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"The Scavenger" A Joule Thief inspired Boost regulator. (unfinished)

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OK. I have done some tests and tried some suggestions. The following are my thoughts, notes, and some data. Note that I'm not perfect, I'm sure there are some mistakes and false assumptions in here. Just know this is an ongoing process with room for improvement.

Decreasing switching times.
I think Q1 switching ON is a non issue under all running conditions. This is because it is always going to switch with the inductor at ~0 current and with a very low voltage across it. This is effectively "zero voltage switching". So even if it lingers in the liner region where it would be wasting power, there is no power AVAILABLE for Q1 to waste at this point in the cycle. Unless anyone has better observations and objections, I'm just going to lay this to rest right here and now.

Q1 turning OFF time is a whole different matter though. First, the switching OFF happens where ALL of the power is gathered. So efficient switching here should be considered a top priority to be sure. Second, I'm not absolutely certain, but I think ~400ns is going to be a hard lower limit for switching OFF Q1. I was reading about BJT switching when coming out of saturation, I guess there remains some charge on the base that takes time to drain. This charge makes it so even if you are driving the transistor as hard as is needed, it's still not going to come out of saturation any faster. This problem is even worse for discrete parts do to larger die size/area for base charge to accumulate in. Now there are ways to fix this, [1] [2] but these methods are always about keeping the transistor from going into saturation in the first place. This is obviously not desired because it would cause other types of loss that would be even worse.

I think the only practical way to fix the issue is to run the circuit at a low enough frequency to keep the switching ratio high. Running at 25KHz gives you a 100:1 ratio, which is plenty efficient, though it is too close to human hearing for my tastes. 50KHz halves the ratio to 50:1, which is still quite decent. And 100KHz gives you about 5% switch transitioning time, which could be rounded up directly to 5% efficiency loss. Some where between 40KHz and 70KHz seems sane to me. That, or come up with another way to fix the problem.

Battery Model.
My battery is assumed to vary from 3 volts, down to 0.7 volts, and is assumed to have around 5 Ohms internal resistance. I choose 5 ohms to model for a very dead battery, which should have quite a high internal resistance. It may be much higher, it may be much lower, I don't really know exactly what is ideal. If anyone thinks these values are unreasonable, then please let me know what you think I should model with and I may change it.

Vin capacitor BEFORE the devices on switch.
I tried this out as it seemed like a great idea. And it did kind of help with starting but not enough to change the circuits minimum Vin much. It appears to only provide enough "boosted current" for a few cycles before it drains out to following the battery. The only way to make it more effective is to make the Vin capacitance impractically large, while making Vout capacitor impractically small. This has more negative effects than positive I'm afraid.

Ringing on L1.
I 100% agree with Mr RB's assessment of this ringing. It is such a low energy that there is no point in adding parts just to deal with it. If there happens to be a "free" way to fix it, then it should be done, if only to make the waveform look cleaner. But otherwise I'm just going to close the issue right here.

Transistors
Keeping with the "use only common parts" credo, I am using "general propose" BJT's. This means 2N2222, 200+ hfE and similar parts. We can for sure get better performance from something like the BC337-40, but the majority of people looking to build this circuit are going to use whatever they have in the junk box at the time. I could instruct people to only use good high gain switching transistors, but I think it might be better to make the design work with basic small signal transistors, but have room to upgrade to better ones for better performance when needed. Also, I myself don't have any really good switching transistors to use for testing, so if this was changed, all information would remain in simulation, assuming I am the only one building the circuit.

Bypass capacitor on Q1
I am playing with the idea of adding an AC bypass capacitor somewhere near Q1. The idea is that such a capacitor would provide strong base current when oscillating, but would lower or remove quiescent current drain altogether when in discontinuous mode. I also played with putting one across R1 to give stronger Q1 base drive during high frequency oscillation, and lower drive during low speed. I did this to try and get Q1 to switch off faster, at the cost of extra Q1 base current. But as I stated above under the section "Decreasing switching times", Q1 switching OFF time doesn't seem to want to go under ~400ns no matter how strong the base drive is. I think this capacitor is only ever going to be useful for either lowering quiescent current, and/or effecting the regulation.

Regulation modes.
When playing around with the circuit, I noticed that there are distinct ways regulation can limit output.

