skill

 

[CapeCAD]

Thanks Craig .. is it possible to breadboard SMD components? I may need to learn a new skill

There are SMD to through-hole adapters for breadboarding.

 

[CraigHB]

Thanks Craig .. is it possible to breadboard SMD components? I may need to learn a new skill

Yes, it is possible and I do breadboard stuff on occasion. For my bigger projects, I actually have my PCBs manufactured. I've been using ExpressPCB. In that case, I may want to test portions of the circuit on a breadboard before spending money on a manufactured board. What I do is make a small PCB to mount the components. Then I put break-away header pins on on the mini-board with required electrical connections so I can push the assembly onto a breadboard. If my camera hadn't died the other day, I'd show you a picture.

Unfortunately, breadboards have a lot of parasitic impedance (reactive and resistive loading) and you somethimes have to take that into account. It's usually not a problem for purely DC stuff, but when you get into things where you need pulsed sources or signals passing through the breadboard connections, it can be a problem.

 

[clutchjunkie]

I have a question if we have any 555 experts here.

In theory, the pulses that come out of pin 3 of the 555 go from - to + in a square-ish wave shape. Could one use a PNP transistor instead of a NPN to basically reverse the duty cycle? The reason I am asking is that if this were possible, it would change the way the potentiometer is used so that a higher resistance # would increase the "effective" power coming out.

Then, if one could tie the resistance of the atomizer to the resistors and pot in the circuit, with a little testing and math one could have a device that operated similar to the darwin. The higher the ohms of the atty, the more "power" that would be sent to the atty by increasing the duty cycle.

Just a theory. I guess one could use a NPN transistor to trigger the PNP mosfet if they had too, if I weren't working 12 hour days here I would test it myself.

 

[CraigHB]

I'm not an expert with the 555, but you should be able to do that, just use a P-channel FET on the high side instead of an N-channel on the low side. It wil reverse the duty cycle. FETs make better switches than transistors generally. You don't have to worry about biasing a FET and unlike the base of a transistor, the gate draws negligible current thereby isolating the control input.

You could do it another way if you wanted to keep using an N-channel FET. I often use a P-channel or N-channel small signal MOSFET to reverse control logic. A problem you run into with using high side switches (P-channel) is you need to bring the gate up to source voltage to guarantee they're fully off. Can be an issue if you're tyring to control the switch with a lower voltage which is sometimes the case (but not here). A small signal MOSFET with a pull up resistor can usually solve that problem, but you add propegation delay and for high frequency apps, you have to consider the reduction in switching performance. Though, high side switches are nice because you don't get any ground offset like you do with low side switch. Using a low side switch for supply power can be problematic for some chips.

 

[bstedh]

I have been looking around for a pre-built PWM circuit that we could use that you would only need to add a POT to, but have had no luck. Every thing I have found so far still needs sever external capacitors and resistors. I have found a couple where the PWM and fet are in the same package giving a little smaller foot print but I was hoping to find something packaged like the OKR where all you need to do is add your adjustment POT and you are done.

Anybody have any ideas of weather or not something like this is available out there? If it is it would make PWM available to a lot of modders who don't do circuit building. =] Even if it was a complete PWM circuit an you add the FET and POT to it would be good.

 

[AttyPops]

I have been looking around for a pre-built PWM circuit that we could use that you would only need to add a POT to, but have had no luck. Every thing I have found so far still needs sever external capacitors and resistors. I have found a couple where the PWM and fet are in the same package giving a little smaller foot print but I was hoping to find something packaged like the OKR where all you need to do is add your adjustment POT and you are done.

Anybody have any ideas of weather or not something like this is available out there? If it is it would make PWM available to a lot of modders who don't do circuit building. =] Even if it was a complete PWM circuit an you add the FET and POT to it would be good.

I could be wrong, but I think the logic is that switching regulators are, in essence, a variation on PWM already. To an extent, this type of PWM is reinventing the wheel. If you think about it, what they do is turn on/off fast. The conditions are different.... they switch based on a voltage comparison, rather than a timed pulse. However, they end up having a frequency just like a 555 based PWM would. Someone with a better understanding of the internals of switching regulators may chime in here.....

That was the reason for my comments in earlier posts of this thread. However, I think that it is cool to built this from a modder's perspective. And, like I said, I'll bet it rivals (if not beats) switching regulators. There's an inherent efficiency loss in the circuit and the mosfet, so IDK how it compares to a dedicated switching reg from end-to-end % efficiency.

Now, the other threads on MCU stuff (mine, and others like the recent AVR one) use PWM also to vary the heat to the atty. The MCU's have that all-in-one thing going for them since they have an internal timer (or can emulate a timer and PWM in code). Of course, you still have the support circuitry and then you have to program them.

