While playing around with a PIC and some LEDs, I was observing some rise/fall times on my scope. With a traditional i/o pin -> resistor -> LED -> Vss setup, I was PWM'ng an LED with a 25% duty cycle at about 60Hz. The rise time (10-90%) at the i/o pin was a bit under 5nS. However, the rise time at the LED/resistor junction was much slower at around 70nS. I'm thinking this rise is tied to the rise in Tj as the LED comes on. Is the brightness ramping up in direct proportion to the slower rise time of the voltage accross the LED, or is inductance in the resistor slowing things down?
I would expect that it's the capacitance of the _forward_ biased junction... huge, much larger than predicted by the classic diode junction model. I implemented such a modified model to GenRad's simulator around 1980 (Fortran :-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Hmmm, ok then. I'm using a 1k resistor, a 5V Vcc and the LED's Vf peaks at just under 3V. Does an extra 65nS sound right for that? None of this is indicative of the LED's output? Is it possible that Vf could peak before the brightness?
Yeah, I've done some Fortran, but very little and quite a while back (~1980) Did scads of COBOL back then though.
Presuming the 65ns is the time constant, that's 65pF, not an outrageous number... but it's probably time to peak, so capacitance is even smaller. Why don't you measure capacitance versus forward drop... you'll need a high-grade bridge to do it. You will find that capacitance rises sharply with forward bias, then drops abruptly as junction current rises.
Fortran and FAP are the only languages I actually have any formal training in... summer of 1960 ;-)
I took a community college course in Pascal around 1990, but it didn't really stick... I was constantly fighting the instructor, who took umbrage with my use of flow-charting.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...
Might take a look (in your abundant free time) at an interpreted Language like Python. Freely available and there are Scientific Libraries so you don't have to re-code the wheel. Not going through the Compile-Link-Run cycle sure speeds things up.
Pascal is tedious. By the time you finish all the declarations, you're too tired to write the executables.
The best programmers I know never took any programming courses.
The slowest PowerBasic compiles I've seen lately are 0.2 seconds, but most are 0.1. By the time you take your finger off the F9 key, the app is running. And it will run useful 30 MHz FOR loops, something interpreters may not do.
I forgot to mention that the fall time at the same junction was about 250nS. The PIC actively drives low with the same "force" as it does when going high.
I was just poking around, noticed the difference in rise times and was curious as to why. I'm more curious as to how fast the LED can ramp up its brightness.
The other day, I was tinkering with an IR photodiode and various remote controls lying about. I had the photodiode reverse biased with 5V against a
1M resistor. IIUC, photodiode response is very fast so I take it that the rise time I see on the scope should pretty much resemble the actual brightness increase as the LED lights up on each pulse. IIRC, the output from the photodiode indicated that the brightness was ramping up over uS not nS of time, but I'll have to recheck that later. I thought LEDs were faster than that.
I can't seem to find a loose IR LED lying around so the above mentioned Vf tests were done with a blue LED. Of course I can't use the IR photodiode with them to compare the Vf rise against the output from a photodiode to see how much correlation (if any) there is. I'll keep looking for an IR LED, I know I have one here somewhere. Perhaps the remote control has some capacitance accross the LED slowing it down.
Formal training????? What's that? ;-)
You're not missing anything, nobody uses PASCAL for anything real. Real work is done with assembler. ;-) I never was much on flowcharts myself, way too much work.
As far as I can tell, Python usually compiles (the terminology is often "compiles") to virtual machine byte code. That's better than actual interpreting, but is still a lot slower than machine instructions. A lot of our stuff is realtime, for test sets and such, so we want fast execution and bare-metal hardware access, which is not the classic "computing" environment.
A lot of Pascal was like that, compiling for a fictitious P-machine. Microsoft did this for PDS Basic and I think still does for Visual Basic. Java works like this too, the advantage being that the runtime system can seriously protect the OS from the code if the code is not truly executable.
The rise is just a portion of the rc exponential... the diode clamps at a volt or so out of a 5-volt charging curve. On the fall, you see the full exponential, all the way down to zero.
That will *very* slow. Dump it into 50 ohms or so.
