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LED Formard current

Discussion in 'Electronic Basics' started by GrahamIT, Aug 8, 2004.

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  1. GrahamIT

    GrahamIT Guest

    Think of an optocoupler, e.g. ISP847 from http://www.isocom.com the MAX
    forward LED current as stated in the data sheet is 50mA, but all the current
    transfer tests are done at 5mA except the Vce(sat) which is done at 20mA,
    confusing or what!!!

    The question is, which LED forward current and voltage would be the best
    choice, so that the output photo transistor was switched hard on to full
    saturation but the LED had as long a life as possible?

    I've been under the impression that a transistors base current necessary to
    only "just" saturate the transistor should be increased by a factor of 5 to
    fully saturate. Therefore as 5mA through the LED would only just saturate
    the phototransistor, an LED current of 25mA would fully saturate it, but
    their Vce(sat) current was only 20mA

    If you can make sence of the question, then you're a better man than me :p
     
  2. LED current is not proportional to the base current of the phototransistor.
    5 mA is probably plenty. If the Vce(sat) is specified at 20 mA LED current,
    then use 20 mA.
     
  3. These two requirements need opposite extremes. You need to define how
    much collector current the output must pass, and what voltage drop is
    acceptable. Then using the data sheet, see if you can extrapolate
    what LED current that will require and if it is low enough for
    acceptable LED stability. This last part is hardest to guess from
    most data sheets. But it is safe to assume that operating at the
    absolute maximum LED current is not a good solution and that staying
    at or below the current where most of the specifications are done will
    give a reasonable life.
    The terms 'just saturate' and 'fully saturate' are not very well
    defined. What voltage drop can you tolerate?
    The forward current transfer ration for the ISP847 is more than 50% at
    5 mA LED current and 5 volts across the transistor (not saturated, at
    all). This is 2.5 mA of collector current. The ISP847D guarantees
    300% (15 mA collector current) under the same input and collector
    voltage conditions.

    At saturation (defined as no more than .2 volts collector to emitter,
    and a 1 mA collector current, 20 mA LED current may be needed. This
    represents a forward current transfer ratio of only 5%, a 10 times
    reduction of the linear case. But it is one point only. If you could
    tolerate .3 volts drop, a much smaller drive may do. You can get some
    idea how the drive varies with saturation voltage from the graphical
    data.

    Based on what I can only assume is typical characteristics (since it
    is so much better than the one data point guaranteed), the guaranteed
    value is very conservative compared to the graph of saturation voltage
    versus input current and for several collector currents.
    You have to start with what you need. How much collector current must
    the device pass, and what voltage drop is acceptable? Then you can
    start the difficult process of extrapolating what LED current may be
    needed based on the fragmentary info on the data sheet.
     
  4. Gareth

    Gareth Guest

    Have a look at the datasheet, there is a graph with the title
    "Collector-emitter Saturation Voltage vs. Forward Current". From this
    graph you should be able to see what Vce(sat) will be for various LED
    forward currents at 25 degrees C. Don't forget to leave some room for
    variations in Vce(sat) with temperature.


    --
     
  5. GrahamIT

    GrahamIT Guest

    Sorry for my ignorance, when I said "just starting to saturate" I meant
    "just starting to conduct", high voltage drop across transistor low
    collector current etc.

    What I actually need is for the output of the optocoupler to be TTL
    compatible, because it is to feed other TTL logic circuits in a CNC
    application I'm working on. This part is for the parallel port breakout
    board so I don't fry my laptop if I get a short from the 36V 9A motor
    supply.

    The circuit so far is described as this.

    Parallel port output > Tri-State buffer / line driver > Opto-Coupler > Drive
    Logic with separate and independent Vcc and GND either side of the
    Optocoupler. I could use another buffer on this side of the opto I suppose.

    The tri-state buffer is needed for power up and down stability, so the CNC
    machine can't move during a reboot, the software I'm using outputs a 12.5kHz
    signal on a parallel port output pin when ready to send data to the CNC
    machine and I'm feeding that into a 1mS timed charge pump to the Tri-state
    enable lines, also because my laptop has 3.3V and 5V output voltages from
    the parallel port, so I thought I'd standardise at 5V. so the breakout board
    could be used on desktop PC's as well, without overdriving the opto LED's.

    25 years ago I was an Army Radio Telecommunications Technician, it's
    surprising how much you forget if you don't use it.
     
  6. Gareth

    Gareth Guest

    There are optocouplers available with logic gate outputs which may be
    easier to use, for example:

    http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=143534&N=401
    I have found that desktop PCs also have 3.3V parallel port outputs, well
    mine does anyway I haven't carried out a detailed survey.

    --
     
  7. Most TTL inputs see anything above about 2 volts as a logic high. See
    the data sheet for a typical LSTTL gate:
    http://www.physics.wisc.edu/graduates/courses/623-f03/ds/74LS00.pdf

    So you can have a tiny bit of leakage and still have a logic high out
    of the transistor. With a 4.7k pull up resistor (i mA to pull down to
    ..3 volts) you can have about a half mA passing through it and still
    have greater than 2 volts out of the coupler.

    Another good way to make a fast logic output that is leakage and noise
    immune is to use two couplers, with their outputs stacked to form a
    push pull totem pole. Drive their inputs alternately. Most TTL
    inputs will treat an open circuit input (neither coupler on) as a
    logic high. You can parallel the pull up coupler with a 10k resistor
    for extra insurance (adds an extra half mA load to the pull down
    coupler).
     
  8. Jamie

    Jamie Guest

    Nah.
    there is Max handling current and there is Max usage currents/
    Normally the lowest!
    basically its like this.
    you really don't want to max out your LED because all your going to
    do is shorten its life spanned, normally 50% of the max forward current
    is a good rule of thumb to follow with LED's, then there's the chart
    that some times is given in spec's to show you where at what point the
    light output starts to drop off the linear scale which is also a good
    point of
    reference.
    and as far as transistors go, yes it is a good idea to fully
    saturate the bias plus alittle more to keep the passive resistance in
    the transistor low so less heat build up will be generated. so if you use a
    open collector scheme to drive the LED's you should either lower the
    feed voltage through the common side of the LED's to prevent over
    current or use a series resistor on each led ..
    remember that you will lose approximately .6 volts via the transistor.
    unless your planning on using Power Fets.
    P.S.
    there are some lower current fets that will generate enough
    resistance when fully one to give you the drop you need for your leds
    thus reducing the part count but remember the heating problem.
     
  9. GrahamIT

    GrahamIT Guest

    Not a bad idea, but they're bloody expensive, I can get all 3 of my octal
    chips for less than 4 of these and I'd need 8 of them for all my outputs.
    The IBM PC parallel port spec. says TTL 5V or TTL 0V toggle on output ports,
    I think the 3.3V comes from using TTL compatible CMOS devices that were not
    around when the original interface spec. was made up.

    I suppose I could use pullup resistors, but that always seems a bodge job to
    me, think I'll stick to the tri-state buffer with schmitt trigger input for
    safety > optocoupler > schmitt trigger buffer. That way is a bit belt and
    braces but should work nicely, I'll let you know.
     
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