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Maximum Ratings

Discussion in 'Electronic Components' started by Mark Aitchison, Feb 10, 2007.

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  1. Hi, three (and a half) somewhat related questions:

    1. Some LEDs have a maximum reverse voltage rating less than their
    normal forward voltage. If I want to work out which lead of the LED is
    the cathode (assuming the length of the lead doesn't help, e.g. it has
    been cut when wired up), is there a risk of destroying the LED when
    testing? I normally measure semiconductor junction breakdown voltages
    with a (pulsed/ramped) very low current source, but that will show
    similar voltages no matter which way around the light emitting diode is,
    surely. If I increase the current to be enough to see light, am I
    risking damage if the diode happens to be reversed during the test?

    2. This raises the question of reverse breakdown currents in general:
    how much reverse current do semiconductor junctions generally stand? Is
    it based on total power dissipation? Is it about the same (assuming
    pulsed) as the forward current limit?? Normally, data sheets only refer
    to pretty low leakage currents while specifying breakdown/"sustaining"
    voltages, and that is appropriate for most people because once you go
    past the onset of breakdown you normally (without current limited
    circuits) get heaps of current soon afterwards. But if the reverse
    current occurs at a lowish voltage, e.g. the LED mentioned above, or a
    base-emitter junction that is reverse-biased, *and the current is
    limited* so we don't have to worry about thermal runaway, what sort of
    current can you use in the test without damage? Should a light emitting
    diode that can stand 30mA forward current be able to stand at least 3mA
    of reverse current? Would a small signal transistor's base-emitter
    junction be able to stand 1mA in the reverse direction? The latter is
    an important problem for me: if I have a simple transistor tester and I
    accidentally get the pinout or polarity wrong, what current limiting is
    "safe" enough for most (all??) transistors... if it exceeds the
    base-emitter reverse voltage rating (and some are only 3volts) how
    limited does the current have to be?

    3. Which leads to the question: how high can the FORWARD base current be
    in general? Some transistor spec sheets don't mention the limit; some
    give something like half the collector current rating, and a small
    number give currents like 1mA (i.e. less than a tenth the maximum
    collector current). Again, thinking about simple transistor testers
    where the base might accidentally be connected to where the collector
    should go, and get more than 1mA, how likely is that to kill some
    transistors? Is there a non-destructive way to tell what current an
    unknown diode can take? Is there are pattern to the base current
    limitation (e.g. a given fraction of the collector current for
    particular types of transistors? Or the current which gives a certain
    forward voltage?) What is behind the current limitations: is it
    connections from the silicon to the pins being too thin? Is it more
    often local effects in the silicon? Heating effects? Sorry, that's lots
    of sub-questions!

    And can running currents (forward or reverse, base or collector or
    anode) do damage other than obviously kill the device (e.g. make it more
    noisy? Increase leakage current permanently??) or is it only
    over-temperature problems than do that?

    Any advice appreciated,
    Mark Aitchison,
  2. Nermal

    Nermal Guest

    The best tool for this test is a transistor curve tracer. You can use a
    precision power supply (sensitive current meter and accurate voltmeter)
    and slowly apply the rated "forward" voltage. If you guessed correctly
    the current will rapidly increase and at around 1 mA you will see the
    light. If you apply forward voltage and do not see a current increase
    the device is connected backwards. Another thing to check for: identify
    the Cathode/Anode connections on one device, the other devices will be
    Reverse current should be limited to less than 100 uA.
    Many LEDs can be over driven in the pulsed mode. Calculate the maximum
    current from the manufacturers maximum power dissipation and do not
    exceed by more than 50%. Rated max power dissipation is given based
    upon die size, heat sinking, and junction temperature for a given life.
    I have seen a bank of LEDs 'go south' because they were over driven.
    There were five or more banks of LEDs in series/parallel with only one
    current limiting resistor. One batch had LEDs with slightly lower Vf
    and these drew most of the current. The high power dissipation caused
    the epoxy to delaminate from around the bond wires and the die. This
    killed all of the LEDs in series.
  3. Henry Kiefer

    Henry Kiefer Guest

    | the device is connected backwards. Another thing to check for: identify
    | the Cathode/Anode connections on one device, the other devices will be
    | identical.

    Simply: No!
    Depends on material/manufacturer.

    - Henry
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