Is there a upper limit as to how fast you can pulse an led say a ordinary red led 2v v forward,20mA current. I'm try to drive it with a signal generator,and pick up the flashes with a sensor..I went through the book that came with my generator and there isn't a word in there about a maximum current caution with the generator I'm using a current resistor but don't know if I'm exceeding the max.the led flashes erratically..Any help? thanks jf
If you don't apply more than the safe continuous current, I don't see how the pulse rate makes a difference. The current limt is set by heat dissipation, so the continuous case is the worst case. Pulse rate and duty cycle become important only if you use more than the maximum continuous current, which is often done to get higher peak brightness in applications like infrared remote controls, where the higher peak intensity improves the signal-to-noise ratio. The noise, in this case, is mostly IR from sunlight and incandescent lamps.
There's going to be an upper limit to how fast any diode switches and it will depend on the signal you drive it with among other things. Is this just a straight signal or modulated carrier? Square wave or something else? Duty cycle?
What you don't say is how fast you are switching - Radio or Audio ranges? What is the output of the sig gen- voltage and impedance? What is the wave shape? etc..
The average remote control for TV's uses a pulse repetition rate of ~40 KHZ, uses infrared diodes (generally rated at 100 milliamps of drive with a higher pulse current lower duty cycle). I'm reading about some 200 KHZ remotes too.
The store bought detectors for that type of signal already incorporate a lot of electronics since they are detecting the modulation of a carrier - the carrier is working at 40KHZ but the intelligence is working at a lower frequency AND it only couples an AC modulated signal to the output. Turn on the 40KHZ and you'd get a spike on the output turn off and get another but to use one for object detection you need to modulate it with a second low frequency to get a continuous output.
Often it is the detector speed that limits performance not the LED. Photo transistors = slow. Photo diodes = fast. Heavy conduction in the detector transistor slows the speed further. To raise speeds of communications a digital signal won't switch the LED on and off but is used to modulate the conduction. Read up on optical couplers to learn more about conduction detectors and speed.
Erratically? - something else is going on. Do you see it visually or in the detected signal? If it is the detected signal - it can be lots of things like ambient light sources interfering, noise caused by alignment or mounting problems etc..
If you can visually see erratic flashes - I'd check to see what the drive signal looks like with a scope. Over driven leds will usually just die if the current is high enough or their lives are shortened but should just look like they are working. Or you could be overloading the sig gen.
If you are using a detector designed for TV remote control - you have to match both the carrier and modulation to get a good signal out.
No telling what a led with a built in flashing circuit would do with pulse drive . . .
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What kind of signal generator are you using? Most don't put out 2 volts, or 20 mA.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
Michael,
Maybe I misunderstood your comment. Where you speaking about a specific line of signal generators?
I have two different signal/function generators here at home (old CREI home-built and BK 4040) and two more at school (Insterstate F34 and HP
33120A), none of which have any problem driving an LED/current limiting resistor (typically 220 ohms, or more) combination. The HP and the BK are rated at 10Vpp into a 50-ohm load and 20Vpp into an open (or 600-ohm) load. Even the old F34 is rated at greater than 2Vpp, depending on waveform, impedance, and whether or not symmetrical about zero.I have used many others over the past too many years and I don't recall having problems driving any small diode, including LEDs.
I did just test the two here at home to be sure the specs weren't mis-typed.
Me too - 20 years worth - retired 1986
Richard
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