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Question on small capacitors...

D

Dave

Jan 1, 1970
0
I hate to be stupid, but what is the difference (operationally) between a
1uF tantalum cap and a 1uF electrolytic (polarized) cap? Can a tantalum be
subbed for an electrolytic? Just wondering. I can get tantalums easily,
right down the street, but not electrolytics. For electrolytics I have to
catch a bus, then a train, then another bus, to go across town.

Thanks,

Dave
 
J

John Popelish

Jan 1, 1970
0
Dave said:
I hate to be stupid, but what is the difference (operationally) between a
1uF tantalum cap and a 1uF electrolytic (polarized) cap? Can a tantalum be
subbed for an electrolytic? Just wondering. I can get tantalums easily,
right down the street, but not electrolytics. For electrolytics I have to
catch a bus, then a train, then another bus, to go across town.

Thanks,

Dave
Tantalums are very good electrolytic caps with one weakness.
If you drive then up to full voltage too quickly, they
sometimes catch fire. Other than that, they are fine.
 
S

Spehro Pefhany

Jan 1, 1970
0
Tantalums are very good electrolytic caps with one weakness.
If you drive then up to full voltage too quickly, they
sometimes catch fire. Other than that, they are fine.

They generally won't catch fire if they explode first.


Best regards,
Spehro Pefhany
 
D

Dave

Jan 1, 1970
0
Winfield Hill said:
Which they love to do. Very inconvenient.

Huh. Thank you for the warnings. Further question: what if the 1uF
tantalum cap is rated at 35V, and being driven at only 9V? Would that make
them any safer?

Do appreciate the help.

Dave
 
E

Eeyore

Jan 1, 1970
0
Dave said:
Huh. Thank you for the warnings. Further question: what if the 1uF
tantalum cap is rated at 35V, and being driven at only 9V? Would that make
them any safer?

Probably a bit. The problem is the rate of rise of voltage. Tantalums fare best
when NOT driven from a low source impedance.

Why do you want to use one anyway and where in a circuit ? I'm puzzled that you
can get tantalums easily and not electrolytics. It's usually the other way
round.

Graham
 
J

John Fields

Jan 1, 1970
0
Probably a bit. The problem is the rate of rise of voltage. Tantalums fare best
when NOT driven from a low source impedance.

Why do you want to use one anyway and where in a circuit ? I'm puzzled that you
can get tantalums easily and not electrolytics. It's usually the other way
round.
 
J

John Popelish

Jan 1, 1970
0
Dave said:
Huh. Thank you for the warnings. Further question: what if the 1uF
tantalum cap is rated at 35V, and being driven at only 9V? Would that make
them any safer?

By quite a bit.
 
E

Eeyore

Jan 1, 1970
0
You're calling the OP a "dumb ass" btw. I simply followed his usage.

"I can get tantalums easily, right down the street, but not electrolytics."

Graham
 
W

Winfield Hill

Jan 1, 1970
0
Probably a bit. The problem is the rate of rise of voltage. Tantalums fare best
when NOT driven from a low source impedance.

Why do you want to use one anyway and where in a circuit ? I'm puzzled that you
can get tantalums easily and not electrolytics. It's usually the other way
round.

Graham

I'm not sure I agree with that. Most, if not all, of
the many failures we've seen were in slow-rate-of-rise
bypass applications, without high ripple currents or
anything else to explain the failures. 35-volt caps
failing when used as +5V cmos 74hc logic bypass with
a slow easy-going linear-mode supply. In an office
environment.
 
D

Dave

Jan 1, 1970
0
Eeyore said:
Probably a bit. The problem is the rate of rise of voltage. Tantalums fare
best
when NOT driven from a low source impedance.

Why do you want to use one anyway and where in a circuit ? I'm puzzled
that you
can get tantalums easily and not electrolytics. It's usually the other way
round.

Graham

Hey there, thanks for the input. I am trying to put together a
build-it-yourself set of instuctions, and the schematic I have come up with
calls for a 1uF electrolytic (didn't know that tantalums were electrolytic,
BTW). I am trying to fix it so the builder can get everything at his or her
local RadioShack, which seems to stock 1uF tantalums but not 1uF (standard)
electrolytics. If need be, I can change the schematic to use the 4.7uF
(standard) electrolytic they stock, but was wondering if I could just use
the tantalum. It is used as the C in the R/C part of a 555 timing circuit.
(555 operating in monostable multivibrator mode, I believe. Still working
on my understanding of that.) If you like, I can email you the schematic,
but I dont wish to post it as I am hoping to eventually sell it along with
the instructions for building it. The tantalum, BTW, will be driven by a 9V
battery, hooked between the negative terminal and the two resistors
(10K/47K) that make up the rest of the timing determinate for the 555. Just
realized I can post the 555 part of the circuit and leave the rest out, as
that is nothing special. Let me know if you want to see it, and I will post
it to abse.

Thanks again.

Dave
 
D

Dave

Jan 1, 1970
0
John Popelish said:
By quite a bit.

Thank you, John. I am still fairly new at all of this, and there is
obviously a lot I don't know...

