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| 04 May 2008 01:43:58 |
| pulsing leds |
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 |
| 04 May 2008 06:46:29 |
| Stephen J. Rush |
| Re: pulsing leds |
On Sun, 04 May 2008 01:43:58 -0500, entropy429 wrote: > 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. |
| 04 May 2008 09:15:07 |
| default |
| Re: pulsing leds |
On Sun, 04 May 2008 01:43:58 -0500, entropy429@yahoo.com wrote: >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 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 . . . -- ----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.pronews.comThe #1 Newsgroup Service in the World! >100,000 Newsgroups ---= - Total Privacy via Encryption =--- |
| 04 May 2008 10:31:03 |
| Michael A. Terrell |
| Re: pulsing leds |
entropy429@yahoo.com wrote: > > 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 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 |
| 04 May 2008 11:53:17 |
| Richard Seriani |
| Re: pulsing leds |
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 "Michael A. Terrell" <mike.terrell@earthlink.net > wrote in message news:481DC8A7.E856AEA0@earthlink.net... > entropy429@yahoo.com wrote: >> >> 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 > > > 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 |
| 04 May 2008 13:00:39 |
| Michael A. Terrell |
| Re: pulsing leds |
Richard Seriani wrote: > > 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. A function generator is a distinct class of signal generator. That is what I was asking. > I have used many others over the past too many years and I don't recall > having problems driving any small diode, including LEDs. RF and most audio signal generators generally don't have enough ouyput to drive to light a LED. -- http://improve-usenet.org/index.html Use any search engine other than Google till they stop polluting USENET with porn and junk commercial SPAM If you have broadband, your ISP may have a NNTP news server included in your account: http://www.usenettools.net/ISP.htm |
| 05 May 2008 14:13:13 |
| Re: pulsing leds |
I tried a blue led and a 147 ohm resistor and seem to be getting a consistent signal. I'm using a instek function generator,I'm just suprised there is no max current info. jf n Sun, 04 May 2008 13:00:39 -0400, "Michael A. Terrell" <mike.terrell@earthlink.net > wrote: > >Richard Seriani wrote: >> >> 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. > > > A function generator is a distinct class of signal generator. That is >what I was asking. > > >> I have used many others over the past too many years and I don't recall >> having problems driving any small diode, including LEDs. > > > RF and most audio signal generators generally don't have enough >ouyput to drive to light a LED. |
| 05 May 2008 22:00:56 |
| Michael A. Terrell |
| Re: pulsing leds |
jfisher864@comcast.net wrote: > > I tried a blue led and a 147 ohm resistor and seem to be getting a > consistent signal. I'm using a instek function generator,I'm just > suprised there is no max current info. jf Measure it. -- http://improve-usenet.org/index.html Use any search engine other than Google till they stop polluting USENET with porn and junk commercial SPAM If you have broadband, your ISP may have a NNTP news server included in your account: http://www.usenettools.net/ISP.htm |