My First Pulse Width Modulator Mod

[Dalton63841]

You know what? I may be overdriving my FET I am using an IRL3103, which works great in a single battery mod. However, it has a rated Gate/Source voltage range of 2-4v, with this setup I will be giving it more like 8v...

So, instead of connecting pin3 of my 555 directly to the gate, I need to put a resistor in there. I need to find out what value will give me the 2-4v range I am looking for.....

Perhaps a 4v Zener diode backwards would work? Been a while since I used diodes for anything, but if I remember correctly, a reversed 4v zener would subtract 4v from whatever is passing through it. So, when the batteries get gown to 3v apeice, it would still supply 2v to the gate. This would also double as protection for the batteries as they would operate from ~4.2v down to ~3v, but once the overall voltage dropped below 6v(~3v apeice) the gate would no longer have a high enough voltage to open.

 

[clutchjunkie]

I just use a 100ohm resistor on my 1404's in their other application, and that would work from 7v-12.4v input voltage. The 3103 seems very similar. If you already have the circuit driving an LED and it's working then you may not need one at all though...

 

[bstedh]

Perhaps a 4v Zener diode backwards would work? Been a while since I used diodes for anything, but if I remember correctly, a reversed 4v zener would subtract 4v from whatever is passing through it. So, when the batteries get gown to 3v apeice, it would still supply 2v to the gate. This would also double as protection for the batteries as they would operate from ~4.2v down to ~3v, but once the overall voltage dropped below 6v(~3v apeice) the gate would no longer have a high enough voltage to open.

I think you would need a resistor with the zener to shunt the excess voltage to ground otherwise all the voltage would still conduct until it dropped below the zeners ratting.

Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point on, the relatively low impedance of the diode keeps the voltage across the diode at that value.

Just a resistor should work as I don't think the gate should draw very much amperage.

 

[bstedh]

here are some schematics and info that relate to what you are doing and may be useful to refine the circuit.
DPRG: A Simple PWM Circuit Based on the 555 Timer
http://discovercircuits.com/DJ-Circuits/images/SIMPLEPWM2.jpg
Pulse Width Modulators (PMW) electronic circuits, page 1

There is some really good info here =]

 

[air1]

FYI I think that the 2-4v rating is what the FET is rated for to guarantee that it is fully on. In other words your target voltage is at least 4v or else that thing will heat up fast. But that is just from memory, just a heads up, and I could be wrong. This is why they have special ones that are for logic ic's that require less voltage to ensure saturation.

 

[asnider123]

Hi Air1 .. I didn't have any heat problem when it was breadboarded, I was just concerned that I might fry the FET. Not that it is such a big deal, I have a bunch more

I think, just for hoots, I might throw a small resistor in the mix, probably won't make any difference but what the heck

Thanks much

 

[air1]

I talked with my electrical guru today. He confirmed what I said. Anything under 4v on that MOSFET results in heat. There is no issue at 8 or 8.4v at the gate. A resistor is a waste of time soldering, space, and money. If you were running real high voltages (higher than vaping) then look at the SPEC sheet. Anyways that's why it ran cool, it was fully saturated.

Now get it in a box, and take some video!!!

 

[asnider123]

I talked with my electrical guru today. He confirmed what I said. Anything under 4v on that MOSFET results in heat. There is no issue at 8 or 8.4v at the gate. A resistor is a waste of time soldering, space, and money. If you were running real high voltages (higher than vaping) then look at the SPEC sheet. Anyways that's why it ran cool, it was fully saturated.

Now get it in a box, and take some video!!!

Thanks for checking that out. Back to the workbench

 

[CraigHB]

Nice one there. That would be the way to do it. People are using the POL buck regulators because they're easy to implement, but normally, you use one of those to minimize ripple and drive other electronics. With an atomizer, we typcially don't care about that. Interestingly, battery units like the eGo and 510 already use PWM in the same manner, but they reduce voltage to a constant lower level with a single battery. There's a reason they do it that way, simplicity and efficiency. Like you said though, the tricky part is measuring equivalent DC voltage. I guess you'd have to do it by feel. I don't know though, a good RMS meter might be able to provide a true reading.

Oh, on the FET, I would recommend the Fairchild FDS6680A. It has low "on" resistance with low gate-source threshhold voltage. It will generate very little heat at high currents even with only 4V on the gate. It also has excellent switching performance.

 

[asnider123]

Thanks for the wealth of info, Craig! I just checked and, I'll be darned, I DO have a True RMS meter, an Ideal 61-361 .. maybe I am getting good readings after all!

