Testing 1N4007 with a megger - Page 1
Forum section: electronics / projects

author: taydin
author_rank: Frequent Contributor
timestamp: October 30, 2019, 09:20:44 pm
content: I have connected a 1N4007 to the megger in reverse polarity and did a ramp test to 2.5 kV. Attached is the voltage/current graph (Current is microamps).As can be seen, at about 1500V, the first degradation of the junction occurs. Right around 1800V, another punch through occurs. Soon after that, at 1850V, the megger declares breakdown because the current hits 3 mA, the megger's limit.After this test, I have connected the same diode again to the megger and started the ramp again. The megger declared breakdown right away, at 100V which is the ramp start voltage.No surprises so far. But here is what surprised me: I have measured the diode after the breakdown using two multimeters, in the "diode check" range, and the diode is measured to be in working condition with BOTH multimeters. I have also measured the diode in the ohms range, and it had a forward resistance of 38 KΩ (a good 1N4007 has a 40 KΩ forward resistance) and a reverse resistance beyond the multimeter limit.So, the diode has basically failed in state where it MEASURES to be working, but fails when under voltage. And indeed, I applied 30V to it in reverse polarity, and it drew about 1 mA.This means, when we measure high voltage diodes, just a multimeter measurement won't be conclusive. The diode needs to be tested with rated voltage applied to it

author: taydin
author_rank: Frequent Contributor
timestamp: October 30, 2019, 09:23:52 pm
content: I did repeat the test with two more 1N4007's, and the graph was quite similar. One breakdown at around 1500V, and another breakdown around 1800V. And same measurement result with the multimeter.

author: Gyro
author_rank: Super Contributor
timestamp: October 30, 2019, 09:42:42 pm
content: You're enjoying playing with your new megger aren't you!

author: taydin
author_rank: Frequent Contributor
timestamp: October 30, 2019, 09:50:10 pm
content: Quote from: Gyro on October 30, 2019, 09:42:42 pmYes, was quite educationalBut it needs to go back at the end of this week. If you guys can think of another test, let me know!

author: thm_w
author_rank: Super Contributor
timestamp: October 30, 2019, 10:06:39 pm
content: Do you happen to have any 1N4005/1N4006 or anything lower rated?My guess is 1N4001/2/3/4 and 5/6/7 are all the same, since the specs are equivalent.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 30, 2019, 10:08:03 pm
content: Quote from: taydin on October 30, 2019, 09:20:44 pmYou can test avalanche breakdown safely *if* you limit current to maybe 100uA, or you limit the time so that the total avalanche energy is less than what the datasheet says is okay.

author: taydin
author_rank: Frequent Contributor
timestamp: October 31, 2019, 06:52:41 am
content: Those are good suggestions, guys! I don't have 1N4001 but I can get some from a local shop here.For the avalanche, I can place series resistor during the test to limit the current. The megger's current limit is fixed at 3 mA.

author: sourcecharge
author_rank: Regular Contributor
timestamp: October 31, 2019, 07:09:14 am
content: Wow, that is very educational.I have about 500 of those 1n4007 diodes, and I have never tested a diode with rated voltage to check if it was bad.What if you put 2 or 3 or 10 in series or even in parallel?Do you think you would find thesame dimm test problem?

author: taydin
author_rank: Frequent Contributor
timestamp: October 31, 2019, 07:56:34 am
content: Quote from: sourcecharge on October 31, 2019, 07:09:14 amPutting the diodes in parallel would just divide the fixed 3 mA current. But the diodes would still degrade. If you look at the voltage current graph, even after 1200V, there is a substantial increase in current. So even at that voltage, the diode probably won't be the same after the voltage is removed.The diodes in series, you would add the breakdown voltage of each diode. The voltage drop across each diode won't be the same because they will have different insulation resistances. The one to reach break down first will just be in standby because current is limited by the other diode. Then that other diode will reach break down and then both will get damaged.

author: taydin
author_rank: Frequent Contributor
timestamp: October 31, 2019, 08:00:10 am
content: And another thing to keep in mind: This type of failure would only occur in the real world where the voltage source is rather low energy. Maybe getting continuously hammered by ESD, or maybe the diode is clamping a small inductor (small relay coil?) and gets continuously hammered with back EMF ...But many times in the real world, the voltage source is high energy, so the diode will probably fail short circuit, and if the current continues, it will melt away and become open.

