Transistor as a current limiter

not really, the PWM just dissipates more energy in the LED's internal resistance than the other plans. you don't get efficiency gains until you use buck regulator, eg: by adding an inductor and a diode to the PWM circuit.

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Jasen Betts
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I pretty much figured that.

I only download headers via NNTP and then, selectively, download bodies when I care to read them.

Probably lots less than that, then, since most of what I get from SED remains headers-only.

Jon

Reply to
Jon Kirwan

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That's admirable, but I fear you still don't understand the operation 
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Reply to
John Fields

yeah, but does your software use XHDR or HEAD? The former pulls a single header from several messages whereas the latter pulls all headers from a single message.

If your killfiles only use four headers, 4 XHDR requests will be more much more efficient than 100 HEAD requests,

Also headers are often larger than message bodies.

Hmm... I can count that.

jasen@gonzo:/var/spool/news/sci/electronics/design$ find -type f -ctime -183 -exec 'sed' '/^$/,$ d' {} ';' | wc 457382 1757014 29908634 Hbout 30 megabytes of headers in the last 183 days.

total message size: find -type f -ctime -183 -exec cat {} ';' | wc 1567860 8477618 70408901 Hmm, 70 megs, looks like I over-estimated total volume

So 40 megs message body, So the header:body ratio is approximately 3:4

How much is quoted content?

find -type f -ctime -183 -exec cat {} ';' | sed '/^ *>/ p;d' | wc 643846 4338102 24717942 Almost 25 megs, well over half. (ignoring the quotes by those who use non-standard quoting styles, mainly Phil A. and one of the political activists whose handle I don't recall)

so, headers are more almost twice the volume of the new content.

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Reply to
Jasen Betts

Interesting. So I went to this site:

formatting link

where it discusses my reader (Agent.) Apparently, by default, it does not use XHDR... but it can.

I'm still learning about this. And thanks for making me more aware.

I can easily believe that. Lots of message bodies are very short.

Okay. Believable, too. I only examine 3 newsgroups on a daily basis. sci.electronics.basics, sci.electronics.design, and comp.arch.embedded. That's it.

Thanks for the results. It makes it pretty clear that I would have quite a long ride with 1000 gig of metered newsgroups. Even if it were a gig a year for the 3 groups, and if I didn't allow myself to die until it expired, I'd be the new Methuselah.

Even adding a binary group, if I don't download things I don't want, should work okay.

Best way to find out is to buy the tiniest package they offer and see how long it lasts. That will be the benchmark, then. Assuming I live long enough to see even that expire....

Jon

Reply to
Jon Kirwan

Nice link. Using a diff-amp in a controlled feedback loop. Thanks.

I do NOT like the capacitor there. When inactive (OFF), C1 charges up to Vcc across it, if C1 is small enough (if C1 is big, then it never reaches Vcc across it.) When active (ON), the common node between C1 and R4 jumps up towards 1.2V below Vcc and in doing so causes the collector of Q2 to be driven below ground, Vcc-1.2V-Vc1=-1.2V since Vc1=Vcc. As C1 discharges it actually borrows current from Q3's collector (rather than from R3) causing a lower current in R3, yielding a lower voltage potential for Q2's base... leading to diversion of R2's current towards Q1 and thus MORE drive current into Q4 through Q7. The upshot is that upon turn-on, the peak current in the LEDs is actually HIGHER than designed and it then gently settles down to the desired level as C1 discharges to a new (lower) potential across its leads. This is spikey behavior instead of a "soft start." Make C1 bigger and it gets worse, too. The sharp leading edge will reach even higher initial currents in the LEDs that way.

So it's value should be kept small. I'm not yet convinced about the danger of oscillations or the source of them in this circuit. There are very low impedances on every node of Q2. It just doesn't seem necessary to me. I don't see how the small Cbc value and the value of Rb' can conspire to make a problem here, even with the collector making a small jump at turn on. And it actually seems to have a downside where I'd rather simply remove it.

I might also invert the whole structure and delete R1 and D1, if my LED supply was a volt or more above the micro rail voltage. The pin output won't be loaded much and will probably be a very close reflection of the micro rail voltage. Of course, I'd adjust values, accordingly.

For example, if my micro were running on 3.6V and my LED supply were 6V (hypothetically) and I wanted 100mA and 5 chains of them (not 4), then I might invert the polarities and use R3=36 ohms, R2=470 ohms, and R4=270 ohms, kill C1, and just drive the base of Q1 directly.

