17 Jul
2007
17 Jul
'07
2:57 p.m.
I'm trying to reverse engineer a board that I have and I've bumped into
a component that I don't understand the markings on. If you would like
to help, a picture can be found here:
http://brutman.com/2007_0717_124331.JPG
I suspect it is a capacitor, but the markings are strange to me. The
capacitor behind it has '104' written on it, so I understand that to be
a 0.1uF capacitor. But the one in the foreground marked '35+' is
strange to me ..
(My apologies in advance - I'm a software person, but I'm trainable.)
The identified capacitor and the suspect capacitor tie together two
ground plane, so I suspect it is for power smoothing.
Thanks in advance,
Mike
17 Jul
17 Jul
2:11 p.m.
Michael B. Brutman wrote:
I'm trying to reverse engineer a board that I have and I've bumped
into a component that I don't understand the markings on. If you
would like to help, a picture can be found here:
I suspect it is a capacitor, but the markings are strange to me. The
capacitor behind it has '104' written on it, so I understand that to
be a 0.1uF capacitor. But the one in the foreground marked '35+' is
strange to me ..
(My apologies in advance - I'm a software person, but I'm trainable.)
The identified capacitor and the suspect capacitor tie together two
ground plane, so I suspect it is for power smoothing.
My guess would be a 1uF / 35V tantalum capacitor, by the shape and the +
polarity marking.
2:11 p.m.
On 17 Jul 2007 at 12:57, Michael B. Brutman wrote:
I suspect it is a capacitor, but the markings are
strange to me. The
capacitor behind it has '104' written on it, so I understand that to be
a 0.1uF capacitor. But the one in the foreground marked '35+' is
strange to me ..
Looks like 1 mfd, 35 vdc to me--with the + identifying the positive
side.
Cheers,
Chuck
2:16 p.m.
"Michael B. Brutman" wrote:
I'm trying to reverse engineer a board that I have and I've bumped into
a component that I don't understand the markings on. If you would like
to help, a picture can be found here:
http://brutman.com/2007_0717_124331.JPG
I suspect it is a capacitor, but the markings are strange to me. The
capacitor behind it has '104' written on it, so I understand that to be
a 0.1uF capacitor. But the one in the foreground marked '35+' is
strange to me ..
(My apologies in advance - I'm a software person, but I'm trainable.)
The identified capacitor and the suspect capacitor tie together two
ground plane, so I suspect it is for power smoothing.
That's a "1" above the "35", so it's a 1uF, 35V tantalum
capacitor, the "+" is
indicating the positive lead.
4:21 p.m.
Ok, looks like that was too easy. The 35 threw me ..
The caps are connected from a power rail (+5) to ground. Is there a
good tutorial somewhere that explains how capacitors are used for
decoupling and smoothing? (I've checked some obvious sources, but I
need more detail, like if you connect a cap to a +5 source and ground,
why doesn't it just discharge completely ...)
Mike
3:54 p.m.
On 17/07/07, Michael B. Brutman <mbbrutman-cctalk at brutman.com> wrote:
Ok, looks like that was too easy. The 35 threw me ..
The caps are connected from a power rail (+5) to ground. Is there a
good tutorial somewhere that explains how capacitors are used for
decoupling and smoothing? (I've checked some obvious sources, but I
need more detail, like if you connect a cap to a +5 source and ground,
why doesn't it just discharge completely ...)
Mike
Get the cap as close to the chip as you can: think of the trace as a
(really low resistance) resistor: the resistance PS-to-cap is is much
larger than resistance cap-to-chip and bingo! low-pass filter.
Joe.
5:14 p.m.
Get the cap as close to the chip as you can: think of
the trace as a
(really low resistance) resistor: the resistance PS-to-cap is is much
larger than resistance cap-to-chip and bingo! low-pass filter.
Actually, it's the inductive reactance of the trace/PSU connections
that matters here. With high-frequency signals -- this doens't mean a
high clock rate necessarily, it means a fast ris-time edge -- you can
get significant voltage drop from very small 'inductors', like the
connction from the chip to the PSU.
-tony
7:16 p.m.
Tony Duell wrote:
I need some books and a double E degree. One more question ...
Some of what I'm reading says put the capacitor near the power supply,
and other things are saying get it as close to the chip as possible.
(With a few rules, I'm simplifying.) If the bypass capacitor is being
used to filer the AC component, wouldn't the answer for a good design be
"do both"?
