Missing protection Diode on inverter - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: January 19, 2021, 05:37:15 am
content: I rebuilt an inverter for a friend. He brought it new locally years ago and had been using it for some time at fairly low current . When he tried to use it closer to its rated 600W it just would not perform and heated up much more than it should given the load . So he gave it to me to have a look at it . A few things I noticed was the soldering of the blade fuses didn't look good at all compared to the rest of the board . It looked like a very poor repair with lifted , chipped traces. Some of the Mosfets also looked replaced .I have suspicions that it had been damaged , "repaired " and resold as new.Putting that aside , I changed out many of the chips on board and checked other components . I also replace the Mosfets on the low and high side since many of them shot anyway. replaced the fuses and included fuse clips . Long story short I got it working again and doesn't get hot under load .Theirs also a polarity protection diode missing and would like to put it back. It's not a great polarity protection but it's probably better than nothing at all.But I'm unsure of the size of diode I should use .What I have10A10 10A rectifierhttps://www.rectron.com/data_sheets/10a05-10a10.pdfSR5100 5A Schottkyhttp://www.secosgmbh.com/datasheet/products/Schottky_Rectifier/DO-27/SR5100.pdf1N5408 3A rectifierhttps://www.vishay.com/docs/88516/1n5400.pdfHER308 3A rectifierhttps://www.rectron.com/data_sheets/her301-308.pdfI really don't want to order another diode unless I absolutely have to.Don't laugh to hard at my solder job . It's way better than what was there originally.

author: octillion
author_rank: Contributor
timestamp: January 19, 2021, 06:00:06 am
content: Look at the time-current characteristic for the fuses and compare to the surge current rating of the diode. The fuses may "win", but if the diode fails shorted (likely) then the fuses may blow before killing anything else. Out of your choices, 10A10 has the highest surge rating. It's not a guaranteed reverse voltage protection.

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2021, 06:07:50 am
content: So I looked it up for you.The fuses (just using a similar type from Littlefuse) have a fusing energy of 1510 A2S each, whereas the biggest diode you have listed has a max surge rating of 664 A2S. Leaving aside the issues with parallel fuses, and perhaps just going with the idea, you need 8 or 9 of the 10A10 diode to have any chance of it reacting to a reverse polarity event by blowing the fuses without any other damage. From your description, it seems that the original protection plan didn't work out.

author: Jwillis
author_rank: Super Contributor
timestamp: January 19, 2021, 09:00:18 am
content: Quote from: bdunham7 on January 19, 2021, 06:07:50 amI'm not a fan of this type of protection scheme.Not sure if my math is right but I figure the I2t rolls in around 13590 and the let through current around 62 Amps . What ever diode was there probably blew up and didn't get replaced when the fuses were . So it probably won't make any difference if it's there or not.