(1) The bias on Q1 base is altered as C2 gets charged from leakage through Zd1. This causes the current level Q1 turns off at to drop as Vout rises. In other words, the current decreases proportionate to the voltage increase.

(2) The oscillation duty cycle changes from an even 50/50 to other ratios as Vout gets up to and beyond 2X Vin. This happens because a 1:1 charge/discharge duty cycle can only ever double the voltage for a given current.

(3) Vout gets near enough to Zd1 reverse breakdown that the spikes from T1 primary momentarily push through the zener. This causes different bursts of counter biasing that limit the current in a staircase pastern.

(4) Finally, Vout reaches full blown zener breakdown levels. This causes any voltage on C2 to keep from discharging through the zeners forward drop. Because of this, C2 can only discharge through Q2 base, holding the oscillator fully off.


The following is a waveform from a circuit configuration that exaggerates each different mode of operation.

View attachment 67336

This is interesting information because if we can make good use of these two different modes of operation, then we can better adjust circuit properties for particular circumstances and applications. For example, if we could skillfully avoid the current limiting mode and go right to fully stopping the oscillation, we could potentially gain efficiency because during discontinuous mode we don't have any switching loss. This would cause more noise on Vout though. Or we could do the opposite, and get less extreme ripple, at the cost of some efficiency. This is very similar to the effects of changing the running frequency up or down, so we could do that also to get each arrangement's benefits.

Load and line regulation
I can't give any load and line regulation simulation figures as I am still to novice at LTspice to be able to know how to make a varying voltage or resistance for a transient analysis. Though admittedly I only gave it about half an hour of trying. However, I have played with load and line regulation on the actual device. Load regulation is easy to understand. It is 99% dependent on the available line energy. If the voltage and current available is fairly high, then the circuit will boost and regulation will be fairly steady (± 0.1 Volts) from no load up to very near battery wattage. After the load current exceeds what the battery can fully supply, then Vout starts to sag. This is almost entirely caused by the battery's internal resistance. The more battery's you have in parallel, the lower the internal resistance, and thus the lower the voltage of each cell can be. Bad battery's tend to have low voltage AND high internal resistance. So combining them in either series or parallel will allow the circuit to boost to full the voltage. Note that the circuit doesn't seem to care if it gets it's watts from volts, or current. As long as the total load watts + system loss is on the line, it will be boosted to the proper regulated voltage. The only times this isn't true is when there are mode changes. As seen above, there are different modes of operation. The one for the transition to 2x Vin has the biggest effect on regulation. I think the total regulation, over all operating modes and conditions is about 0.3 volts. Could be better, could be worse. And all this will probably change as the circuit is tweaked more also.

Precision adjustable voltage reference, ("super zeners")
It somewhat goes against the "use only common parts" credo, but we could use a Tl431 like precision zener. The advantages is that the TL431 and similar is 100% adjustable, so we could use a trimmer pot and potentially get a tunable voltage from a single board, without changing any parts. Also, they have much more exact reverse breakdown voltages than real zeners do, making regulation a more ON/OFF thing. The disadvantage to this is that it is harder to simulate (I STILL don't have a working model for the TL431) and it would add one or two more resistors, needed to set the voltage. Also, such a change might have unintended side effects, such as the fact that a zener has a reverse AND forward mode of conduction. The TL431 may not like behaving this way, which could change the way the circuit currently operates.

That's what I got so far. Let me know what you think.
-()blivion
 
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Mr AL said:
...I did a similar thing for the two transistor circuit quite a long time ago which only requires an inductor. It's somewhere on the web :)
The circuit uses two transistor, but doesnt require a specially hand wound transformer, only an inductor.
If you are interested i'll try to find it.

I second that I'd like to see it too. I originally tried to get a 2 tran- boost circuit working well at <1v but with no end of heartache I gave up on it pretty fast. If tackling it again i think the best solution is to go for a feedback winding like ()blivion has done, in order to get good operation from a wide range of Vin.