Depending on photodiode area, expect from 1pF to 1000pF or more, even reverse biased. It's actually quite common to have time constants in the 10s of microseconds with a 1 MegOhm load. It's extremely easy to check: use 500K and see if things speed up a factor or 2. If so, it's all about your photodiode load. Paul Mathews
That glorious F9! Wrote a PB prog last night for a customer. It looks for exact matches of contiguous code segments in ROM dumps from two different products. All found code runs then stored in a third file plus some other statistical info accrued during the searching. From sitting down it took a whopping 2 hours and 60 lines. My last 'real' PB prog was over a year ago but being Basic it takes little time to come up to speed. A quick check on the "binary file open" syntax and I was in. A few dozen F9's later and voila!, finished. A far, far cry from the C compiler I watched last week, that chuntered away to itself for an eternity as it compiled and linked and did other strange stuff that only "modern" compilers can and must do. Difference is, I'm looking for a result and not to secure my career.
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The blue LED is clamping the voltage at just under 3V max with a couple of mA of current. When the PIC pin is brought low, it seems to me that I should see the 3V fall to zero in the same amount of time. In fact, I see the 3V drop to 0V in about 3-4 times the amount of time. This is why I thought that I might be looking at heating/cooling factors.
This seems odd to me and doesn't really match my experiments. I have the diode on top of the resistor with the diode reverse biased by the 5V supply. At the junction of the photodiode and resistor, I am taking my measurements. Under ambient conditions here in the house, I get an "idle" voltage of about
60mV. When the remote control is activated, I get pulses that peak at 5V naturally with the remote control about 1' away.
I don't see why the large resistor would be slowing down the first (rising) edge of the pulse. I can understand that it would slow down the falling edge though since the photodiode needs to bleed the 5V off thru it. I was under the impression that the resistor I chose would set the "gain" of the photodiode and this is indeed what seems to happen. If I lower the resistance to say 100R, the sensitivity plummets to the point that I can barely get 100mV out of it and that is with the remote control jammed against the photodiode. The rise time is even worse than before. BTW, I was actually using a 100K not a 1M resistor with the photodiode. A 1M resistor increases the gain of the photodiode substantially and results in an even faster response, even though the voltage peaks are much higher. It also increases the "idle" voltage to about 300mV at the junction of the photodiode and the 1M resistor supporting the bigger resistor = bigger gain theory.
I did some more basic tinkering and was surprised to see what the rise and fall times for an unterminated 1K resistor hanging on the PIC pin was compared to what you see at the pin (
With the 1K resistor, easily. With the 100R resistor, not legally. ;-)
Just so everything is clear, I got two things going on but each one has interesting aspects.
1) A PIC driving an LED at 25% duty cycle for detection by an IR photodiode. I noticed a slower voltage drop rise across the LED vs. the actual rise time of the driving pin. That seems to mostly be due to the actual resistor itself and not the LED so it's quickly losing its appeal.
2) IR photodiode "on top of" a 1M resistor to ground. The photodiode is reverse biased so that I'm using it in "photoconductive" vs. photovoltaic mode.
I'm trying to determine how fast an IR LED illuminates vs. the rise in current flow thru the LED. IOW, trying to measure the lag, if any. I understand that LEDs can be quite fast in response and I was trying to see if I could actually measure the lag/rise-time in response of different IR LEDs. So far it's not working out too well. Either the LEDs illuminate much slower than the current flow or my photo-diode is not nearly fast enough to measure.
A reasonable first guess, perhaps, but under closer scrutiny not well explaining the scene. One clue to resolve the issue: if the LED light emission is filtered with a narrowband filter at the spec'd wavelength, appropriate to the bandgap of the LED junction, the desired very fast light risetime and falltime is observed. All the "slow light" is at longer wavelengths. Unfortunately, this slow light is often the source of a significant fraction of the LED's overall emission.
Naaah! The "slow" risetime is "at the LED/resistor junction"... an electrical issue, not a "slow light" issue.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Since this is just an experiment versus real life, i.e. you could have a really inefficient circuit, why not experiment with a current source feeding the LED, with pulsing done by shunting the current with a N- fet. LEDs are really current devices, and the pic output plus resistor isn't a very ideal current source. To be a bit more specific, get a bench supply of say 20V (pretty common). If you want 20ma in the LED, then use a 1kohm resistor in series with the LED. The mosfet source and LED cathode go to ground. The mosdet drain goes to the anode of the diode. Drive the gate with the pic.
With a current source, you can estimate the capacitance directly from the rise time.
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