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ScotchTapeLord

Wed Oct 06 2010, 09:14PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

The higher the frequency, the less inductance needed, yes. That is because when you first apply voltage, the inductor blocks all current, and then lets it rise. The sooner you cut it off, the lower the current at time of turn-off, so the lower the output voltage (V = L*(di/dt)). Also, the more inductance, the slower the current rises, so you have to wait longer to cut off the supply (you cut it off less frequently: lower frequency).

BTW, Don't go so far to limit current. The current drawn is determined by frequency and inductance values. Perhaps you should implement passive limiting for your prototype, but once it's working, let the boost converter limit it itself. If you have passive limiting, it will only create losses (inefficiency) and slowwwer charging... which was, if I recall, what this whole fiasco was trying to solve!

Artikbot

Wed Oct 06 2010, 09:58PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Alright so... I have to get an inductor slightly bigger than the needed one, but not too big or it will be slow saturating aka inefficient at high frequencies, right?

No current limiting... I'm forcing it to 2Amp at the moment of assembly just to see if soething burns out or what's happenin. When it starts to run, I'll let it go without limiting, the whole circuitry should limit itself.

Yeah, higher efficiency was one of the drawbacks of using the stock chargers XD
I started all this stuff because I wanted fast and efficient chargers :P

Okay, tomorrow morning we'll try and see if anything blows up :)

Will keep informing!

Thanks!

GhostNull

Wed Oct 06 2010, 10:07PM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

lol, typo >.<

For passive limiting, (filament) Light bulbs are always a good choice. Talked about it a bit here:

Artikbot

Wed Oct 06 2010, 10:38PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Wo-ah is all I can say right now.

So, putting in a (small) 25W or so 230V bulb will act as a resistance? This is a hell of good!

If I swap it for an halogen one... Will it be too much inefficient? I mean, those things eat a lot of power and heat up like small suns inside crystal cans. Do you suggest any specific type of bulb? I suppose incandescent 230V low power ones will do the trick placed right before the inductor.

Thanks again for that weird yet useful trick!

GhostNull

Thu Oct 07 2010, 12:16AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Not quite, you need a light bulb rated around what you are using
In your case, you are using a 7.4v power source so you need a bulb rated for around that voltage. A main light bulb might work, look at the power of your boost converter and use a light bulb around that value

ScotchTapeLord

Thu Oct 07 2010, 01:09AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

A bulb for mains voltage will probably have too high of a resistance since it's meant for 120 or 230V depending on where you live. Best to test it with a multimeter. You only want a few ohms for such a low voltage...

Another neat trick is to use a mechanical pencil refill clipped with alligator clips. They're about 3 ohms each. You should shield it from your eyes because if everything fails and your supply gets shorted across it, it may act like an unshielded fuse! Which can be bad for your eyes, but good for your circuit as you have just cut the power! It's a good idea to wear goggles when experimenting, by the way!

;D Function over form!

Artikbot

Thu Oct 07 2010, 07:31AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Right, so 12V lamp will suffice in this case. Resistance increases when the lamp lights up right?

I'll check pencil leads too, used them once to act as resistors for a music project... I was using a lead between two conductive spots to create higher or lower pitched tones :P

Well. Seems today is a bad day to go and get the parts because dad has taken the car and I have no way of going to the store. Damn it.

Thanks!

Edit: I've been doing some calculations for the inductors since they still bother me, and... Tadaaa!!! Changing timer resistors to 10k R1 and 47k R2 I rise my frequency to almost 13.5kHz, which is great because I only need a ridiculous 65+ uH inductor. And I have a 125uH lying around here that is capable of driving 8Amp natively. I can even lower frequency to 10kHz and need an 85uH inductor, much better for the capacitor on the timer to be less "stressed".

This is starting to see the light :)

GhostNull

Fri Oct 08 2010, 09:07AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

ScotchTapeLord wrote ...

The higher the frequency, the less inductance needed,

That is, for the same current output. In a boost converter. The output voltage is determined by the input voltage and duty cycle. The frequency and inductance determined the current consumption/output. So you don't really need to worry about getting an exact inductance. A lower inductance will *only* mean higher current levels and as long as the current levels do not go beyond the rating of the components there is nothing wrong. So winding your own inductor should be fine.

And the resistance of the light bulb does go up when lit up.

Artikbot

Fri Oct 08 2010, 09:44AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Yeah, it's great. i have an inductor capable of sustained 8Amp, my diodes are rated at 3Amp 100oV and I don't expect getting more than 25-30W juice out of the charger, so I'm far below component ratings.

Let's hope they have the components today evening, because I called them two hours ago and they still hadn't received it.

Bunch of shit, because i ordered them last friday and they're coming from 700km abroad, shipping from there usually takes a couple of days, not five. Well, i'll start de-soldering more capacitors and see if I need more than I have.

I was wondering... If efficiency corresponds, 4.8mF should be enough to pierce a fried tomato can? In case I need more juice, can I add 100uF capacitors to the abttery too? I will be doing banks of 8 capacitors each, with independent charging balance via paralleled resistors. So essentially, I'll have 5 banks of 960uF each. If I could use the 100uF capacitors aswell, I could build a pair of banks rated at 800uF, leading to a total capacitance of 6400uF. This sounds to me like a hell of beefy, maybe I'll need to build another coil... 10 layers of 14AWG wire, that sounds devilish :P

Thanks!

GhostNull

Fri Oct 08 2010, 11:29AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

With a straight singe stage non controlled pulse you'll probably get no more than 5%
Look here for the math (big post with quote box)

You'll have to do your own reascearch on the amount of energy it takes to pierce a fried tomato or anything. I have no idea.

The rectifying diode, right after the inductor, should be ultra fast. Be sure of that or it might not work very well.

edit: WAIT! I found an error!!!! Your timing capacitor is 1pF!!!!! I needs to be higher!! At the least 1nF!

1pF is EXTREMELY low. Stray capacitance (capacitance between wires/connections) will heavily throw off your timing. If you put two 5 cm wires close together the capacitance would be higher then that, at least 50pF. That's 50 times your timing capacitor. You want your timing capacitor to be at least 1nF/0.001uF so it will not be effected by stray capacitance.

Also, don't expect the design to work exactly as in the sims. You MUST leave room for error, I suggest you put some trimmers or potentiometers in series with the timing resistors in the 555 part so you can adjust the resistance and therefore timing.
Another thing you might want to consider is a fail safe resistor for the MOSFET, this is a 100kOhm (high value) resistor going directly from the MOSFET gate, to ground. So is the 555 stuffs up or something fail safe resistor will turn off the MOSFET.

Sorry I didn't see this earlier

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