I found this tiny PCb on ebay, it's a single chip and inductor and diode on a PCB for $3.49;
**broken link removed**
however the fact that a tiny SMD SMPD regulator chip exists does not mean that it's silly to design a two-transistor version. There are a lot of people who would make one for fun and satisfaction, especailly when it can be made from junkbox parts or parts from any old radio or TV. You can buy nice cheap TV dinners but some people still like the satisfaction of cooking for themselves. ;)

()blivion said:
(re fixing Q1 turnoff time)... I think the only practical way to fix the issue is to run the circuit at a low enough frequency to keep the switching ratio high. Running at 25KHz gives you a 100:1 ratio...

I agree that lowering frequency is a good option, and trying to make nice efficiency SMPS usually ends up with good quality inductors and reduced operating frequency to cover for not having perfect digital switching.

But it's not the only option? Maybe a small cap from feedback winding direct to Q1 base, bypassing the new resistor which was added to decrease off state current. That would give a faster turnoff (edit, I noticed you mentioned it too). Also using more turns on the feedback winding would provide more dynamic base drive during turnoff?

()blivion said:
...Keeping with the "use only common parts" credo, I am using "general propose" BJT's. This means 2N2222, 200+ hfE and similar parts. We can for sure get better performance from something like the BC337-40, ...

Good point. :) I'd still try it with BC337-40 myself, to get the stats and see if it makes a significant difference to efficiency or minimum Vin.

()blivion said:
(re cap across R1) ...But as I stated above under the section "Decreasing switching times", Q1 switching OFF time doesn't seem to want to go under ~400ns no matter how strong the base drive is. I think this capacitor is only ever going to be useful for either lowering quiescent current, and/or effecting the regulation. ...

I think you have hit the inductance issue? There's probably a limit to how fast that transfomer can produce the feedback pulse. I would try increasing or decreasing the turns on the feedback winding... Decreasing might allow the winding to produce that base current pulse faster.

Your chart of regulation operation is cool. That discontinuous mode looks great, and it could be very efficienct for low load current applications. That is a good achievement.

At this point, apart from proving the design through the whole operating range there's not that much needed to improve it? One thing to think about would be some type of RC delay, preferably adding to the OFF period. That would give a forced longer off period, so causing a longer on period to regulate and reducing the frequency. It would also mean greater field strength in the transformer during turnoff so would mean better feedback energy to give harder switching?

(edit) Actually looking at the circuit you could do something very similar to the "cap dumps into cap" thing I did on my 2tran buck regulator. If you put a small cap from Q1 collector to the top of C2 in the killer regulator... When it goes to turn Q1 off the cap will dump energy into C2 so there will then be a longer period where the Q2 is turned on (RC delay caused by C2 and R4). Likewsie it should extend the length of the ON period too.
 
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Finished?

OK, so I think I am just about finished designing this. It looks like I have taken it as far as it can reasonably go. Everything else is up to the specific application, parts available, and person building it. So here is the more or less final schematic...

View attachment 67440

Pros.
Will start and run from 3.2 volts, down to 0.7 volts, with up to 5 Ohms cell ESR.
Uses every last drop of a cells energy, even past 0.7 volts. Reducing waste.
Can supply 500mW of usable power under optimal conditions.
Has greater than 95% efficiency under most conditions. 98% with careful tuning.
Uses the most common of parts. And not very many of them.
Is fairly small and light. The inductor is the largest part.
Has good regulation, and acceptable ripple for many things.
All operational parameters can be 100% adjusted for almost any need or condition.
Simple enough to be built by the novice with high likely hood of success.
Virtually no possibility for electrocution.
Should be more ESD and EMP safe than typical IC based DC-DC converters... LOL.
Has natural reverse battery protection... I think ???

Cons.
Doesn't like to start with high load and low input power.
Can become inefficient when running under non optimal conditions.
Doesn't have any safeties. No over current, over temperature, or under voltage.
Need to run at low frequency's means bigger inductor, and more likely to make noise.
Needs a some what odd transformer/inductor. Where winding phase is important.
Transformer inductance is important, and not easy for the average person to measure.
Can really only use BJTs, where MOSFETs could yield better performance and power.
Can't have great efficiency, power, low ripple, and great regulation all at the same time.
Possibly for over volt on Vout if the regulator section fails. Multiple points of failure.
Miniaturization is limited by the transformer. 15x15x10mm seems like the practical limit.
Can't be made to work well with voltages very close to or higher than target Vout.
New design, not 100% tested or proven. Inaccuracy's could pop up without warning.

