I imagine in my ignorance that the transistor heats up most when it's half-on (still has some internal resistance), is that right? Slow switching means it spends more time in that zone so it heats up more...?
No idea, sorry. I pulled it out of a junker PC.
Beam spread is ok (might even be better for what I'm doing...)
That's the nature of the millicandella rating though. It is easy to make a poor performer sound good simply by narrowing the focus of the spot of light.
The same total flux of light from a led, is brighter when the light energy is concentrated in a smaller beam. The led chip and total flux output can be the same, and the manufacturer just offers different beam angles by changing the shape of the lens.
You, no doubt, have an idea of the size of the area you want to illuminate. You select based on the distance from the source and illumination needed and choose the angle and candela output to match.
I have some 25 leds up on a pole illuminating a ramp I use to load my kayak in the dark - narrow angle, high output leds work better for that application.
I don't think so. I can do the calculations, and achieve accurate results from them, without taking saturation into account. I'll try and give a more detailed account than I have, if interested.
Jon
"Breaking down" and going bzzzzt internally because the voltage is too much. It doesn't make the transistor better. ;)
Jon
Obviously...
So you think the LEDs this guy sells are just narrower focus than the ones I have, ie. total number of photons is approx. the same for all the 2.0V @ 20mA LEDs out there?
Okay. Here's another shot at the general idea. I'll write this in ASCII characters. You'll need to use a fixed-spaced font to read it, such as Courier or Courier New or Prestige Elite. Notepad will do that, so will many newsgroup readers. Even google can do it, if you look around for the option on the page.
I'll assume 6 LEDs and 3 batteries:
Tweek your transformer to get in the vacinity of 20-50kHz. The LED current will vary somewhat less, closer to 50kHz, but down at 20kHz it should look fine, as well. C1 should be at least 35V, but 50V is safer.
The value for R1 can be as you see fit. I'd stay at or above 1k and you can try it upwards of 3.3k. I tweeked things so that 2.7k appears to work fine.
LED current will definitely droop as the battery voltages also droop. Nothing in there to help that problem. As battery voltage goes down, so does base current. Lower base current, lower peak collector current. Lower peak collector current, lower energy stored and transferred. Etc. But I think it will move down by the ratio of voltages, so no worse than 1.1/1.5 or about 3/4ths.
What you need to add is C1, R2 (not strictly necessary, I just added it so you could play with some values there), and D8. Some folks may recommend a schottky diode for that, because they present a somewhat lower voltage and they switch fast. But probably you can use what you have available.
Oh, and D7!! I added that to protect your Q1 base-emitter from reverse voltages. That will help protect your poor BJT.
If you try this, let me know what happens!
Jon
Tweak!
Jon
If you read 'default' more closely, you'll see him write, "They also have some impressive mcd numbers ALONG with fairly large beam spreads." Note that he includes "___ALONG with___" here and describes the idea of large beams. So, actually, I think 'default' is saying that these seem to be pretty impressive not just for having high numbers but also for having those numbers while also having wide spreads.
So to the opposite, I think. I don't think he thinks they get their numbers from a narrower focus. He feels pretty positive, I gathered.
Jon
I think you _are_ mistaken.
No. Saturation of the core isn't required. This would work on an air core transformer, I believe.
....
The base current starts out unaided by any induced voltage on the base winding, but still turns the BJT on. Once that happens, though, the collector drops immediately to near zero (there is no collector winding current to speak of, at that point.) This places a near-fixed voltage across the collector winding, which allows the collector current to rise according to V/L. Almost from the first moment this takes place, there is an induced voltage in the base winding due to the rate of flux change on the collector winding which adds to the battery voltage (if wired with the correct orientation, of course) and this increases the base current to it's initially 'highish' value of (2*V_bat-Vbe)/R.
At first, the Vce of the BJT remains very close to zero because the Ic/Ib is well below 10. (The BJT is severely saturated.) But as Ic rises along the ramp of V/L, while at the same time Ib remains close to fixed, it eventually reaches the point where Ic/Ib goes over 1, then goes over 10, then goes over 20, etc. During this time, the Vce rises, too. As that happens, the induced voltage on the base winding declines due to a falling rate of flux change. That reduces the base current, but does so exactly at the point when higher Ic requires more, not less, Ib. In other words, the aiding voltage by the base winding is falling and _reducing_ base current right at the point where Vce is rising and reducing the voltage across the collector winding.
Suddenly, the beta just isn't enough and the BJT attempts to reduce Ic. As soon as it 'tries' to do that, though, the collector winding immediately responds by reversing its polarity as the only possible response to allow a reduction in Ic (V/L must reverse its sign.) But that immediately causes the base winding to also reverse its polarity and __oppose__ the battery voltage that is struggling to drive current into the base. The whole thing collapses with the battery voltage opposed by an overwhelming reverse polarity, base current goes to zero, base voltage goes below ground, etc. The BJT is off at this point.