Dave
 
J

John Larkin

Jan 1, 1970
0
By quite a bit.

Ratios like 3:1 seem to be pretty good. But tantalums are so erratic
that it's hard to draw hard conclusions.

A recent LTC datasheet claims that "tantalums don't explode when used
across the outputs of regulators" which I know to be nonsense... they
certainly do explode across the outputs of LM1117's, even derated 2:1.
The same LTC datasheet, as I recall, alludes to some other upcoming
appnote about tantalums.

Polymer tantalums are apparently better than the classic MnO2
detonators.

John
 
J

John Larkin

Jan 1, 1970
0
Thank you, John. I am still fairly new at all of this, and there is
obviously a lot I don't know...

There's a lot we all don't know. As you gain experience in electronic
design, you begin to not know at a much higher level.

Like Woody Allen said, "now that I'm famous, I'm strinking out with a
much better class of women."

John
 
R

Rich Grise

Jan 1, 1970
0
I hate to be stupid, but what is the difference (operationally) between
a 1uF tantalum cap and a 1uF electrolytic (polarized) cap? Can a
tantalum be subbed for an electrolytic? Just wondering. I can get
tantalums easily, right down the street, but not electrolytics. For
electrolytics I have to catch a bus, then a train, then another bus, to
go across town.

For what it's worth, I've never had a problem with tantalums. As far
as operationally, 1 uF is 1 uF. And polarized is polarized. :)

Personally, I suspect that the people who blow up tantalums did it
because they'd installed them backwards, then blame the tantalum
so they don't get caught being clumsy. ;-)

IOW, go for it.

Cheers!
Rich
 
R

Rich Grise

Jan 1, 1970
0
Which they love to do. Very inconvenient.

What in the _world_ do you guys do to your capacitors? In 40 years,
(or however long there have been tantalum caps), I've _never_ seen
one explode or burn.

I think you guys are installing them backwards, and are too embarrassed
to admit it. ;-)

Cheers!
Rich
 
J

John Fields

Jan 1, 1970
0
You're calling the OP a "dumb ass" btw.

---
Not at all, since he was ignorant of the fact and asking for help
as, obviously, a novice. You, on the other hand, professing to be
knowledgeable of these things should have corrected him graciously
and sent him on his way somewhat better informed.
 
J

John Fields

Jan 1, 1970
0
Hey there, thanks for the input. I am trying to put together a
build-it-yourself set of instuctions, and the schematic I have come up with
calls for a 1uF electrolytic (didn't know that tantalums were electrolytic,
BTW). I am trying to fix it so the builder can get everything at his or her
local RadioShack, which seems to stock 1uF tantalums but not 1uF (standard)
electrolytics. If need be, I can change the schematic to use the 4.7uF
(standard) electrolytic they stock, but was wondering if I could just use
the tantalum. It is used as the C in the R/C part of a 555 timing circuit.
(555 operating in monostable multivibrator mode, I believe. Still working
on my understanding of that.) If you like, I can email you the schematic,
but I dont wish to post it as I am hoping to eventually sell it along with
the instructions for building it. The tantalum, BTW, will be driven by a 9V
battery, hooked between the negative terminal and the two resistors
(10K/47K) that make up the rest of the timing determinate for the 555. Just
realized I can post the 555 part of the circuit and leave the rest out, as
that is nothing special. Let me know if you want to see it, and I will post
it to abse.

---
If you're using two resistors like this: (View in Courier)

VCC>--+-------------------------+
| |
[Ra] +---------+ |
| 7|_ |8 |
+-----------O|D Vcc|--+
| 6| _|4 |
+--[Rb]-+----|TH R|O-+
| 2|__ |3 |
+---O|TR OUT|--|-->
| | GND | |
[C] +----+----+ |
| 1| [0.1µF]
| | |
GND>----------+---------+-------+

then the circuit is an astable (not a monostable) multivibrator and
Ra and Rb are the resistors you describe.

Since the value your 1µF capacitor is pretty small, I suggest you
substitute a 0.47µF polyester (Mylar) capacitor and double the
values of your resistors. This will cut the leakage current through
the cap to virtually zero and since most aluminum electrolytics have
tolerances of either +/-20% or -20 +80%, your 10% Mylar will give
you better timing accuracy right off the bat, if that matters.
Also, polyester capacitors are just inherently better capacitors
than electrolytics of _any_ sort, so you'll have a better product.
 
Q

qrk

Jan 1, 1970
0
What in the _world_ do you guys do to your capacitors? In 40 years,
(or however long there have been tantalum caps), I've _never_ seen
one explode or burn.

I think you guys are installing them backwards, and are too embarrassed
to admit it. ;-)

Cheers!
Rich

Boy, I wish I had your touch! One summer was spent repairing a bunch
of circuit boards where most of the failures were tantalum caps on the
power supplies. Of course, they were operating just below their rated
voltage. Ahh, fond memories of college years. Now days, I'm finding
bulging Al electrolytics leaking their guts in consumer products.
Icky!!
 
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