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jluckey(at)pacbell.net
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 Posted: Tue Jun 01, 2021 8:37 am Post subject: Diode Failure Current vs Time Graph |
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Hey BobN,
In your vast array of data & test results, do you have a current vs time graph of a solenoid spike-suppression diode (1N5004?) short circuit?
I'm wondering what the peak current is before it is destroyed.
Somewhere I have an old PC based O-scope but I don't remember what its frequency range is (or even where it is), but how many samples per second would be adequate to catch a failure like that.
I would use something like a 150A shunt (I think I have one lying around). Do you think that would be adequate? Would a shunt like that remain linear at say 100% overload? Or, would we get even close to 300 amps?
Purely an academic exercise (electrical nerd inquiring mind(s) want to know;)
-Jeff
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nuckolls.bob(at)aeroelect
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| Posted: Thu Jun 03, 2021 4:44 pm Post subject: Diode Failure Current vs Time Graph |
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At 11:35 AM 6/1/2021, you wrote:
| Quote: | Hey BobN,
In your vast array of data & test results, do you have a current
vs time graph of a solenoid spike-suppression diode (1N5004?) short circuit?
I'm wondering what the peak current is before it is destroyed. |
Interesting question and one that can probably
be deduced by experiment. Setting up the experiment
would be tricky.
It's not so much a "peak current" as it is a "watt-seconds"
issue . . . very much time dependent.
To destroy a diode you need to either (1) MELT the
barrier-hill layer between the p/n elements. This
requires a watts/second injection of energy at some
rate that exceeds the devices ability to reject
the resulting generated heat. For our 1N400X series
friends, this could be something on the order of 5A
for Seconds . . . or 100A for milliseconds.
(2) you can punch out the barrier-hill layer with
reverse voltage that exceeds that layer's ability to
stand it off. Again, it's an energy rate function,
how much reverse current flows at the junction's
breakdown voltage (watts) for sufficient time
and rate to produce melting.
Most people don't know it but ordinary junction
diodes can be pressed into service as high voltage
zeners. I used to use them as 700VDC regulators
to bias up Geiger Muller tubes in radiation
detectors. I had to hand select diodes from a lot
to find the ones that offered 700V operation at
about 50 microamps . . . or about 35 milliwatts
dissipation. Well below the thermal threshold for
damage.
Getting back to your question:
Download this data sheet . . .
http://www.aeroelectric.com/Mfgr_Data/Semiconductors/1N4001.pdf
Here we see a forward surge current rating of 50A
with an interval of 1/2 of the 60H sine wave of
8.3 milliseconds. The data sheet says you can do
this one time (generally interpreted to mean very
low duty cycle). You could probably hit the device
with this pulse once every second for a duty cycle
of less than 1%.
On this same data sheet we see thermal ratings
citing 3W maximum dissipation. 175C maximum
junction temperature and 50C/W thermal resistance
to ambient.
50C/W X 3W is a 150C rise which is added to
25C ambient to get to the 175C max operating
temperature.
My hypothetical 5A 'overload' cited above would
probably produce a voltage drop of over 1V. That's
5 Watts dissipation for a potential 275C junction
temperature. Somewhere above 175C the junction
will fail after sufficient time to produce the rise.
The 30A rated surge pulse would obviously heat
it up much faster but the 8.3Ms limit would
be conservative.
These numbers suggest some parameters for setting
up your experiment. They also validate our selection
of the 1N400x series devices as contactor coil spike
catchers. A charged capacitor will deliver discharge
voltage at no higher value than the capacitor was
charged to. Similarly, an inductor delivers discharge
current at the same value to which it was chaged.
A 3A starter contactor coil would 'hammer' its
catch diode at 3A peak for no more than 5-10
Milliseconds. Well inside the surge current limits
suggested by the data sheet.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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