Miscellaneous notes:
(1) I have tried all sorts of ideas to improve switching OFF time, nothing reasonable will take it below ~400ns. Thus the only viable option still seems to be keeping the frequency low. The current parts minimize frequency during most normal use conditions.

(2) The quiescent (waste) current that the system draws during oscillator shutdown mode is almost entirely dominated by the DC current through R1, and Q2. Increasing impedance of this path increases efficiency, but lowers max power.

(3) The parts where chosen to give the best balance of efficiency and good regulation, at the cost of a frequency closer to the audio and higher ripple on Vout. Increasing R1 and Vout filter cap will have the opposite effect. More filter stages can also be added.

(4) Any transistors will work for average use. Even small signal transistors with low-ish gain switch acceptably well under most normal conditions. High performance switching transistors are preferred.

Anyway, I think it's ready for making an article for entry into the circuit section now. If there are any objections, let me know. And thanks for your input.
-()blivion
 
Nice going there! Are you going to show some charts of Vin or Iout regulation or efficiency etc? They are nice to have on a SMPS project.

Also some info on your inductor type like inductance spec, and a photo of the toroid etc to give people an idea of the size to look for and roughly how many turns of X gauge wire they need etc. Also the size of recommended inductor will help to finalise a PCB.

If you don't want to do charts then I'll try and get some time over the next few days to put one together and chart it. :)

(edit) As a suggestion, you could preplace the zener with a lower voltage zener and a forward biased LED (high brightness type). Then when the output is at full voltage and the voltage regulator is working the LED will light, showing the boost is operational. The LED should not hurt efficiency as it uses pre-existing current.

Considering there may be some risk of the circuit not starting up (especially if it's using a bad battery) having a "boost ok" LED would be a nice touch.
 
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Nice going there! Are you going to show some charts of Vin or Iout regulation or efficiency etc? They are nice to have on a SMPS project.

I would like to, but I'm not sure exactly how to go about it. The simulation differs from reality, and I don't have a real good way to chart certain aspects of a real world device. My Ampere meter is too flimsy and would interfere with the measurement too much to be considered reliable. So I would have to calculate based on impedance, which is what I have been doing so far.

Just from simple observation, Vin appears to have almost no effect on Vout over the entire range until around 0.8 Volts. Iin is about the same story unless both Iin and Vin are some what low at the same time. Bottom line, when it comes to line regulation, as long as the total power is available on the input, and the load doesn't change the circuit will boost to target Vout ± 0.05 Volts, over all of Vin.

The load and output current is a different story though. Just doing a quick and dirty test with two mostly dead batteries in series, I found that there is only a 0.1 Volt change in output voltage from no load, all the way down to 470 Ohms Load. Then going from 470 to 235 Ohms, it dropped by about 0.5 volts. And even farther with any more load. So... when device load exceeds a certain point, there is a knee that it will cross and the Vout regulation will drop significantly. More input power will extend this knee for larger and larger loads. At some point, you need to use a bigger inductor and or better transistors to extend the knee any farther. But this is for power levels at or above 150mW, and I expect most DIY battery powered devices will not draw more than this on average anyway. Otherwise, you will need to accept larger losses from R1 and Q2.

As for the minor drift of 0.1 Volts, this is most likely because the zener doesn't break down exactly perfectly with so little current going through it. This could be fixed by using a TL431 or similar, as they have nearly perfect breakdown. One other thing, zeners are rated based on a certain bias current, usually around 1mA. This circuit doesn't use zeners in the same way as would be normal though, the current needed to trip Q2 into regulation is usually much much smaller than this bias current. So the accuracy of your zeners breakdown voltage may be compromised when using it for this circuit. I'm using a 4.7 Volt zeners that makes the output 5.5 Volts for my actual device. Careful tuning of Q2 base resistors can get you exactly 5 Volts if you want perfection. But your also going to want to do some other things and they are going to hurt efficiency and power in the end.

Also some info on your inductor type like inductance spec, and a photo of the toroid etc to give people an idea of the size to look for and roughly how many turns of X gauge wire they need etc. Also the size of recommended inductor will help to finalise a PCB.

