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Old 10th April 2003, 02:30 PM   #1
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Default What is this capacitor for?

I'm about to build a gainclone, and I don't have that cap ( encircled in the schematic), so could I use the caps in the picture, 100V, 2.2 microF?

I do have 10 microF caps, but their only 25 Volts rated, so they might blow up if I apply +- 30 V to the circuit right?

/Christian
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Old 10th April 2003, 02:42 PM   #2
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Default The DC blocking...

Quote:
I'm about to build a gainclone, and I don't have that cap ( encircled in the schematic), so could I use the caps in the picture, 100V, 2.2 microF?
Quote:
I do have 10 microF caps, but their only 25 Volts rated, so they might blow up if I apply +- 30 V to the circuit right
This capacitor is for the amp to have at DC a low gain (1)..and don't amplifie any DC that can appear at the input..

You can use your 10uF 25 Volts capacitor without fear..

regards
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Old 10th April 2003, 04:52 PM   #3
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You can use your 10uF 25 Volts capacitor without fear..



I would suggest that you check that it's a non-polarised cap , since that cap will see an AC signal in this circuit and polarised caps tend not to like reverse polarities.....

small point, but I felt it worth mentioning.

ray
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Old 10th April 2003, 05:09 PM   #4
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One may also point out that in case of errors in the circuitry
the output may in the worst case have a DC voltage equal
to one of the rail voltages, which will also be the voltage over
the capacitor. The capacitor is not rated for this situation. On
the other hand, there are probably more serious things to
worry about than en exploding capacitor in that case. As
somebody else pointed out, however, if it is a polarized
electrolytic cap. even a few volts DC on the output may
kill it. However, this situation should not arise either in a
working amplifier.
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Old 10th April 2003, 05:29 PM   #5
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Default The explosive capacitor...

Quote:
One may also point out that in case of errors in the circuitry
Quote:
the output may in the worst case have a DC voltage equal
Quote:
to one of the rail voltages, which will also be the voltage over
Quote:
the capacitor. The capacitor is not rated for this situation.
In that case the capacitor will see a diference of potencial between his ratings 25 volts and the rail (30 volts) of 5 volts...
this 5 volts across the 1k Ohm resistor can have a maximal current of 5 mA...so the cap will not explode...

But of course to use a 35 volts or more rated capacitor doesn't hurt!!

Quote:
if it is a polarized
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electrolytic cap. even a few volts DC on the output may
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kill it.
That i don't understand...why a few DC volts in a polarised electrolitc capacitor.... can kill it???
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Old 10th April 2003, 05:32 PM   #6
tiroth is offline tiroth  United States
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Because it could be +rail or -rail. Reverse biased.
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Old 10th April 2003, 05:37 PM   #7
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Default Re: The explosive capacitor...

Quote:
Originally posted by Tube_Dude


In that case the capacitor will see a diference of potencial between his ratings 25 volts and the rail (30 volts) of 5 volts...
this 5 volts across the 1k Ohm resistor can have a maximal current of 5 mA...so the cap will not explode...
I don't what strange theory you are applying here? If you have
30V DC at the output, you will get 30V DC over the capacitor.
You are right, however, that the resistors will limit the current
so it will take some time before the capacitor is charged to
30V. It is also true that 30V will probably not be enough to
make a 25V capacitor explode, but if it is an electrolytic it doesn't
hurt to be on the safe side.

Quote:

That a don't understand...why a few DC volts in a polarised electrolitc capacitor.... can kill it???
If the DC has wrong polarity with respect to the capacitor.
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Old 10th April 2003, 05:57 PM   #8
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Default The voltage ...or the current...

Quote:
Because it could be +rail or -rail. Reverse biased.
But what cause a capacitor destruction is the current across it and the consequent eating of the dielectric...in the case of reverse biased with 30 volts DC at the output...the reverse current will be +- 1,5mA ...(30V across 21K)...not much!

The voltage rating of a capacitor must not be exceded expecialy when conected to a voltage source...because this will cause a high DC current thru the capacitor that will heat it...but when the capacitor is feed with a resistor...this ratings become less stringent...expecialy the danger of explosion of the capacitor!!!

But as i have said...use a 35 volts if that gives better sleep...
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Old 10th April 2003, 06:36 PM   #9
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Default Re: The voltage ...or the current...

Quote:
Originally posted by Tube_Dude


But what cause a capacitor destruction is the current across it and the consequent eating of the dielectric...in the case of reverse biased with 30 volts DC at the output...the reverse current will be +- 1,5mA ...(30V across 21K)...not much!
You are neglecting that the capacitor gets charged and the
voltage over it will raise to 30V eventually. The exception
would be if the leakage current is so big that the current
through the resistors cannot charge the capacitor to more
than the rated voltage (25V)., or if it breaks down before
reaching 30V. As far as I Know, a capacitor breaks down
when the internal voltage becomes too high, so you get sparks
between the plates, damaging the dielectricum. It thus has
nothing to do with charge currents.

Anyway, we are probably being overly theoretical here. A
25V capacitor is probably quite OK in practice, since it will
not take any high voltages under normal operation.
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Old 10th April 2003, 07:02 PM   #10
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Default When the theorical meet....

Quote:
Anyway, we are probably being overly theoretical here. A
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25V capacitor is probably quite OK in practice, since it will
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not take any high voltages under normal operation.
I 100% agree with you.!!!

Anyway this is what i said i my first post...

Cheers!

PS: I have seen some japonese constructors even with +-45 volts rails use 25 Volts rated elect. capacitors in series with the lower arm resistor of the feedback voltage divider...maybe they are pushing their luck to far!!
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