After the collector winding reverses voltage, current is driven now through the LED as the reversed-sign V/L allows the current through it to decline along a ramp (the LED maintains a fairly fixed, but gradually declining V/L.) At some point, the LED is no longer able to accept much current (non-linear decline, as well) and the collector winding's field entirely collapses and has no energy remaining. It's voltage goes to zero, so does the induced voltage on the base winding, the battery is now able to generate some current into the base of the BJT, the BJT turns back on, a voltage is applied to the collector winding, the collector winding induces a renewed aiding voltage on the base winding, the base current rises a bit, and the whole cycle repeats.
No saturation of a core invoked here.
But I disagree. And, it appears, so does LTSpice where I don't have to add any saturation effects to get it to oscillate just fine.
Hope that helps. Or, if you find good fault with my reasoning, it will help me. Either way, it's all good.
Jon
I added D8 and a big scary electrolytic capacitor* I found in my box at C1 (I don't exactly have a big selection of components to play with and that's the only capacitor which looked likely to do anything...)
The capacitor adds a cool effect - the LEDs fade up/down when I switch the thing on/off.
But ... it works! I think I'm pretty much at full brightness with the capacitor in there.
Looking on the scope, the voltage at the first diode is perfectly flat (to be expected).
Best of all ... I measured the output current and with 4.2V input it was 106mA for six LEDs, that's an average of 17.6mA each - right on target.
With some half-depleted batteries I got 104mA across the LEDs from 3.8V input.
With two half-depleted batteries I got 70mA from 2.5V
- still quite respectable.
I'll be thinking of him...
So far so good... now I want to work on the size of the ferrite bead (I want this to fit in a more discrete package).
[*] Electrolytic capacitors always make me nervous...
I'm definitely interested... :-)
Which leads to another question before I go parts-shopping tomorrow ... what sort of capacitor is best for this? There's a zillion different types.
Electrolytic for cheapness, 10uF or bigger, 35V or higher. I don't think it matters that much, otherwise. You can strip something out of some electronic device (the same one you got the transformer core out of?) Don't shop if you don't have to....
Jon
Yes, that's right. Which means you need to keep the frequency low enough so that the switching time is small compared to the on time.
If you have a scope, you could look at the voltage between the emitter and collector. It should be close to a square wave, with steep slopes on the rising and falling edges, for best efficiency.
What part of the PC? If it was somewhere in the power supply, it's probably okay.
-- Greg
It means that there is a high enough reverse voltage being applied to the base-emitter junction for it to break down like a zener diode, which could damage the transistor.
-- Greg
Interesting -- there appear to be more subtle things going on in this circuit than I thought!
But it seems like saturation would have much the same effect -- the flux suddenly stops rising, causing the base voltage to fall to the point where transistor can't sustain the current and a flip-over occurs.
So the circuit will still work if saturation occurs, just in a slightly different mode.
It might even work slightly better in that mode, if it causes the transistor to cut off more sharply.
In any case, it seems like it would operate more predictably, since the period and maximum current would depend on the saturation flux of the core rather than some rather uncertain transistor parameters.
If you just pick some random transistor, then it's only by luck that you avoid overdriving the LED. Seems like a rather hairy way to design a circuit to me!
Another thought: For driving several LEDs in series, maybe it would help to use a separate secondary winding with more turns for the output. That would give a current step-down relative to the transistor current and allow the output duty cycle to be increased for the same output current.
E.g. suppose you have 4 LEDS in series and want to drive them at 20mA max. If you use a 4:1 turns ratio and arrange things so that the primary charges up to 80mA over an on-time of t, it will then deliver
20mA initially to the secondary, ramping down to 0 over a time of 4t before the stored energy runs out. So the LEDs are driven at a duty cycle of 80%.In contrast, without the current step-down, the peak transistor current has to be limited to the maximum LED current, and the LED duty cycle falls in proportion to the number of LEDs -- which means you can never get more light out of series LEDs than you could from a single LED.
Does any of that make sense?
-- Greg
There's dozens of capacitors within easy reach but they're all soldered to PCBs and only have 1mm legs... :-(
Wasn't from the PSU. I just opened a PSU and pulled a couple of smaller ones out. They're made of blue ceramic or something and they stick a lot harder to a magnet than my big one does. I'll give them a try when I can get some better wire.
PS: Hard disk magnets ... crazy strong. I nearly lost a finger.
No. The leds this guy sells specify beam angle, and has a range of beam widths.
I'm very happy with the tricolor leds I got for a project. I'm using them for power indicators and the wide beam lets me see them from anywhere in the room with any ambient light level.
His price was 60 cents a led versus $2.80 from a US importer of the same part.
Only negative comment I wish he'd link to the actual data sheets for the Leds instead of the abbreviated ones he provides.
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