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I am wondering if someone can explain what Maximum Average Forward Rectified Current means for a diode. I see that this is marked as 1 Amp in the datasheet for the 1N4001. I have not been able to find a clear explanation of what this means online.

This came up when I was doing stress testing on a circuit I am designing. I was driving the circuit under it's maximum conditions for long periods of time. In these tests 1.7 Amps of current passed through a 1N4001 diode for over 10 minutes. The diode did not get hot or show any signs of distress. Someone later pointed out the 1 Amp in the datasheet, but clearly this doesn't mean the maximum current that can pass through the device, or there would have been some sort of negative effect.

Can anyone explain what this attribute means for a diode, and why the circuit operated without any negative effects at the higher current?

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    \$\begingroup\$ The fact that you didn't observe a negative effect doesn't mean there wasn't one. The reliability of the diode may be greatly reduced and it could fail tomorrow. Furthermore, you may have just gotten lucky...the next 99 diodes you try may fail catastrophically when you pass 1.1A through them. The maximum current spec doesn't mean the diodes will fail when you exceed that current, it means they won't fail if you stay below the spec. \$\endgroup\$ – Elliot Alderson Sep 13 '18 at 0:35
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    \$\begingroup\$ Something must be really wrong with your measurements. Typical 1N4001 has D=2mm and L=4mm, it is a fairly small device. At 1.7 V the forward voltage is about 1.1V, so the dissipated power is nearly 2W This is huge power for a device that small, unless the legs are soldered to massive pads/wires. The device gets fairly hot, Tcase goes to 70C, I just happen to have an old sample of 1N4001 and run the test, it burns fingers. \$\endgroup\$ – Ale..chenski Sep 13 '18 at 1:19
  • \$\begingroup\$ The diode got very hot if the current was actually passing through it. Maybe you didn't notice because it cooled relatively quickly. \$\endgroup\$ – Spehro Pefhany Sep 13 '18 at 1:19
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At 25°C ambient and 1.7A the junction temperature would likely have been in the region of 150°C, depending on what was happening with the leads. If they were short and soldered into large areas of copper the temperature would be less, if long and with thin wires attached, then higher.

That's not a high enough temperature to immediately destroy or cause obvious damage to the diode, but within perhaps 30-50°C of what would. Rectifier diodes tend to be quite rugged and resistant to damage from short-term overloads, which is why we often don't worry about start-up surges (eg. for charging filter caps) damaging the diode. It's capable of surges in the several ampere range for a second or two if Ta is low enough, and 30A peak 1/2 sine wave- non-repetitive- which means rarely and allowed to cool between hammering it.

Running a diode that hot (and whatever caused the manufacturer to limit the current to 1A average regardless of Ta) is bad for reliability. You should be operating nowhere near the limits if you want to create things that work indefinitely. Maximum junction temperatures in the 100°C to 120°C (when ambient is at a maximum, with lower average temperatures) are more reasonable, and lower is generally better.

From a datasheet:

enter image description here

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