I let out the magical smoke on a buck converter today

[Silver_Is_Money]

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?

[Jwillis]

One of two possibilities. Over voltage and/or crappy capacitors. I'm leaning towards crappy capacitors.

[Silver_Is_Money]

One of two possibilities. Over voltage and/or crappy capacitors. I'm leaning towards crappy capacitors.

It was rated for 4V to 38V on the input side.

[CaptDon]

(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.

[CaptDon]

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!!

[Silver_Is_Money]

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.

I soldered the input wires onto it myself.  Red wire to +IN, and black wire to -IN.

[Silver_Is_Money]

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.

[Bud]

.  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..

Then you will know why it had zero reviews 

[MarkT]

I see no mention of a fuse on the output of the LiFePO4 battery...

[Silver_Is_Money]

I see no mention of a fuse on the output of the LiFePO4 battery...

So 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?

[SeanB]

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.

[Silver_Is_Money]

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.

To install it backwards I would have had to cross the wires.  I didn't cross them.

[CaptDon]

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.

[pcprogrammer]

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.

[Silver_Is_Money]

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.

What resistance range are you looking for from the potentiometer before powering up?

[pcprogrammer]

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

[Silver_Is_Money]

Inline socket and an ATO automotive fuse of 7.5A will work.

I've ordered the ATO fuse holder and 7.5A fuse.  Thank you for this!  Also ordered was a new (and different) buck converter.

[hneve]

Check the converter specs to make sure it can handle the input voltage of your LiFePO4 battery.

[Silver_Is_Money]

Check the converter specs to make sure it can handle the input voltage of your LiFePO4 battery.

Off 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.

[pcprogrammer]

Check the converter specs to make sure it can handle the input voltage of your LiFePO4 battery.

Off 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.

That sounds like a big one, like what I just ordered today 

https://nl.aliexpress.com/item/1005006365806926.html

I 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 

[David Hess]

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.

How 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.

[Silver_Is_Money]

How 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.

I'll bet you're correct in assessing that it was an aluminum electrolytic capacitor.  The 1 piece housing cover was definitely aluminum.

[Silver_Is_Money]

Check the converter specs to make sure it can handle the input voltage of your LiFePO4 battery.

Off 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.

That sounds like a big one, like what I just ordered today 

I 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 

I'll have to start using Aliexpress going forward.  That price is way less than Amazon.

[MarkT]

That sounds like a big one, like what I just ordered today 

https://nl.aliexpress.com/item/1005006365806926.html

I 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 

Very sceptical about its ratings - 20--50W perhaps, not 300!

[David Hess]

I'll bet you're correct in assessing that it was an aluminum electrolytic capacitor.  The 1 piece housing cover was definitely aluminum.

It 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.

[pcprogrammer]

That sounds like a big one, like what I just ordered today 

https://nl.aliexpress.com/item/1005006365806926.html

I 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 

Very sceptical about its ratings - 20--50W perhaps, not 300!

I will find out when it arrives. 

[Silver_Is_Money]

It 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.

It looked very much like these:  https://www.mouser.com/c/?marcom=113882612

[Silver_Is_Money]

Follow up and closure.  My new and more robustly built buck converter has  arrived, and it is working admirably.

[pcprogrammer]

 

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.