The board I posted about is not optimal in this regard. It's an ISA bus
to PCjr bus adapter, so there is no TTL logic anywhere on the board -
the individual ISA cards have the TTL (and hopefully bypass capacitors).
In this application was the designer just trying to 'clean up' the
voltage sources for the TTL on the cards a little bit?
Mike
Get the cap as
close to the chip as you can: think of the trace as a
(really low resistance) resistor: the resistance PS-to-cap is is much
larger than resistance cap-to-chip and bingo! low-pass filter.
Actually, it's the inductive reactance of the trace/PSU connections
that matters here. With high-frequency signals -- this doens't mean a
high clock rate necessarily, it means a fast ris-time edge -- you can
get significant voltage drop from very small 'inductors', like the
connction from the chip to the PSU.
-tony
11 p.m.
Michael B. Brutman wrote:
Tony Duell wrote:
I need some books and a double E degree. One more question ...
Some of what I'm reading says put the capacitor near the power supply,
and other things are saying get it as close to the chip as possible.
(With a few rules, I'm simplifying.) If the bypass capacitor is being
used to filer the AC component, wouldn't the answer for a good design
be "do both"?
Tony is exactly correct; what degrades the performance of bypass
(decoupling) capacitors most directly is the series inductance of PCB
traces (and wires), not DC resistance. For good high frequency
performance, small bypass caps should be as close as possible to a
devices power/ground pins to minimize the inductance.
That being said, as you mention there is not just one function of bypass
capacitors in a design. Generally speaking, small bypass caps (eg,
10nF-100nF) across a devices power/ground pins serve the function of
tying both the DC power and ground rails together at high frequency,
shunting noise and providing a very low impedance path between the rails
for signal return currents.
Larger bulk capacitors (10uF and larger) provide short term charge
storage to handle the current transients of the loads that the power
supply cannot. Placement on the PCB of these larger bulk caps is usually
not critical, and most times is done near the power supply or its
connection to the boards (like near the fingers on DEC boards).
So to answer your question, usually 'both' are required; larger caps
near the power supply connection, and smaller caps per device
power-ground pin pair (as a general rule of thumb).
Get the
cap as close to the chip as you can: think of the trace as a
(really low resistance) resistor: the resistance PS-to-cap is is much
larger than resistance cap-to-chip and bingo! low-pass filter.
Actually, it's the inductive reactance of the trace/PSU connections
that matters here. With high-frequency signals -- this doens't mean a
high clock rate necessarily, it means a fast ris-time edge -- you can
get significant voltage drop from very small 'inductors', like the
connction from the chip to the PSU.
-tony
18 Jul
18 Jul
4:42 p.m.
New subject: dying capacitors Re: Help identifying a capacitor
Tony Duell wrote:
>> Get the cap as close to the chip as you can: think of the trace as a
>> (really low resistance) resistor: the resistance PS-to-cap is is much
>> larger than resistance cap-to-chip and bingo! low-pass filter.
>
> Actually, it's the inductive reactance of the trace/PSU connections
> that matters here. With high-frequency signals -- this doens't mean a
> high clock rate necessarily, it means a fast ris-time edge -- you can
> get significant voltage drop from very small 'inductors', like the
> connction from the chip to the PSU.
>
> -tony
In my 5 years of TV repair only one I needed your experts to figure
out and inprove the reliablity of these sets as they are premium
series.
I have a bad time with PARTICULAR switching power
supply design that likes to make a diode HOT also capacitor on that
output to self-heat as well. This makes frequent repairs that lasts
from 9 months to 2 years at the most on that capacitor.
Even this
capacitors is designed for high freq SMPSes.
This is usually found in JVC AV-xx980, 820, 985 series, more worse if
it is equipped with guide + module.
This SMPS is based around STR-F6626 IC. I don't fault this if it's
designed according to the Sanken suggested circuit design and I
really liked these STR-xxxxx series as they tend to gracefully shut
down without any blow ups. Only this JVC made this extremely
complicated and problematic when it finally give me problems like
this with hot diode and self-cooking capacitor.
On hot side, there is 2 extra circuits that
sanken sample circuits don't use: one regulator fed with DC from
SMPS transformer and there is extra circuit for the off/on standby
mode that ties into the error/feedback regulation pin on the STR IC.
This pin is also fed from the optisolator for the feedback from cold
side again complex too. Two circuits sharing this optisolator.
One is for 13V supply used for standby and supply for many low
voltage devices so it takes fair current but I doubt this is a issue.
And 135V error feedback circuit using SE135 IC that is disabled when
in standby.
Pretty complicated.
