Transistor as a current limiter

That's been discussed.

Jon

Oh... well.... Then I will most certainly be dead before I could use up a 1Tb purchase.

Jon

Over the years I've downloaded at least 1Tb from alt.binaries.e-book.technical

Very likely I'll be dead before I can read that many books & magazines.

Hmm. Haven't looked at that one. And yes, it looks like one way or another I'll be dead (and still have more capacity to download.)

By the way, I did get the list of currently suppported groups at astraweb located at:

For anyone else wondering like I was.

No information at all on how long they retain things, though.

Jon

--
When you originally stated that you were using a 12V supply and that 
the LED Vf was 2.2V, I - quite naturally, I think - assumed that 
that's what you were working with. 

As a consequence, I posted a circuit which would give you the 
current-limiting and PWM capabilities you were looking for.

--
Au contraire. I've just been on an asshole-free sabbatical. 

Had I not been, I would have taken exception to the latest of your 
idiotic posts on abse where you demonstrated your moronosity by  
stating:  

"SiC  rectifiers are supposed to be all the rage, but I've heard they 
have a large Vf, and I've no idea how much junction capacitance." 

Perusing a data sheet in order to glean that sort of data would seem 
to be de rigueur for someone interested in the art. 

But you? 

Sounds to me like you're the one who's got the rag on.

--
To what avail?

Damn! I had forgotten Ian Field even existed >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Sorry for not responding to your posts earlier. I was a bit overwhelmed by the number of replies to what seemed to be a relatively straightforward query. After the current mirror circuit, which seemed ideal, the point of the additional suggestions seemed lost to me, especially since there was no explication of how they would be superior.

You are suggesting using a resistor as a current limiter, and a transistor as an on/off switch for PWM. This doesn't seem ideal to me. Firstly, it requires multiple components per chain (though there is an obvious variant where there is a single power transistor switching the supply to all the chains). But more importantly, a resistor is a fragile current limiter: the current will depend on fluctuations in the supply voltage and the I-V curves of the leds (which, I hear, are prone to individual variations). And if a single led shorts, the current to the others will increase, decreasing their lifetime as well. I doubt these issues will be very relevant for my toy project, but I want to learn to do things properly.

The reason I asked about using a _transistor_ as a current limiter is that transistors are mostly immune to such fluctuations at the collector, and only their (relatively low) base voltage needs to be well-regulated. This seems superior to using a resistor, and there is the added advantage that the same transistor can also be controlled to provide e.g. PWM.

However, I recently learned that PWM is not actually very efficient with leds: it seems that leds provide more light per watt when run at constant low currents than when pulsed at a high current. PWM, though, provides better linearity and stable color. But even if I want to use adjustable current instead of PWM, the current mirror still works: just attach a voltage-controlled current source to the mirror. So I think I'm going to stick with that one.

But thanks for your suggestions, regardless.

Lauri

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Trust JF to stir things up!

Knowing what a shit stirrer you are, I'm in no great hurry to provide you with ammunition.

--
Everything was nice and cordial until you showed up and started 
flaming for no reason at all but to throw some shit into the game. 

You're a mean, reprehensible person who's only goal is to toot his own 
horn no matter what the cost.

--
Then just shut the fuck up altogether.

Run along now - it must be time for your hourly crack pipe.

Your statement here needs clarification. It's not as nuanced as it probably should be.

PWM is simply a method of using a duty cycle (from 0% to

100%) to adjust the apparent intensity. It is not necessarily the case that it is less efficient with LEDs. It may be. But not necessarily.

Let's say you are using PWM to adjust the brightness of a single LED. Normally, at 100% duty cycle the LED is at "normal and desired" brightness. Now you use PWM to reduce this intensity. In this case, quite to the opposite of your conclusion, PWM is actually a MORE efficient method than others. In this case, your nominal power required is 100% when operating at full brightness and will correspond to the duty cycle % when operating at other brightness levels. So operating at 10% duty cycle will require 10% of the power. Other methods would dissipate 90% elsewhere so that 10% would be dissipated in the LED and would be HORRIBLE, by comparison. So you would WANT to use PWM, here, to save power.