author: mzzj
author_rank: Super Contributor
timestamp: October 31, 2019, 09:09:31 am
content: Failed components like these are always real pleasure to try to find. Oldschool CRT monitors often had components that would measure ok with DMM but leak too much in real circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 01, 2019, 08:27:34 am
content: Hm, wonder if the avalanche withstanding capability varies with manufacturer.I would expect a good diode to just simmer in avalanche at say 1200V or thereabouts. Rather nasty, the results seen here!Tim

author: mzzj
author_rank: Super Contributor
timestamp: November 01, 2019, 10:25:29 am
content: Quote from: T3sl4co1l on November 01, 2019, 08:27:34 amOr you could interpret the results that leakage current increases at 1350v and diode enters avalanche knee at around 1.8kVIt is indeed "simmering" with the 3mA current limit on the megger and 1.8kV applied = 5.4W !If OP still has time to play with the megger it would be interesting to know what happens if you test couple of diodes with the current limited to 0.5mA or so. (less than 1 Watt power at avalanche)

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 01, 2019, 11:17:40 am
content: Quote from: mzzj on November 01, 2019, 10:25:29 amBingo! That's why you need to really limit the current (or the time spent) in avalanche for a runty little 1A diode which has a nominal Pd rating of maybe 1W... (and that's if you keep the legs extra long and don't place it next to anything else that gets hot).

author: taydin
author_rank: Frequent Contributor
timestamp: November 01, 2019, 12:00:59 pm
content: Ok, attached are the results with 10MOhm in series of the diode, ramping to 2.5 kV, reverse polarized. The Y axis is again microamps.I have also added the pdf output that has each individual voltage measurement.

author: taydin
author_rank: Frequent Contributor
timestamp: November 01, 2019, 12:08:41 pm
content: After the above test, I did another ramp, this time 5 kV. But the test stopped at 4.8 kV, because either the resistor broke down or the diode. Not sure which, will find out

author: taydin
author_rank: Frequent Contributor
timestamp: November 01, 2019, 12:11:05 pm
content: I measured the resistor, it is still 10 MOhms. And just to make sure the diode is OK, I ramped it up to like 800V and it didn't breakdown. So I suppse the resistor arced internally, but the megger shut off the voltage before the diode would get killed. The resistor is 1/4W metalfilm.

author: taydin
author_rank: Frequent Contributor
timestamp: November 01, 2019, 12:12:32 pm
content: So can we conclude that limiting the current prevents the junction from sustaining permanent damage under avalanche?

author: taydin
author_rank: Frequent Contributor
timestamp: November 01, 2019, 12:18:30 pm
content: Nope, we can't ...I picked 3 new 1N4007 from the bin and measured their insulation resistance at 1 kV. They are 65 GOhm, 87 GOhm, and 102 GOhm.Then I measured the diode that just underwent the ramp test. It has an insulation resistance of 260 MOhms ... So it has permanently degraded.

author: tooki
author_rank: Super Contributor
timestamp: November 01, 2019, 01:22:20 pm
content: Quote from: taydin on October 30, 2019, 09:23:52 pmSo... they all failed well beyond their maximum ratings. And this is a surprise why?Look also at the derating graph in the attached datasheet. As temperature rises, reverse current rises dramatically.Quote from: thm_w on October 30, 2019, 10:06:39 pmWell, they can be the same, or not, depending on manufacturing method. E.g. the Vishay ones in the attached datasheet could very well just be binned dies of the same design, since they list identical specs other than breakdown voltage, while another brand's might not be. See also:https://www.eevblog.com/forum/chat/why-1n400x/https://electronics.stackexchange.com/questions/84471/what-is-the-difference-between-1n4001-and-1n4007-other-than-their-maximum-revers

author: iMo
author_rank: Super Contributor
timestamp: November 01, 2019, 02:43:09 pm
content: It is a common practice manufacturers produce, for example, 1n4007 chips only, marking them 1n4001-1n4007. The same happens with transistors, memories, mcus, etc., etc.They also buy chips from other vendors and package them under their brand, when in need.The most expensive item in semiconductor business (except r&d) is the production of the sets of masks for photo-litography, the production of a silicon die costs almost nothing. You could save say 90% costs making the 1n4007 only.

author: taydin
author_rank: Frequent Contributor
timestamp: November 01, 2019, 03:42:28 pm
content: Quote from: tooki on November 01, 2019, 01:22:20 pmThe surprise is that the diode MEASURED to be working fine with a multimeter after it broke down around 1850V. I think I made that clear in my first post.

