Small capacitor across a big capacitor.

D

David Farber

Guest
I'm servicing a JBL S400 amplifier. There was a shorted capacitor in the power
supply. A .05uF 250V cap was the culprit. My question is that this cap is in
parallel with a 15,000uF 80V cap. What's the point of this configuration? I
think this subject might have been discussed before but I don't remember the
reasons you would design a circuit like this.

Thanks for your reply.

David Farber
David Farber's Service Center
L.A., CA
 
"David Farber" <farberbear@aol.com> wrote in message
news:20030821140919.06077.00001987@mb-m03.aol.com...
I'm servicing a JBL S400 amplifier. There was a shorted capacitor in the
power
supply. A .05uF 250V cap was the culprit. My question is that this cap is
in
parallel with a 15,000uF 80V cap. What's the point of this configuration?
I
think this subject might have been discussed before but I don't remember
the
reasons you would design a circuit like this.

Thanks for your reply.
Click to expand...
As some say in the RF business, one for the mouse and one for the rat.
Large capacitors are not as effective for bypassing higher frequencies
(stray inductance, self resonance, and so on).
 
This is done for high frequency noise bypassing. I know that in
theory the large cap is huge compared to the small one, and when the
direct analysis is done, it is like "WHAT!". But, the internal
resistance of the lead ins from the large cap is enough to allow high
frequency noise to not be supressed.

You just have to put back the small cap, and things will be safe and
of proper specs. Any small cap from about 0.047 to about 0.1 uF will
work. Make sure that the voltage rating of the cap is the same or
higher.

Jerry Greenberg
http://www.zoom-one.com

--

farberbear@aol.com (David Farber) wrote in message news:<20030821140919.06077.00001987@mb-m03.aol.com>...
I'm servicing a JBL S400 amplifier. There was a shorted capacitor in the power
supply. A .05uF 250V cap was the culprit. My question is that this cap is in
parallel with a 15,000uF 80V cap. What's the point of this configuration? I
think this subject might have been discussed before but I don't remember the
reasons you would design a circuit like this.

Thanks for your reply.

David Farber
David Farber's Service Center
L.A., CA
Click to expand...
 
The mouse bit the dust!
Rat is still in business....
;-)


"Jerry Greenberg" <jerryg50@hotmail.com> wrote in message
news:460a833b.0308211306.7d0e3498@posting.google.com...
This is done for high frequency noise bypassing. I know that in
theory the large cap is huge compared to the small one, and when the
direct analysis is done, it is like "WHAT!". But, the internal
resistance of the lead ins from the large cap is enough to allow high
frequency noise to not be supressed.

You just have to put back the small cap, and things will be safe and
of proper specs. Any small cap from about 0.047 to about 0.1 uF will
work. Make sure that the voltage rating of the cap is the same or
higher.

Jerry Greenberg
http://www.zoom-one.com

--

farberbear@aol.com (David Farber) wrote in message
news:<20030821140919.06077.00001987@mb-m03.aol.com>...
I'm servicing a JBL S400 amplifier. There was a shorted capacitor in the
power
supply. A .05uF 250V cap was the culprit. My question is that this cap
is in
parallel with a 15,000uF 80V cap. What's the point of this
configuration? I
think this subject might have been discussed before but I don't remember
the
reasons you would design a circuit like this.

Thanks for your reply.

David Farber
David Farber's Service Center
L.A., CA
Click to expand...
 
"Jerry Greenberg" bravely wrote to "All" (21 Aug 03 14:06:47)
--- on the heady topic of "Re: Small capacitor across a big capacitor."

Isn't anyone just the least bit curious why a small 0,05uF cap rated at
"250 volts" on an 80V circuit went and shorted out in the first place?!

JG> From: jerryg50@hotmail.com (Jerry Greenberg)

JG> This is done for high frequency noise bypassing. I know that in
JG> theory the large cap is huge compared to the small one, and when the
JG> direct analysis is done, it is like "WHAT!". But, the internal
JG> resistance of the lead ins from the large cap is enough to allow high
JG> frequency noise to not be supressed.

JG> You just have to put back the small cap, and things will be safe and
JG> of proper specs. Any small cap from about 0.047 to about 0.1 uF will
JG> work. Make sure that the voltage rating of the cap is the same or
JG> higher.

JG> Jerry Greenberg
JG> http://www.zoom-one.com

JG> --

JG> farberbear@aol.com (David Farber) wrote in message
JG> news:<20030821140919.06077.00001987@mb-m03.aol.com>... > I'm servicing
JG> a JBL S400 amplifier. There was a shorted capacitor in the power >
JG> supply. A .05uF 250V cap was the culprit. My question is that this cap
JG> is in > parallel with a 15,000uF 80V cap. What's the point of this
configuration?
I > think this subject might have been discussed before but I don't remember
the
reasons you would design a circuit like this.

Thanks for your reply.

David Farber
David Farber's Service Center
L.A., CA
Click to expand...
.... Techs would rather pee on an electric fence for the light show
 
Subject: Re: Small capacitor across a big capacitor.
From: Rich Webb bbew.ar@mapson.nozirev.ten
Date: 08/21/2003 6:55 PM Pacific Daylight Time
Message-id: <u4takvktpfk9kldq2m2nfshs0hp1edu4v3@4ax.com

On 21 Aug 2003 18:09:19 GMT, farberbear@aol.com (David Farber) wrote:

I'm servicing a JBL S400 amplifier. There was a shorted capacitor in the
power
supply. A .05uF 250V cap was the culprit. My question is that this cap is in
parallel with a 15,000uF 80V cap. What's the point of this configuration? I
think this subject might have been discussed before but I don't remember the
reasons you would design a circuit like this.

Capacitors are not "pure" devices and their capacitance varies with what
you throw at them. Some materials and configurations are better (more
capacitance for a given size) at low frequencies but the point where
they change their characteristics and become inductive instead of
capacitive (their self resonance freq) is also lower. The .05 guy was
there to take care of the high frequency noise.

--
Rich Webb Norfolk, VA
Click to expand...
That makes good sense to me.

Thanks for your reply.

David Farber
David Farber's Service Center
L.A., CA
 
Sometimes caps just fail. I assume you're suggesting that the cap
failure is indicative of a bigger problem, and you may be right... or it
may have just failed.

Sam



Asimov wrote:
"Jerry Greenberg" bravely wrote to "All" (21 Aug 03 14:06:47)
--- on the heady topic of "Re: Small capacitor across a big capacitor."

Isn't anyone just the least bit curious why a small 0,05uF cap rated at
"250 volts" on an 80V circuit went and shorted out in the first place?!
Click to expand...
 
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