I have two rectifier diodes, a 1N4007 and a 1N5391. The first one is rated at 1A while the second is rated at 1.5A. What does the current rating mean? Will I damage/overheat either if I connect a device (such as a wall wart/barrel connector) that has an output of 2A or more?

  • \$\begingroup\$ size of junction, case and heat radiation affects T rise and thus I max, bigger rating is better. you can run 1N4007's in parallel if that's all you got. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 24 '17 at 15:33
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    \$\begingroup\$ @Tony, no, you can generally not run silicon diodes in parallel due to negative tempco. \$\endgroup\$ – winny May 24 '17 at 16:32
  • \$\begingroup\$ @winny yes and NO . you can if they are thermally coupled then Vf matches but I forgot to mention that. TY My Rule of Thumb is Rja/Pd < k and k depends on % of Pd used, if towards 100% then Rja must be optimally low. contrary if %Pd is 50% it is very reliable with no thermal runaway, Would you like to see 65Watts of 4S12P LEDs in parallel running off 12.5V on heatsinks 1x4" for the last 6 years with no series R except AWG16 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 24 '17 at 18:34
  • \$\begingroup\$ @Tony That's a very special case. Do you think the three 1N4007 you suggested are going to be that thermally coupled? LEDs these days have much more ESR than your diode. \$\endgroup\$ – winny May 24 '17 at 19:36
  • \$\begingroup\$ you obviously have not my detailed analysis of diodes and ESR \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 24 '17 at 21:06

The rating is the continuous current limit of the Diode. If you use these with a 2 A or 6 A power supply makes no difference ....it's the amount of load current that flows that is the limitation.

For example if you have a 2 A power supply but are only using a load that draws 1 A, you may be fine with the 1N1007 (though it will get warm/hot).
If you used a 1N1007 in an automotive project (the battery is capable of 100's of Amps) the same applies ....as long as the load current for your project (flowing through the Diode) is less than the continuous current rating for the Diode, everything will be fine.

So the current rating of the supply is irrelevant, it's the current rating of your load that must be considered, and must not exceed the Diode rating.

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The key is to read the datasheet carefully. The 1A or whatever rating is under some particular set of conditions that may be optimistic for your situation. In the case of the 1N4007 (what I assume you mean, since a 1N1007 is an old germanium diode) a "1A" rectifier, the rating is specified as follows:

enter image description here

So we are talking average rectified output current with the lead temperature maintained at 75°C 9.5mm from the case. Note that they effectively define this as the ambient temperature which is.. rather dubious. There is an obvious danger if you have long leads and are running close to the maximum current. On the plus side, the actual RMS current may be significantly higher than the average rectified output current.

A graph on the datasheet shows linear derating to zero at 150°C (again that's the lead temperature 9.5mm from the case).

If you are running at higher current (or even close to 1A) I suggest you use a higher rated part such as 1N5407 or a Schottky 3A diode which would run cooler (because less voltage drop) eg. 1N5821. The 1N5821 datasheet I linked is a bit more honest in that it refers to Tl (lead temperature) rather than ambient temperature, however it is specifically the cathode lead and only 1/32" (0.8mm) from the case! There is detailed thermal design material in that datasheet, and it's pretty straightforward, just simple arithmetic.

Of course if your adapter can supply 2A but you are sure your load can never draw more than (say) 0.5A then a 1A diode is fine.

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    \$\begingroup\$ Strangely, I read 1N4007, you read 1N1007, but the OP wrote 1N0007. \$\endgroup\$ – Andrew Morton May 24 '17 at 18:34
  • \$\begingroup\$ @AndrewMorton Autocorrect? \$\endgroup\$ – Spehro Pefhany May 24 '17 at 18:42
  • \$\begingroup\$ It was a typo on my part; I have since corrected to 1N4007. Thank you. \$\endgroup\$ – Agi Hammerthief May 25 '17 at 8:20

Read the rest of the datasheet.

Vishay's datasheet for 1N5391, for example, has these two charts:

enter image description here

So a continuous current of 2 A is not allowed. Even 1.5 A is not allowed if your operating environment does not keep the lead temperature below 75 C.

Up to 50 A current surges may be allowed if the surge duration is short enough and the surges are infrequent enough.

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  • \$\begingroup\$ Those datasheets mean nothing to me; they're full of terms and symbols I don't understand. \$\endgroup\$ – Agi Hammerthief May 25 '17 at 8:24

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