author: jaromir
author_rank: Unknown
timestamp: November 01, 2019, 07:06:14 pm
content: Quote from: tooki on November 01, 2019, 01:22:20 pmThis is indeed true. Let me share a bit of insider info.More than decade ago, I worked for major semiconductor manufacturer other than Vishay. For simple rectifying diodes, they manufactured probably three dozens of diode types from one type of silicon. Silicon wafer was doped and gold was sputtered from both sides, making huge "mother diode" (with reverse voltage higher than 1kV) with 30cm diameter PN junction. This basic material was cut into smaller pieces, with variety of perhaps 5 sizes and shapes. For 10A diodes the pieces were larger and hexagonal in shape, 3A diodes were smaller sqares, 1A diodes were the same, with smaller lead cross-section - the limiting factor wasn't current density in silicon or leads, but heat dissipation. When buying larger diode, you are paying for copper too, not only silicon.All diodes were manufactured as the "highest grade" of the family. For example, all 1N400x were manufactured as 1N4007, the "mother diode" was doped to have reverse breakdown voltage well above 1kV and vast majority of the diodes from manufacturing line had indeed breakdown above 1kV. There were no "mother diodes" for lower breakdown voltage. Lower voltage bins were filled at final diode testing by diodes which failed 1kV test. If those bins weren't full enough (the manufacturing yield was quite good), it was filled from 1N4007 bin. As an example, 1N4006 datasheet states breakdown voltage higher than 800V and 1N4007 (Vbr>1000V) fulfills this criterion. Diodes with higher than 1kV Vbr rating were made as stacks of 1kV chips in one package.Vast majority of customers were aware of this and ordered 1N4007 exclusively. Reason for having lower grades diodes comes from past, when lower manufacturing yield made binning more reasonable. Nowadays it isn't as attractive and all grades are sold for the same price.

author: floobydust
author_rank: Super Contributor
timestamp: November 01, 2019, 08:55:15 pm
content: There was a time 1N4005-1N4007 and 1N5405-1N5408 used to be spec'd with higher Vf, lower junction capacitance, longer reverse recovery time, higher thermal resistance - it depended on the manufacturer and their datasheet. The doping or construction was different.It is in antique databooks the difference but now pretty much the same. It might be that sorting/binning the parts costs more than simply making the 1,000V part, or semi processes have improved.Almost twice junction capacitance1N4005-1N4007 Diodes Inc.aww its obsolete now.Vishay S1Anote 800V, 1,000V part differencesSo they aren't all the same but who really designs that tight nowadays.It was discussed herehttps://electronics.stackexchange.com/questions/84471/what-is-the-difference-between-1n4001-and-1n4007-other-than-their-maximum-revers

author: tooki
author_rank: Super Contributor
timestamp: November 01, 2019, 10:30:48 pm
content: Quote from: taydin on November 01, 2019, 03:42:28 pmI dunno, IMHO if a part has been so grossly abused, it’s suspect, period. And my years of working as a computer tech taught me that parts often semi-fail, leaving them in marginal states where they sorta work.

author: taydin
author_rank: Frequent Contributor
timestamp: November 02, 2019, 10:15:14 am
content: Yesterday I have also tested a 1N4001. Same behavior as the 1N4007. Broke down at about 1800V, measures ok with a multimeter. So indeed, the 1N4001 is a 1N4007 in disguise.This begs the quesion. In the old days, when they were classifying the diodes according to their voltage rating, how did they determine the breakdown voltage? I guess there's gotta be a way to detect the onset of avalance without damaging the diode.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2019, 11:13:27 am
content: Compare Vf?Tim

author: jaromir
author_rank: Unknown
timestamp: November 02, 2019, 11:23:07 am
content: Quote from: taydin on November 02, 2019, 10:15:14 amThey are measuring Vbr even today, not just in old days. Breakdown voltage is measured at few orders of magnitude lower current than your limit, for example Vishay declares Vbr at 5uA - seehttps://www.vishay.com/docs/88503/1n4001.pdfYou mentioned 3mA before (I don't know how well is the current limited in your Megger, it's not designed to test semiconductors, after all) - that is quite a lot for a small diode at 1,8kV, no wonder they got damaged, especially on repeated measurements.The Vbr parameter is measured using current source with high compliance voltage, measuring voltage on the diode.

