[Johnthurston]

Can anyone explain to me the theory behind C2 (tantalum capacitor 10µF 25V) in the power supply section of the mini3?

The "back story" is, I have built a mini3 which sounds great, but while screwing around with my external power supply, I managed to cook C2 (pop!). Now I'm wondering if I have a suitable substitute in my pile of parts

John Thurston
Juneau, Alaska

 

[pixeljedi]

Quote:

Originally Posted by Johnthurston /img/forum/go_quote.gif Can anyone explain to me the theory behind C2 (tantalum capacitor 10µF 25V) in the power supply section of the mini3? The "back story" is, I have built a mini3 which sounds great, but while screwing around with my external power supply, I managed to cook C2 (pop!). Now I'm wondering if I have a suitable substitute in my pile of parts John Thurston Juneau, Alaska

PM me your address, I have a few extra on hand

 

[amb]

Most 3-terminal IC voltage regulators perform best with an input bypass capacitor located close to the regulator itself, particularly if the rectifier and bulk cap(s) are located far away. For the Mini³, these will typically be in a wallwart over a long length of wires.

All 78xx/79xx series datasheets show this cap in the "application" schematics but don't usually provide much explanation. STmicro's 7800/7900 datasheet is an exception and has a good description of this cap in section 6.1.

Usually a cap of around 0.22uF-0.47uF is shown in the examples, but I prefer to use a 10uF cap instead, and a tantalum cap has lower impedance at high frequencies than an electrolytic cap of similar basic specs, which is desirable for this purpose.

Be careful, though. Tantalum caps usually fail catastrophically when subjected to over-voltage or reverse polarity.

 

[diditmyself]

It seems to me that tantalums have a tendancy to blow even without being exposed to over-voltage or reverse polarity. I've used 10 uF 35 V and 25 V tantalums in many amps including Mini3. All of them have blown. None of them were exposed to reverse polarity and no voltages above 24 V (regulated). I'll never use tantalums again. I don't want to set the house on fire while charging my amps. Nowadays I use polyester caps instead.

 

[sandbasser]

The "crazy" (crazy in quotes

) thing about tantalums is that the "+" leg is marked not the "-" leg of polarized electrolytics. Leads to dummies like me putting them in backwards.

Luckily, very luckily, I've never had one blow. YET

 

[diditmyself]

That's another reason not to use them, and the high price is one more.

I have to admit that most of my blown tantalums were old surplus ones bought for cheap in a bag with mixed values, but the one in Mini3 was brand new.

 

[Johnthurston]

Quote:

Originally Posted by amb /img/forum/go_quote.gif All 78xx/79xx series datasheets show this cap in the "application" schematics but don't usually provide much explanation. STmicro's 7800/7900 datasheet is an exception and has a good description of this cap in section 6.1.

Ahh. That is a much better explanation than I have been able to find elsewhere. Thank you for the pointer.

Quote:

Usually a cap of around 0.22uF-0.47uF is shown in the examples, but I prefer to use a 10uF cap instead, and a tantalum cap has lower impedance at high frequencies than an electrolytic cap of similar basic specs, which is desirable for this purpose.

Is there anything wrong with substituting a 10uF electrolytic in its place? I'm more likely to have this in the bottom of my box than a tantalum.

Quote:

Be careful, though. Tantalum caps usually fail catastrophically when subjected to over-voltage or reverse polarity.

You don't say! Now that I have studied the board with my loupe, I can see the polarity marking on C2 and am willing to say that I done put it in backwards. No wonder all the smoke (and other stuff) came out.

John Thurston
Juneau, Alaska

 

[amb]

Quote:

Originally Posted by Johnthurston /img/forum/go_quote.gif Is there anything wrong with substituting a 10uF electrolytic in its place? I'm more likely to have this in the bottom of my box than a tantalum.

Of course an electrolytic would "work", but as I mentioned, a cap with lower high frequency impedance is preferable, as mentioned in the STmicro datasheet linked above.

 

[SiBurning]

Quote:

Originally Posted by amb /img/forum/go_quote.gif Be careful, though. Tantalum caps usually fail catastrophically when subjected to over-voltage or reverse polarity.

They short when they fail. If you're really lucky, it'll heat and explode quickly opening the circuit again before too much damage can occur elsewhere and before any fuse trips. That's if you're really lucky. (Note that the short will protect the mini3 electronics, if not the connectors, but that "protection" probably won't last long.)

But, yeah, they rock otherwise.

If you're the type, it might be fun to destroy a few on purpose at different voltages to get a feel for them. Wear eye protection. They blow chunks. At a buck a pop (pun intended), it's not exactly cheap entertainment. Paper electrolytics are a lot more fun to blow up, and cheaper.