I don't really have any set inductance specs. I don't have an inductance meter so I just go by trial and error. I have used all kinds of cores from a tinny ferrite bead that probably have only a few uH to quarter sized toroids, and all sorts in between. So pretty much anything goes. If I had to put numbers to the inductor, I would say make it more than 100uH/use more than 20 turns. Ceramic cores are better than laminated iron cores. I will probably put up some pictures when I finally make an article for this project.

As for the wire, it only needs to handle 1 amp. But using multiple strands of finer gauge enamel coated magnet wire could improve high frequency efficiency by reducing copper loss. That's assuming your go over 100Khz, otherwise it doesn't matter, just needs to handle the current.

If you don't want to do charts then I'll try and get some time over the next few days to put one together and chart it. :)

You would probably be better at it than I am. For stuff like this I'm more of a whatever works trial and error kind of guy.

(edit) As a suggestion, you could preplace the zener with a lower voltage zener and a forward biased LED (high brightness type). Then when the output is at full voltage and the voltage regulator is working the LED will light, showing the boost is operational. The LED should not hurt efficiency as it uses pre-existing current.

Considering there may be some risk of the circuit not starting up (especially if it's using a bad battery) having a "boost ok" LED would be a nice touch.

OK, tried it. Works and is still reasonably well regulated. here is a picture.

View attachment 67495

Oddly the LEDs get brighter when there is a load. You would think it would be the other way around. oh well.

Anyway, this circuit still needs proven, someone with real instruments needs to build and measure it properly. Then we can have a better idea of what it's actually going to be like with real parts. All I know is the one I built works reasonable well. And the simulation makes it seem like it's extremely efficient. It will be up to the particular builders wants and skills to be able to make it fit any particular goal. It's a rather plastic design from what I have seen.
 
Thanks for the photo, stuff like that helps a lot. I can see now the rough toroid size anyway. And WOW that's an old looking breadboard! I have not seen that style since the 70's. Nice layout too, and congrats on using two LEDs as the zener as an easy solution because everyone has LEDs but not many hobby people have a good selection of zener values at hand. :)

I don't have good instruments by lab standards but have a few decent multimeters and can run up a couple of charts for you. I'm pretty busy for the next few days anyway so I'll wait for your project page and have a look in then.
 
I can see now the rough toroid size anyway.

That toroid is well and away over sized actually. It can be substituted for something much much smaller.

And WOW that's an old looking breadboard! I have not seen that style since the 70's.

Yeah, the thing is basically as old as I am.

I'm pretty busy for the next few days anyway so I'll wait for your project page and have a look in then.

I'm also somewhat busy right now. Should be for at least a little while before I post this circuit as an article. I probably won't stop making the odd post or two though when I "need" to.
 
Well... What was supposed to be only a few days turned into a few weeks. But I'm back now and ready to end this.

I'm creating an article from this as an addition to the circuits page right now. There is not much else to be done or said that hasn't been covered here. So I'll probably just pick through this thread and copy past all the relevant bits to the article and post a link back to here. Seems like the best way to go.

If anyone would like to build one of these and scope it, or as RB suggests chart out the Load-Line regulation, please do post here so we can add it. It would be very useful information as I only have guesses as to how efficient it is and limited equipment to actually prove it.

The most important thing is turn off time of Q1. But any tests at all would be appreciated. Thanks.
 
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()blivion,

I was going to do your line and load sims, but for some reason I can't get it to run. Could you post your asc file so I can fix it?
 
I was going to do your line and load sims, but for some reason I can't get it to run. Could you post your asc file so I can fix it?

Oh! Yeah... I definitely should do this, Here you go.
 
Just randomly came across this topic on google after trying to create my own regulating 5V joule thief. I'm curious, how did the design go? Did you ever post a picture of the schematic? I don't think I saw one on any of the pages...
 
Very cool, thanks! The design I played around with happens to be almost identical to this one! Except this one looks a good deal better. I am curious though, in mine, the winding ratio of the transformer is 5 to 1 rather than 1 to 1. ( 5 on the base, 1 on the collector). I have seen so far that a step up there allows for R1 to be a higher value, effectively lowering the quiescent current through R1 when the oscillations have been turned off by Q1. Also, it usually allows for a lower startup voltage. I am curious if anyone here has tested that?
 