author: mzzj
author_rank: Super Contributor
timestamp: November 02, 2019, 09:29:49 pm
content: Quote from: jaromir on November 02, 2019, 11:23:07 amRead somewhere (usenet?) that back in the day silicon diodes would fail catastrophically at overvoltage. Controlled avalanche diodes are bit later improvement.https://groups.google.com/forum/#!search/controlled$20avalanche$20diode$20series/rec.radio.amateur.homebrew/XLRgaCb7XQE/tO2M_k_fleYJ

author: taydin
author_rank: Frequent Contributor
timestamp: March 06, 2020, 07:38:04 pm
content: I just wanted to revisit this with new information.Earlier, I had done a ramp test to 2.5 kV on a 1N4007 diode with a series 10 MΩ resistor. The diode had failed at a voltage about 2 kV and later measurements showed that the diode had permanently degraded. So my conclusion at that time was that, even with a series resistor, a high enough voltage causes the diode to fail.But there is more to this story and that statement isn't correct. When looking at the datasheet of 1N4007, the worst case leakage current is specified to be 50 uA (at rated voltage and 100 degrees celsius temperature). This means that the manufacturer guarantees that the diode will NOT GET damaged under these conditions.But when I made the ramp test, at 2 kV, with a 10 MΩ resistor, the resultant current is 200 uA, well beyond the 50 uA that the manufacturer specifies. If I had used a 40 MΩ resistor, the diode would not have failed at 2 kV.

author: taydin
author_rank: Frequent Contributor
timestamp: March 06, 2020, 07:45:41 pm
content: So I did another test, but now using a lower voltage BAW27 diode (rated reverse voltage is 60V). The worst case leakage current is specified to be 50 uA (rated voltage, 100 degrees celsius temperature).I used my 2450 SMU to source 60V and measured the leakage current. I waited until the current settled at about 20 nA. Then I configured a 50 uA current limit and started to increase the reverse voltage. The leakage current increased to about 100 nA at 100V. And finally at 110V, the current hit the 50 uA limit.Then I reduced the voltage back to 60V, and indeed, the leakage current returned back to 20 nA.So the conclusion is that an avalanche is not harmful to the diode if the current stays below the worst case leakage current specification.

author: zitt
author_rank: Regular Contributor
timestamp: March 06, 2020, 08:11:25 pm
content: Wonders to self; how this breakdown "knee" is temperature related.... If running at 120C; is the breakdown voltage higher or lower. what about -40 C?

author: taydin
author_rank: Frequent Contributor
timestamp: March 06, 2020, 09:11:52 pm
content: Quote from: zitt on March 06, 2020, 08:11:25 pmDon't know. But the reverse leakage is temperature dependent. For the BAW27, the reverse leakage current is 100 nA at 25 degrees and 50 uA at 100 degrees.

author: mikerj
author_rank: Super Contributor
timestamp: March 06, 2020, 10:29:02 pm
content: Quote from: taydin on November 01, 2019, 12:08:41 pmQuote from: taydin on November 01, 2019, 12:11:05 pmQuote from: taydin on November 01, 2019, 12:12:32 pmQuote from: taydin on November 01, 2019, 12:18:30 pmIf the resistor broke down then you didn't limit the diode avalanche current to the expected value.

author: taydin
author_rank: Frequent Contributor
timestamp: March 07, 2020, 09:32:05 am
content: Quote from: mikerj on March 06, 2020, 10:29:02 pmThe resistor broke down at 4.8 kV, which is way past the 2 kV point where the diode avalanched.

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2020, 10:35:28 pm
content: Quote from: taydin on March 07, 2020, 09:32:05 am250µA might have been too high for the diode though.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 09, 2020, 09:38:17 am
content: To resistor worked as resistor (but not as air gap) you have to obey it rated voltage.1/4W resistor usually is something about 250V rated (300V peak).So for your tests you need to use 4-10 series connected at least 1 W rated resistors.

author: rodpp
author_rank: Frequent Contributor
timestamp: March 09, 2020, 11:30:17 pm
content: I did the reverse breakdown voltage test on a 1N4007. My sample knee was around -1600V:And the forward IV curve before and after the test was the same, so no damage:If the reverse current is lower than the datasheet specification, no damage should occur with the diode.

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2020, 08:42:07 am
content: Quote from: rodpp on March 09, 2020, 11:30:17 pmFirstly, please consider those using a lower resolution by not posting such large images and PNG is much better suited to charts, than JPG, which is meant for photographs.Yes, you tested it at a much lower current than the original poster and just because the forward VI curve is unchanged, it doesn't mean it's not damaged. The only way of knowing that is to test the reverse breakdown voltage again.

author: rodpp
author_rank: Frequent Contributor
timestamp: March 10, 2020, 10:14:14 am
content: Thanks for the tip about the figures.Regarding the diode test, I did both (forward and reverse) using a curve tracer that apply a retified sin wave repeatedly. Each graph has around five half waves, but during the test was applied much more repetitions. It is clear in the reverse breakdown test graph, where it is possible to see various traces at the knee.