 

[Dalton63841]

Oh, on the FET, I would recommend the Fairchild FDS6680A. It has low "on" resistance with low gate-source threshhold voltage. It will generate very little heat at high currents even with only 4V on the gate. It also has excellent switching performance.

If only I had a corporate email address. Some samples of that part would come in handy.

 

[CraigHB]

I have one, but it's because I need one. Though, if you think it's worthwhile, you can get yourself a business email by going to Network Solutions then utilize their domain name + web mail solution. It works out to about $50 a year, but if you order a lot of samples, it's worth it. I easily order more than $50 a year in free samples and some of them are not available in small quantities from the usual places like Mouser and Digikey.

 

[Dalton63841]

I have one, but it's because I need one. Though, if you think it's worthwhile, you can get yourself a business email by going to Network Solutions then utilize their domain name + web mail solution. It works out to about $50 a year, but if you order a lot of samples, it's worth it. I easily order more than $50 a year in free samples and some of them are not available in small quantities from the usual places like Mouser and Digikey.

Thanks for the info. I don't need anything like THAT. Just 1 thing here and there.

 

[bstedh]

This thing still working good? I am putting together a digikey order and planning to add parts to put one of these together.

 

[bstedh]

Nice one there. That would be the way to do it. People are using the POL buck regulators because they're easy to implement, but normally, you use one of those to minimize ripple and drive other electronics. With an atomizer, we typcially don't care about that. Interestingly, battery units like the eGo and 510 already use PWM in the same manner, but they reduce voltage to a constant lower level with a single battery. There's a reason they do it that way, simplicity and efficiency. Like you said though, the tricky part is measuring equivalent DC voltage. I guess you'd have to do it by feel. I don't know though, a good RMS meter might be able to provide a true reading.

Oh, on the FET, I would recommend the Fairchild FDS6680A. It has low "on" resistance with low gate-source threshhold voltage. It will generate very little heat at high currents even with only 4V on the gate. It also has excellent switching performance.

here is a surface mount version from DigiKey for $1.06
FDS6680ACT-ND
I am assuming this will work well???

Dose the frequency listed for the 555 matter? Their pricing ranges from $0.50 to $4.00.

 

[asnider123]

This thing still working good? I am putting together a digikey order and planning to add parts to put one of these together.

I have put it on the back burner for a while. It worked fine on a breadboard, got a little frustrated trying to cram all that stuff in a 3AA box. I think I will get a 4AA box, it will have plenty of room to do what I want to do.

In the mean time I am fooling with a few bazaar stealth mods just to pass the time.

 

[bstedh]

Where would you put a tactile switch. I am planning on putting this in an EGO style switch body so no main power switch...

 

[asnider123]

Where would you put a tactile switch. I am planning on putting this in an EGO style switch body so no main power switch...

My switch will be killing the positive batt input, I was planning to use a tactile switch for space preservation, but now that I decided to use a larger box, I think I will use a horn button instead, it has a much better amperage rating

 

[breaktru]

simple circuit involving a 555 squarewave oscillator driving a MOSFET. It oscillates at a constant frequency, you adjust the duty cycle to give you a "virtual" varying voltage determined by the setting of your pot. It will run at 10% min to 90% max duty cycle. So with 8V of batt, it should vary from about .8v - 7.2v. Since it's not pure DC, it is kinda tricky measuring the output.

This FREEware software may help calculating the resistors for the 555 Timer:
Free 555 Timer Calculator

The PRO version: http://www.schematica.com/555_Timer_design/555_Timer_PRO_EX.htm

 

[asnider123]

Back to the drawing board.... I haven't finished the first one yet, but thought I may try out a couple more circuits and decide which one will go into the mod. I got a little frustrated miniaturizing it down to one board, I'm getting old and the eyes don't work as good as they used to, you understand ....

I am gonna try some modifications to my original design. This design is called "Yet Another Simple PWM Generator" and, according to instructables.com, it is has very wide range of duty cycles. It's intention is to provide a fader circuit for LED displays and, according to the author, has a range of 0% (always off) to 100% (always on).



Another possibility is this design, by a vaper in Japan. I downloaded it from one of the forums, can't remember which



They are both very similar in the 555 PWM generator portion. Also, both are very simple (which is good because I am simple too).

Question: (for either circuit): what effect does changing the capacitor have on the circuit. I have a decent stock of all parts except for caps, which I am struggling to get together.

I also found a Brit site (DOCTRONICS home) that is a wealth of 555 info.
555 link: 555 timer

They have a neat 555 component calculator at 555 component selector