I am curious if anyone here has tested that?

I simulated various kinds of turn ratios, nothing really worked much better than 1:1, even though theory would suggest otherwise. If the turn ratio is high on the base, with the effect of letting you raise R1, then what happens is you end up having really bad start up performance at low cell voltages. I played with ideas to get around it like putting a speed up cap across the resistor R1 to offset this, but it had little effect. Lower base-emitter voltage drop would fix this, but that would also require "exotic" transistors, which would go against the design methodology of the circuit.

If you figure anything interesting out, I would be glad to add it to the circuits page and give credit to you. I just got burnt out on playing with it, and as far as I could test it worked as described, so I considered it mission complete. But that doesn't mean there aren't improvements that can be made. So if you want to, feel free to go nuts with the base circuit.
 
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I simulated various kinds of turn ratios, nothing really worked much better than 1:1, even though theory would suggest otherwise. If the turn ratio is high on the base, with the effect of letting you raise R1, then what happens is you end up having really bad start up performance at low cell voltages. I played with ideas to get around it like putting a speed up cap across the resistor R1 to offset this, but it had little effect. Lower base-emitter voltage drop would fix this, but that would also require "exotic" transistors, which would go against the design methodology of the circuit.

If you figure anything interesting out, I would be glad to add it to the circuits page and give credit to you. I just got burnt out on playing with it, and as far as I could test it worked as described, so I considered it mission complete. But that doesn't mean there aren't improvements that can be made. So if you want to, feel free to go nuts with the base circuit.

I'm curious why reality departs from theory in that aspect. Are the simulators just not accurately programmed or are they missing something? I've been changing some circuits around vigorously in several simulators and they all seem to suggest putting a step up on the base would help alot. I posted the results over on this forum.

In LTSpice, I saw power gains anywhere from 200% on up, increasing higher and higher with lower start up voltages.

For example:

**broken link removed**, vs. circuit with step up.

Gave an increase from 8.75mW up to 52.5mW, a 600% increase in power on the load.

Or this example, with an even lower start up of 0.7V

**broken link removed**, vs. circuit with step up

So that one gave a gigantic increase, from .55mW up to 19.25mW, a 3500% inrease.


Then, I replaced the 2n3904 with a ZTX1048A, and saw even more power gains. From that first example where it went from 8.75 to 52.5, with a ZTX it went from 52.5mW up to 156.25mW

That does seem kind of nuts and too good to be true.

Anyways, those circuits were comparing step down vs step up rather than 1:1 to step up because someone over there thought step down was the way to go, which definitely doesn't seem true at all.
 
Sorry for the double post, but I can't seem to edit my original.

Also ()blivion, I found this comment on another thread that led me to believe a step up would definitely help.

"I’ve been playing when them too, and found that if you have more turns on the coil going to the BJT’s base, you can run the battery to a lower voltage, I’ve been able to push it down to a bit more then 0.2v before the LED stopped glowing (~20 turns on the LED, 100+ turns to the base, 2N2222 BJT)."

I guess I will just have to play around with stuff and see what happens
 
Sure.

Here is the circuit from the article. I consider this the "finished product".
Finalized.jpg

Here is something similar to the above, only with a slightly different regulator design. The problem here is you can get excessive base current when you significantly over shoot regulation. And without the base resistor it doesn't really preform any better.
Better practice.png

Here is a design variation with an extra resistor on the regulation transistor, the idea was to try and lower quiescent current when in shutdown. It didn't really work out very well as it would just waste more current in other places and balance out.
Sep 18 schematic.png

Finally, here is a design where instead of shunting current away from Q1 base, I tried to block it. This would (in theory) provide the lowest possible quiescent current. I wasn't able to get this to work respectably though, input regulation was far to unstable. If the voltage reference was on the input, and a voltage divider from the output were used, or if the transistors were reconfigured in some interesting way, then it might work better.

Better theory.png

In the end, if it could somehow be made to work, the above would be the best way to go. It should have a quiescent current of nearly zero.
 
"Here is the schematic of this version..."


()blivion;
Is it my browser who can't display it, or is this -for now- only an empty placeholder?

Edit:
After posting this message, the images appeared. Please disregard......
 
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