I have a 18 V cordless drill battery that I would like to use to power some small 3.3-5 V components such as LEDs etc. So to convert the 18 V coming from my battery to 5 V I bought a LM2596 Step-Down Adjustable DC-DC Switching Buck Converter. Almost identical looking to the one in this link.
I connected the “IN+” and “IN-“ pins to my battery with nothing yet connected to the output pins. And then after a few seconds a hole cracked open in the “100 50V VT” capacitor between the two input pins and a jet stream of smoke started to pour out of the capacitor.
What did I do wrong? According to the data sheet it is supposed to handle 3.2 V-30 V which is within the range of my battery. I checked my battery with a multimeter and it outputs more or less exactly 18 V. Are you not supposed to hook up DC-DC converters to an input power source without an output connected?
larryd:
Maybe you hooked it up wrong.
Maybe the cap was faulty.
Maybe the cap was installed incorrectly.
Maybe you got a bad board.
So you are saying it should be perfectly fine to connect my 18V battery directly to the input pins without anything connected to the output and that I should try to return it at the store where I got it?
Yes it should normally be fine and you could certainly return it. But first check that you didn't connect the battery the wrong way round. The symptoms certainly sound like a capacitor subjected to reverse voltage.
slipstick:
Yes it should normally be fine and you could certainly return it. But first check that you didn't connect the battery the wrong way round. The symptoms certainly sound like a capacitor subjected to reverse voltage.
Steve
Ok, thank you for clarifying that for me Steve. I’ll see if the store will accept my return then
srnet:
I hope you had the switcher covered when you connected it up for the first time.
Some capacitors can eject red hot beads quite a distance if they are faulty or reversed.
Oh. No, it wasn’t covered, I had no idea it could be dangerous to power up small DC components. Fortunately I didn’t experience any hot projectiles from the capacitor, but it did smell truly horrible and toxic
petters:
Ok, thank you for clarifying that for me Steve. I’ll see if the store will accept my return then
There should be enough of the capacitor left to tell if it was assembled reversed by accident, check that, otherwise the sucpicion would be that it was wired up backwards.
srnet:
There should be enough of the capacitor left to tell if it was assembled reversed by accident, check that, otherwise the sucpicion would be that it was wired up backwards.
How do you mean I should check that? Just hook it up again and make 100 % sure its not reversed and then see if it works or not?
Yes, for the input side I just grabbed two wires without considering their colors. I’m pretty sure I connected the orange wire to - on my battery and the brown one to +. But since my choice of wire colors is counterintuitive its not impossible that I subconsciously felt obligated to reverse the connection and did so. As soon as it started to smoke I pulled out the battery so now I have no way of knowing for sure how it was connected.
Let's see.
OK, that's an aluminium electrolytic, not a tantalum. Dark stripe for this is negative, so it has been assembled correctly.
Given your reverse choice of wire colours I would say it is pretty much certain that you connected it in reverse. While it is possible (but inconvenient) to replace that capacitor, one could not vouch as to whether the IC was damaged or not, so I would call it a write-off. (Don't know what the LM2596 datasheet specifies about tolerance to reverse polarity, but I am not going to read it all!)
Paul__B:
Let's see.
OK, that's an aluminium electrolytic, not a tantalum. Dark stripe for this is negative, so it has been assembled correctly.
Given your reverse choice of wire colours I would say it is pretty much certain that you connected it in reverse. While it is possible (but inconvenient) to replace that capacitor, one could not vouch as to whether the IC was damaged or not, so I would call it a write-off. (Don't know what the LM2596 datasheet specifies about tolerance to reverse polarity, but I am not going to read it all!)
I got a new identical one today and hooked it up in the same way. This time it worked just as intended and didn’t blow up. So yes, I probably made a mistake and reverse-connected the old one.
petters:
Oh. No, it wasn’t covered, I had no idea it could be dangerous to power up small DC components. Fortunately I didn’t experience any hot projectiles from the capacitor, but it did smell truly horrible and toxic
Reverse connected electrolytics heat up and undergo steam-pressure explosion. The casings are deliberately
weakened to cause it to burst relatively safely (not too violently), but the result is a cloud of aluminium
compounds, particles of aluminium foil and hot steam.
Reversed tantalum can be even worse and can explode hot and start fires if over-voltaged.
A close visual inspection of any board before it is first powered on is advisable, even if you didn't
make it yourself - all sorts of problems can be present (misplaced components, missing components,
tombstoned SMT parts, dry solder joints, solder bridges, etc etc, any one of which can be a show
stopper.
If you had tested it on a current-limited bench power supply you'd have been able to spot the high
current draw before it burst. A cordless drill battery will provide lots of power without question.
