I let out the magical smoke on a buck converter today - Page 1
Forum section: electronics / beginners

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 03, 2024, 12:04:31 am
content: I connected a new and right out of the box buck converter to my 13.6V LiFePO4 battery (rated at 8Ah) today, and before I could grab the screwdriver with which to dial it down to 5V on the output side it exploded what I presume to have been an electrolytic liquid capacitor. There was no load connected to it. And I was cautious to connect the buck converters input positive and input negative to their respectively corresponding + and - LiFePO4 battery terminals. What did I do wrong?

author: Jwillis
author_rank: Super Contributor
timestamp: February 03, 2024, 12:32:29 am
content: One of two possibilities. Over voltage and/or crappy capacitors. I'm leaning towards crappy capacitors.

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 03, 2024, 12:34:50 am
content: Quote from: Jwillis on February 03, 2024, 12:32:29 amIt was rated for 4V to 38V on the input side.

author: CaptDon
author_rank: Super Contributor
timestamp: February 03, 2024, 12:42:29 am
content: (A) You may have hooked it up backwards even though you are sure you didn't (seen that before) (B) Cheap chinese junk with no quality control and Wang Hung Wo soldered the capacitor in backwards, or (C) Wang soldered the input leads on backwards. I have bought several non-adjustable chineseum power supply blocks that were 10 to 36VDC in and 5VDC out at 5 amps and so far after 4 years they all still work. The unit I incorporated them into works from 12 volt car systems, military 24 volt systems and connected across one 32 volt battery of a 64 volt locomotive. Just what I needed.

author: CaptDon
author_rank: Super Contributor
timestamp: February 03, 2024, 01:04:37 am
content: Another weird possibility, if it has a white and a black 'input' wire the black could be the hot or + in this case! I actually ran into that with some off the shelf locomotive supplies!! They were exported to the U.S.A. and I think the engineering team must have looked at the U.S. domestic wiring where white is neutral / ground and black is hot. They were inverters designed for locomotive use. 72VDC in 120VAC 60Hz out at 3KVA. We thought they were dead since they did nothing when connected. Fortunately they had reverse polarity protection which is a required specification for locomotive battery operated equipment. This is because there are actually idiots who jumper a dead locomotive backwards!!! The fireball of molten copper clamps has actually set clothing on fire!!

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 03, 2024, 01:11:41 am
content: Quote from: CaptDon on February 03, 2024, 01:04:37 amI soldered the input wires onto it myself. Red wire to +IN, and black wire to -IN.

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 03, 2024, 01:29:53 am
content: Aluminum electrolytic capacitors are polar specific, so my best guess is that (as @CaptDon mentioned) the capacitor that exploded was likely wired to the board backwards. All I found on the floor afterwards was the aluminum capacitor cover.Indeed it was cheap Chinese junk. I paid $6.99 for it on Amazon. It had zero reviews. I just left one explaining that it exploded. I would have given it zero stars, but one star was as low as I could go. The Amazon reply was that it will take several days of review before my response gets posted for public consumption. I'm betting it won't be posted at all.Fortunately I was off scrounging for my tiny potentiometer screwdriver when it went bang, as I could have been injured.

author: Bud
author_rank: Super Contributor
timestamp: February 03, 2024, 02:44:45 am
content: Quote from: Silver_Is_Money on February 03, 2024, 01:29:53 amThen you will know why it had zero reviews

author: MarkT
author_rank: Frequent Contributor
timestamp: February 03, 2024, 09:32:09 am
content: I see no mention of a fuse on the output of the LiFePO4 battery...

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 03, 2024, 10:21:27 am
content: Quote from: MarkT on February 03, 2024, 09:32:09 amSo I can right the ship moving forward, what would be the proper recommendation in this regard? My maximum intended combined users demand will be not more than 65 Watts. My battery states maximum permissible users draw is not to exceed 10 amps. A 5 amp fuse perhaps? What type of fuse?

author: SeanB
author_rank: Super Contributor
timestamp: February 03, 2024, 01:39:12 pm
content: Inline socket and an ATO automotive fuse of 7.5A will work.Will guess either they had left the output voltage set to all it can give, and there is a 10V capacitor on the output, or it was put in bacwards, or it was just defective.