But let's say you are using PWM because you are multiplexing a complex LED display. So, here, let's say you are multiplexing by a factor of 5 because you have 5 columns (or rows -- pick your terminology) to operate. In order to achieve a "nominal 100% brightness" in a column, you must drive it at 5 times the nominal current. So if the current is

20mA, nominally, you need to run them at 100mA, but at 20% duty cycle. The other columns will also be operated at 20% for their nominal brightness level. To adjust their brightness from 0% to 100%, you would PWM them from 0% to 20%. In this case, because the LED voltage will be higher at 5X the nominal current, the power dissipated will be more than just 5X nominal. But the brightness is determined by the current, not the power. So in this case one could argue that PWM wastes some power that, had the LEDs had individual drivers and weren't muxed instead, would be less for the same effect. But you pay this price because of the convenience and reduced cost.

Keep in mind that as far as human perceptions go, so long as the repetition rate is high enough that the brain cannot follow it, the brightness perceived will be based upon the average (integral) of the incident light flux. It will not depend upon the pulse intensity. (There are other factors, such as the spatial size and spatial frequency, position on or off axis of the eye, and the surrounding intensities nearby that also affect perception... but let's keep this focused.) If you reduce the repetition rate, then at some point the brain starts to perceive the peak pulse and will show a change in brightness perception, along with it. But it's also confounded by the fact that the pulse is being noticed as well and that usually isn't desired. So the best rule of thumb to stick with is that your average current value represents the perceived flux.

The only other rule to keep in mind is that human perception is logarithmic, so halving the average current does NOT reduce the apparent brightness in half. To maintain a constant rate of decline in brightness, you must multiply the duty cycle by a constant factor for each time unit. So, for example:

100% 50% 25% 12.5% 6.25%

Would yield constant changes in apparent brightness. If you achieved those values with fixed time intervals between them, a human would perceive a "smooth" diminuation in brightness that appeared linear.

Regardless, PWM is by itself not necessarily inefficient. And even when you must pay a small price in efficiency because you are multiplexing and have to use higher pulse currents because of that, it's still better than the alternative of paying for individual drives for every LED and not terribly inefficient, anyway. Other alternatives would be either excessively expensive for very little gain, or simply worse.

Of what? Again, note that human perception is logarithmic. So while PWM can easily be controlled linearly, it's not going to be perceived that way if that is how you use it. In fact, PWM would be more usable as a brightness control if the hardware could PWM accurately in a logarithmic way -- it's actually a pain to use PWM correctly for brightness control if you care about human perceptions of relative brightness. It's advantage is that it is just easy to apply, is all.

I guess I should mention something else just to complicate things (and agree with you.)

If you operate a monochromatic (single) LED at differing currents (and, consequently, different voltages to achieve it), then you will get a different wavelength distribution out of it, too. If you use a spectrophotometer you will see the shifts. This also means the "color perception" of a single color LED shifts a little.

So one could argue that keeping the LED current fixed while PWM'ing to achieve brightness shifts is actually achieving "stable color" for these reasons (the peak current remains fixed, just the duty cycle changes.)

But if you are comparing, say, 20mA vs 100mA, then there is also a color shift in doing so. So I take your point that PWM achieves "stable color."

It's what I'd probably do (or use an IC, if I could find one that is likely to exist for a while, is readily available, could handle the dissipation, and is cheap enough.)

Jon

It sort of depends - first of all, many people take total energy for the light output, so energy lost in a series resistor or linear regulator as taking away from efficiency.

The most basic PWM is therefore more efficient than dropping some of the voltage to limit the current.

I've seen various application notes for LED pulse operation with peak pulse current significantly higher than the max average LED current, the total dissipation is not exceeded because of the very low duty cycle - the accepted wisdom is that persistence of vision makes the LED appear to be much brighter than the *average* (calculated) power would suggest.

There is a *don't exceed* current no matter how narrow you make the pulses, so read the appnote carefully before doing.

Yeah, as far as I can tell, all the binaries groups are available. that one certainly is.

--
?? 100% natural 

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

currently almost 5 years

--
?? 100% natural 

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the protocol is basically short ascii commands and clear text messages over TCP, so there's not a lot of overhead.

how much of the messages you ignore (or killfile) get downloaded depends to a large part on the capabilities of your reader and the server.

sci.electronics.design is about 200 megbytes per year of message content (including headers) (I have local retention back to January so I doubled it) 40000 messages each year and perhaps 200 bytes of NNTP overhead on each, for another 8 megabytes, and on top of that TCP overhead, so maybe 220 megabytes per year for S.E.D

--
?? 100% natural 

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