I'd start the design by estimating the total base currents required for Q3 through Q7 (600mA/beta=200, or about 3mA.) Then I'd double that for the diff-amp pair to 6mA. Given 3.6V drive and an estimated 0.7V drop on Q1, I get 2.9V at Q1's emitter. So 2.9V/6mA is 483 ohms. 470 is a standard value and gives a slight increase in estimated current, so I'd use that value instead. I also want 3.6V across R3 at 100mA, so 36 ohms. Changing that to 33 or 39 ohms would change the LED currents by 10% or more. Luckily, 5% resistors do come in 36 ohms. So I'd stick with that. Then, since R4 should be (assuming 3mA in each branch, hopefully) at about one 100mA sized Vbe (about 110mV added to 700mV as a guess), so R4 should be about 0.81/3mA, or 270 ohms. Nice. Standard value. So that's where I'd get my values.

I'm still stuck trying to understand C1's benefit.

Jon

Reply to
Jon Kirwan

ABSE content in the last 6 months $ find -type f -ctime -183 -exec cat {} ';' | wc -c

11504635

11.5 megs since 14-January.

$10 for 25G, probably enough.

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Reply to
Jasen Betts

It wastes 25% of the supply power just to sense the LED current.

And it depends on transistor matching. I don't see how it avoids thermal runaway by pulsing.

Usual EDN standard of quality. They need filler between the ads.

It probably keeps the loop from oscillating.

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John Larkin

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Read the article; he states why.
Reply to
John Fields

A current mirror wastes current. That's what they do.

All current mirrors depend on matching Is, less with degeneration. From my limited experience, this isn't a big problem for LED driving if using parts from the same reel. And it's pretty easy to test for, anyway.

Yeah. If one of them heats up more than others, it's Vbe declines by 2.3mV/C or something like that and this pulls the others down with it, lowering their Ic. And if the feedback BJT lowers its Ic the feedback circuit supplies more drive current... and so on. I've epoxied parts together with modestly okay effect here. But I've also bought arrays on ICs, too (which at the prices I pay aren't exactly matched well but again aren't terrible, either.)

Degeneration would seem to be a partial answer here, too.

A point in this circuit is low overhead drivers. What would you do if designing a discrete, bipolar solution here? I'd learn something from a good answer.

:)

That's what they say. But I still don't see it, probably because I'm blind and not necessarily because it doesn't do that. It's why I said I can't see it -- hoping someone would explain it in detail to me. If you can see the exact mechanism here, I'd like to see a detailed explanation.

Jon

Reply to
Jon Kirwan

He appears to depend upon a slow enough operation with low enough duty cycles to allow cooling back towards ambient. Saying, "The thermal-runaway effect does not have time to develop."

Yes. As the article states. But as I point out, the effect of a larger valued C1 is to cause the leading edge of the pulse to have a noticeable higher current edge that gradually drops back to the design value. Small values for C1 would have little impact on it. Ahyway, he doesn't mention that behavior and instead says it "prevents transient oscillations" but I don't see how they occur as a result of switching on and off.

Jon

Reply to
Jon Kirwan

anyway

Reply to
Jon Kirwan

I should explain more about my experiences, I suppose.

I've no doubt at all that if one of the BJTs carries a different Vce than others in the chain, and if the currents are enough, thermal runaway just happens. The BJT with the larger Vce is going to have its Vbe decline steadily with some bad news.

But LEDs present fairly consistent Vce's to their driving BJTs. (Usually a very low Vce, in fact, if you are keeping the margins down to minimize wasted power.) The only substantially different Vce is the current mirror BJT that sets the current and is under loop control. It's almost always the case that the control BJT is the very hottest one, more so than any of the LED driver BJTs, since it has the highest Vce.

If the control BJT heats up, then the response by the rest of the system is to have lower Ic's. And since its dissipation is fixed by design (its Vce is set and so is its Ic), it settles at some resulting Vbe and the other BJTs exhibit less Ic as they have a lower Vbe drive, less Vce, and are cooler. So no runaway.

Jon

Reply to
Jon Kirwan

I read the article. It doesn't make sense.

And in-between.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

That's not much different from running DC at the same average current.

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John Larkin                  Highland Technology Inc 
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John Larkin

formatting link

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

So as Q3 heats up, all the LEDs get dimmer.

Ic goes 2:1 with 25 mV of Vbe change. Vbe goes 2.5 mV/deg c. So it won't take many degs of temperature difference to seriously affect LED currents.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

Yeah. But it's not runaway. And besides, it's fixable with ease with a slight design change.

Jon

Reply to
Jon Kirwan

Not an explanation of the oscillation issue, which is what I seek.

Jon

Reply to
Jon Kirwan

To you, perhaps.

He stated - and I paraphrase - that with a low enough duty cycle, the tempco of the switch(es) wouldn't cause runaway.

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>>>>I'm still stuck trying to understand C1's benefit. 
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Reply to
John Fields

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