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 03, 2024, 02:07:37 pm
content: Quote from: SeanB on February 03, 2024, 01:39:12 pmTo install it backwards I would have had to cross the wires. I didn't cross them.

author: CaptDon
author_rank: Super Contributor
timestamp: February 03, 2024, 05:58:11 pm
content: When I was working in a QA/QC first time start-up of newly manufactured boards we had a multitude of capacitor explosions. I think they were 4700uf at 25VDC with around 16VDC applied. The shrink wrap with the manufacturer capacitance / voltage / polarity indications was put on incorrectly. It was so random and yet so frequent we had a test jig to apply around 5 volts and show leakage current. If indeed they were marked backwards the leakage current would reveal it within a few seconds. This extra step took an operators time but it was still better than suffering a big capacitor explosion, the P.T.S.D. of co-workers, and the repairs involved to put the board out to the market.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 03, 2024, 08:33:07 pm
content: Some of those buck converters from aliexpress can have double functions. Buck as well as boost. If so it can blow up because of to high an output voltage due to the potentiometer being on the wrong end of the range.I have had this happen once with a small boost converter. Not due to the potentiometer being set wrong on the start, but due to me turning it up to see what would happen. It meant one converter less to play with.Before powering any of these converters up I check the potentiometer first with a multimeter and look at the datasheet to make sure it is ok.

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 03, 2024, 08:57:00 pm
content: Quote from: pcprogrammer on February 03, 2024, 08:33:07 pmWhat resistance range are you looking for from the potentiometer before powering up?

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 04, 2024, 06:17:03 am
content: Depends on the datasheet. Sometimes there is a formula in it to calculate the needed division between resistors to set the wanted output voltage. Not in the attached one, but it can be derived from the other specifications.For instance in the attached datasheet on page 3 there is a example circuit with the resistors R1 and R2 for the feedback input. The feedback voltage is 1.25V for this one. This means that the voltage over R1 needs to be this 1.25 volt.Vfb = Vout * R1 / (R1 + R2)In the example R1 = 3.3K and R2 = 10K and Vout = 5V.So to calculate the actual Vout in this case the formula needs to be reworked to:Vout = (Vfb * (R1 + R2)) / R1

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 04, 2024, 01:59:49 pm
content: Quote from: SeanB on February 03, 2024, 01:39:12 pmI've ordered the ATO fuse holder and 7.5A fuse. Thank you for this! Also ordered was a new (and different) buck converter.

author: hneve
author_rank: Regular Contributor
timestamp: February 04, 2024, 02:15:09 pm
content: Check the converter specs to make sure it can handle the input voltage of your LiFePO4 battery.

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 04, 2024, 03:12:50 pm
content: Quote from: hneve on February 04, 2024, 02:15:09 pmOff the top of my head it states 4V to 36V on the input side, and 1.2V to 34V on the output side. 300W rating.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 04, 2024, 06:07:52 pm
content: Quote from: Silver_Is_Money on February 04, 2024, 03:12:50 pmThat sounds like a big one, like what I just ordered todayhttps://nl.aliexpress.com/item/1005006365806926.htmlI bought it for less via the choice page, you know, order at least three items and get free shipping. Payed only 2,69 for it.When it arrives I will check everything before powering it up

author: David Hess
author_rank: Super Contributor
timestamp: February 04, 2024, 07:57:07 pm
content: Quote from: Silver_Is_Money on February 03, 2024, 12:04:31 amHow sure are you that it was an electrolytic liquid capacitor? I assume that you mean aluminum electrolytic capacitor?Solid tantalum capacitors are known to explode when the applied surge current is high, which is what happens if a battery is connected, so they are not recommended as input capacitors in that type of application.

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 04, 2024, 08:24:55 pm
content: Quote from: David Hess on February 04, 2024, 07:57:07 pmI'll bet you're correct in assessing that it was an aluminum electrolytic capacitor. The 1 piece housing cover was definitely aluminum.

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 04, 2024, 08:28:08 pm
content: Quote from: pcprogrammer on February 04, 2024, 06:07:52 pmI'll have to start using Aliexpress going forward. That price is way less than Amazon.

author: MarkT
author_rank: Frequent Contributor
timestamp: February 04, 2024, 09:15:09 pm
content: QuoteVery sceptical about its ratings - 20--50W perhaps, not 300!

author: David Hess
author_rank: Super Contributor
timestamp: February 05, 2024, 03:15:11 am
content: Quote from: Silver_Is_Money on February 04, 2024, 08:24:55 pmIt could have been an aluminum polymer capacitor, but either way it should not have exploded unless the voltage rating was too low, or the polarity was backwards.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 05, 2024, 07:33:50 am
content: Quote from: MarkT on February 04, 2024, 09:15:09 pmI will find out when it arrives.

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 05, 2024, 11:01:03 am
content: Quote from: David Hess on February 05, 2024, 03:15:11 amIt looked very much like these:https://www.mouser.com/c/?marcom=113882612

author: Silver_Is_Money
author_rank: Regular Contributor
timestamp: February 16, 2024, 02:23:31 pm
content: Follow up and closure. My new and more robustly built buck converter has arrived, and it is working admirably.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 16, 2024, 04:52:30 pm
content: Mine came yesterday and it works, but I have not fully tested it yet. Can't make out which MOSFET's they used without disassembling, so careful performance testing is needed to see if it can handle the 300W stated on aliexpress.