# Maximum Ratings

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

1. ### Mark AitchisonGuest

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?

Mark Aitchison,
ZL3TQE

2. ### NermalGuest

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
identical.
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 KieferGuest

| 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