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Author Topic: When Tantalum caps fail, are they supposed to light up like a match?  (Read 1974 times)
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Texas
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Tantalum haters aside (LOL). Did you check that 12 volt power supply with a volt meter. Lots of them are rated at 12 Volts, but can/will put out a good bit more voltage. If in question, I would at least double the voltage rating for the cap.
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Good luck, Jack

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Fireworks are banned here....


I'm ordering these now smiley

Fireworks are banned where I am too.  I just drive to Indiana or Wisconsin.  Anything goes there...  smiley-mr-green
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Tantalum haters aside (LOL). Did you check that 12 volt power supply with a volt meter. Lots of them are rated at 12 Volts, but can/will put out a good bit more voltage. If in question, I would at least double the voltage rating for the cap.


It's an Agilent power supply and 1), it says 12.0V.  Two, I had actually tested out the board for continuity, voltages at power pins, etc, before placing the more expensive parts.  I always do.  At that point the 12V rail was 12V and the 5V rail after the 5805 was 5V.  So no power issues there.  It ran for a good 5 minutes prior to fireworks.  Can a backwards cap survive that long before igniting?  I'd just test this myself but I don't want to end up with that smell throughout the house two nights in a row.   smiley-cry

Everyone says no Tantalum on power supplies, which I take it you think putting them on the 7805 is a bad idea.  What caps would you suggest?  I had a pair of 10uF tantalums that I provided a link for earlier in this thread.  What's the best bet?
« Last Edit: August 14, 2013, 07:26:31 pm by JoeN » Logged

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I ran for a good 5 minutes prior to fireworks.  Can a backwards cap survive that long before igniting?  
Yes.  When reverse biased, a value-metal cap actually breaks the oxide down.  Until the oxide is thin enough to cause a short, it won't break down.

Everyone says no Tantalum on power supplies
There is a big difference between Tantalum-MnO2 and Tantalum-Polymer.  Polymers are actually preferred for power supplies... Switching power supplies...

A 7805 isn't sensitive to ESR since it is a linear supply.  In fact, 10uF is well beyond what most manufactures recommend anyway.  You could easily get away with ceramics or aluminum Electrolytics, and they'll be cheaper.
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I ran for a good 5 minutes prior to fireworks.  Can a backwards cap survive that long before igniting?  
Yes.  When reverse biased, a value-metal cap actually breaks the oxide down.  Until the oxide is thin enough to cause a short, it won't break down.

Everyone says no Tantalum on power supplies
There is a big difference between Tantalum-MnO2 and Tantalum-Polymer.  Polymers are actually preferred for power supplies... Switching power supplies...

A 7805 isn't sensitive to ESR since it is a linear supply.  In fact, 10uF is well beyond what most manufactures recommend anyway.  You could easily get away with ceramics or aluminum Electrolytics, and they'll be cheaper.

I will be doing that from now on.  Will .1uF be enough for both sides of the 7805?  I have .1uF, .22uF, and .47uF multilayer ceramics.  But the only value I have in appreciable quantities at this moment is .1uF which I have at least a hundred of.  Maybe 10 each of the others in a kit.  I have aluminum electrolytics in 1uF and 10uF - open packs of 200 that I have only used a few of so far.  They are so big!  smiley-cry
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Will .1uF be enough for both sides of the 7805? 
Depends now what you are driving with the regulator and what the datasheet says...
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Quote
P.S.
I just remembered...  I've had a couple of regular electrolytics fail on a home project.    They were probably over-voltaged...  Maybe 1000uF/16V on a "nominally" 15V circuit.   One was on the positive power supply and one on the negative...  Since they both failed at the same time, obviously age had was also a factor.   The thing hadn't been powered-on for a few years, and when I turned it on I didn't see what happened because they were inside the enclosure.  It made an "expensive noise" (as my dad used to say smiley-grin ) and some really nasty smoke came out...  I had to go outside 'till the smoke dissipated.    (It wasn't actually an expensive failure....  I replaced the caps and it was good as new.)

If they were running that close to the rated voltage, they might have been OK if they'd been re-formed. IE, regrow the oxide layer.
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I've actually seen a picture in an electronics book of what happens when you insert a relatively large cap in backwards. Basically, it was on a breadboard, and the cap was gone, and part of the board was melted. If you want a project to look "hardcore" or something on a breadboard... Blow up a cap and get scorch marks on your breadboard!

(My company, its affiliates, me, or my dog are not responsible for any damage done from exploding capacitors on your breadboard. Use at your own risk. Actually, just don't blow up caps.)
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I've actually seen a picture in an electronics book of what happens when you insert a relatively large cap in backwards. Basically, it was on a breadboard, and the cap was gone, and part of the board was melted. If you want a project to look "hardcore" or something on a breadboard... Blow up a cap and get scorch marks on your breadboard!

(My company, its affiliates, me, or my dog are not responsible for any damage done from exploding capacitors on your breadboard. Use at your own risk. Actually, just don't blow up caps.)

Well I do have scorch marks on this board now.  Maybe the cap really was in backwards.  This is how I reworked it:



The way this was before was the connector in the rear, the two tantalum caps in the middle, the regulator in the front.  I ended up reworking it to this.  The regulator is on the side with the two new .33uF and .1 uF caps behind it.  There is also a 100uF cap on the connector to even out DC usage on the motor driver circuit which is not pictured.  The datasheet for the motor driver specifies that cap.  Maybe it should be closer to the actual H-bridge, but I put it here.

Burn marks are clearly visible.   smiley-lol
« Last Edit: August 24, 2014, 03:38:00 pm by JoeN » Logged

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I agree with James, new tantalum are far better than old ones! I've used a lot in my audio stuff for decoupling and power supply too (if you take a look of many linear technologies datasheet you will see many times tantalum are raccomended), never a problem, just be sure don't reverse polarity and stay well over max voltage rating (I use 35V ones on 15V supply), also avoid put too much near heatsink. In counterpart I got several problems with electrolitics, mostly Elna and prolly fakes buyed on ebay, they exploded by projecting the aluminium case as a bullet.
I changed a lot of old tantalium from some old audio stuff (mostly the band colored ones), they was all shorted, so I spent a fortune with desoldering stuff. A friend of mine told me that some old tantalium has a chemical problem than result in a secure short after a while so better change anithing that is older than 10 years.
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