When new, JVC sets usually lasts several years then this C926 1000uF
35V finally gives out on that 13V standby/supply line and STR-F6626
shuts down in self-protection. Also this diode (around 3A RU3YX
diode) runs hot. Even hotter in standby! Same thing with capacitor,
it is not heated by that diode, the capacitor also self-heats but not
as severe. Eventually cap fails.
I have tried all kinds of capacitors, 105C, 85C, jellybean cap,
special cap, even low ESR, even high freq. They simply die only on
that 13V supply. Farther from this on same 13V supply is not a
problem. Even I resorted to replace this diode with TO-220 diode
with small heatsink, still get hot.
There is other cold side 3 outputs also, the two outputs are directly
fed by their seperate diodes and have no issues with heat, and 3rd is
via relay to a diode and a capacitor for the 135V supply for
horizontal circuit. Again, no issues with this.
Only the 13V capacitor/diode is the thorn!
There are plenty of makes that successfully used STR-F66xx series
with no issues but just this particular! Oh my.
Help?
Cheers, Wizard
6:39 p.m.
I need some books and a double E degree. One more
question ...
I have the former, but not the latter :-)
Some of what I'm reading says put the capacitor near the power supply,
and other things are saying get it as close to the chip as possible.
(With a few rules, I'm simplifying.) If the bypass capacitor is being
used to filer the AC component, wouldn't the answer for a good design be
"do both"?
YEs.. Typically a 0.1uF per chip (maybe per power-coonnection on a chip)
and a larger capacitor near the power input.
Remember that capcaitors have self-inductance, whci hcan be considerable
for some types. That's why you don't use aluminium electrolytics for
decoupling at the IS pins.
As I've said to many designers starting out '0.1uF capacitors are cheap,
my time in finding a nasty logic glitch isn't'. In other words, it's hard
to over-decouple, if in doubt, add the capacitor. Maybe if you're in
production you want to save a few cents on each PCB (but to be honest,
I'd ratehr may a few _bucks_ more per PCB and have something that works
properly), but not for experimentaly designs.
-tony
20 Jul
20 Jul
12:44 a.m.
Thanks for all of the help folks ...
As it happens, right after I started getting responses to my query on
the capacitor my email service went bonkers. The virtual host somewhere
out in cyberspace where my email is processed had a non-virtual hardware
problem, thus impacting my ability to read and reply. Things seem to be
back to normal now.
If anybody is interested I have a fairly good pinout for an ISA bus
adapter for a PCjr.
A friend of mine and I collaborated on a version in 2003 that 'almost'
worked for me, and I wasted a lot of time trying to figure out what the
problem was. Recently I decided to revisit it and after the last two
days I figured out that even though the capacitors are probably nice,
the thing that was holding me back was the lack of an OSC signal from
the Jr. The cards that I was using (SCSI cards) were dependent on it,
and we left that pin on the ISA slots not connected because we didn't
have that signal available. An NE1000 Ethernet card (which does not use
OSC) is working fine on the system so the adapter we designed is 'close
enough' even with the capacitors.
(The adapter with the capacitors where the picture came from was a
commercially available adapter that I was reverse engineering to try to
find out what the problem with our homebrew adapter was.)
Thanks again,
Mike
7:16 p.m.
A friend of mine and I collaborated on a version in
2003 that 'almost'
worked for me, and I wasted a lot of time trying to figure out what the
problem was. Recently I decided to revisit it and after the last two
days I figured out that even though the capacitors are probably nice,
the thing that was holding me back was the lack of an OSC signal from
The OSC signal was the 14.3xxx MHz master clcok signal in the PC and XT.
IBM specifically state in the Techref that it has no gauranteed pahse
relationship to any other signal. In the expansion unit, it's provided by
an oscillator circuit on the expansion backplane, in the PC/AT, it's
provided by a separate oscialltoer circuit (i.e. not in any way
associated with the CPU/system clock) on the motherboard.
It was used, of course, by the CGA card as the master clock for the video
and colour cubcarrier timing. Some other cards used it, often divided
down, as a master clock.
the Jr. The cards that I was using (SCSI cards) were
dependent on it,
and we left that pin on the ISA slots not connected because we didn't
have that signal available. An NE1000 Ethernet card (which does not use
I think I'd jsut ass one of those 4-pin oscillator cans and be done with it.
-tony
17 Jul
17 Jul
2:27 p.m.
Michael B. Brutman wrote:
That's a tantalum capacitor. 1uF, 35V, and the positive side is the one with
the line on it.
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