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

 

[Stephen J. Rush]

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.

 

[default]

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.com The #1 Newsgroup Service in the World! >100,000 Newsgroups
---= - Total Privacy via Encryption =---

 

[Michael A. Terrell]

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

 

[Michael A. Terrell]

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

 

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.

 

[Michael A. Terrell]

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