You may find a dedicated IC for it. IDK. I think people just use the switching regulators.

 

[bstedh]

That's kinda what I thought.

I was looking at putting one together but most of what I was planning would require buying more tools =] than I have right now. And that would cut into my modding budget. Right now all I have are standard hand tools, Dremel, and a couple of soldering irons. I can put it together with standard components but was hoping to go surface mount to fit it in an epower size switch body.

I can do it with the switching regulator but wanted to try something new. =] But that's OK. My OKR is working so good I may stick with that circuit for now. Maybe I will come into some extra cash in the near future and I can expand my tool base to give me more options.

 

[AttyPops]

Yeah. As a side note, the PWM idea doesn't, in and of itself, regulate the voltage to a constant value due to the battery's power curve. In other words, it still results in a power curve, just like a resistor would (but with less wasted energy). In order to actually modulate the atty to a set "pseudo-voltage", you'd have to read the v-in and calculate the proper duty cycle adjusting it every second or so. The switched voltage-reg uses a voltage reference to keep it level (I think). It auto-adjusts.

 

[clutchjunkie]

I think price is another issue bringing interest to DIY pwm circuits like this. I can get 20 555 chips for $6, add another $6 for 10 mosfets (or more, depending on which one you go with) and you are right around the price for one TI chip.

Except you can now build at least 10 mods, or have backups in case you do something like I did and burn out a ti ptr08100. Turns out they don't like 8.4v pumped into the Vout pin. I was wondering what that smell was.

Anyway, it comes to around $7 for the whole circuit if you add it all up, resistors, pot, caps, switches, everything but the batteries and atty connector. Thats cheaper than the murata chip even, and you still have to add switches and caps to that.

 

[AttyPops]

Yup. I think that's part of it. The venerable 555 chip should go down in history as one of the most versatile chips made. asnider123 and others are having fun with this. Someone else had a 555 thread too. IDK how he came out.

As for me, I use a $2.50 (that's overpriced) 5v linear regulator and "just say no" to the rest. lol. Efficiency .... bah humbug. That's what battery chargers are for.

But circuits like this are sure fun to play with. Learn a lot too.

 

[CraigHB]

For a down and dirty VV mod, you sure can't beat the simplicity and inexpense of a linear regulator. However, that efficiency does make a pretty big difference in battery run time. When I went from an asynchronous to a synchronous PWM controller I got a modest improvement in the niehborhood of 10%. Even that relatively small amount makes a noticable difference in run time for me. Compared to a linear regulator, it's like 40% more efficiency.

The POL switching modules are relatively expensive, but it costs a lot more to make them yourself so there's that. The one I currently use is more than double the cost. Here's a rough breakdown;

2 10u caps - $1 (the SMD ceramic ones get expensive when they get into the high values)
2 100u SMD low profile tantalum polymer caps $10 (ouch, I'm working on using ceramics instead)
1 1u inductor $2
TPS43000 PWM controller $5
Various caps, resistors, small signal FETS $1 (for compensation and control)
3 Power FETS $3 (one for on/off control)
Digital pot $2

Total $24, but without the digital pot it's $22 since you would need that with a POL regulator as well. If I can work out the cermaic output caps, I can get it down to $18, but it's hard to compensate those because of their less predictable ESR characteristics. That's why I'm using the more expensive ones.

I make my own for a couple reasons. I want to use a single battery with boost but none of the POL ones have high enough power output. I want it on the same board as the rest of the stuff (controller, charger) to keep the profile as low as possible. I can get 5A at 6V with the ones I make, but the highest I would ever go is a little over 3A with a dual coil, but that's still too high for any of the POL modules I've seen.

 

[AttyPops]

Yeah, I wasn't really expounding the "virtues' of linear regs. I just threw that one in as an aside. I give these mods about 3 years before battery technology makes the whole thing moot.

What I really want to know is how efficient PWM like this is vs an adjustable switched regulator, end-to-end. Although as stated above, the reg would adjust itself keeping a constant voltage over the battery curve.

 

[clutchjunkie]

Which digital pot are you using that is only $2? And where do I get them, more importantly.

 

[CraigHB]

I'm using a Microchip MCP4152-103E/MS and I was wrong, it's only a dollar. I did that off the top of my head without looking anything up so my estimate was very rough. However, that's an SPI (serial peripheral interface) pot so you need to use a µcontroller to interface it. Not very cost effective to use a controller soley to drive a pot.

I've seen a digital pot that has an incremental input in this forum, but I don't recall the thread. That's the only one I've ever seen you could control with nothing but a switch and pull-down. I'm sure there's others out there, I just haven't seen them. You'd think there'd be a big call for something like that. I strongly dislike using mechanical pots for user input, they just suck so horribly, wear out and get jumpy. I'm sure others feel the same way.