Decoupling capacitor

[Ziguy]

Hi,

I currently design an small electronic circuit. The circuit is mainly a 5V
regulator and an accelerometer connected to a comparator (LM393) that drive
a MOSFET switch. Do I need decoupling capacitors for the accelerometer and
the comparator? I thought that maybe just one 0.1 uF capacitor after the
regulator would do the job... By the way, this is a automotive application,
the output of the accelerometer is filtered to 1Hz and the board has about 1
square inch.

Thanks

 

[John Larkin]

On Thu, 24 Jul 2003 12:00:21 -0400, "Ziguy"
<robertlatour54@hotmail.NOSPAM.com> wrote:

Hi,

I currently design an small electronic circuit. The circuit is mainly a 5V
regulator and an accelerometer connected to a comparator (LM393) that drive
a MOSFET switch. Do I need decoupling capacitors for the accelerometer and
the comparator? I thought that maybe just one 0.1 uF capacitor after the
regulator would do the job... By the way, this is a automotive application,
the output of the accelerometer is filtered to 1Hz and the board has about 1
square inch.

Thanks

One cap should be OK for a small thing like this, but be aware that
many regulators will oscillate if they don't have enough output
capacitance. See the reg datasheet.

Other posters will give you the three-bears line: big cap plus medium
cap plus small cap. Ignore them.

John

 

[John Popelish]

Ziguy wrote:

Hi,

I currently design an small electronic circuit. The circuit is mainly a 5V
regulator and an accelerometer connected to a comparator (LM393) that drive
a MOSFET switch. Do I need decoupling capacitors for the accelerometer and
the comparator? I thought that maybe just one 0.1 uF capacitor after the
regulator would do the job... By the way, this is a automotive application,
the output of the accelerometer is filtered to 1Hz and the board has about 1
square inch.

Thanks

If there is no other sources of noise being powered from this
regulator, there is little need for lots of capacitance on the 5 volt
bus. Your idea of .1 uf sounds pretty good. The bigger problem will
be noise from the 12 volt side that gets past the regulator. I would
worry more about what you do about the + or - 50 volt spikes that will
occasionally appear there. Perhaps, a diode to prevent reverse
voltages from getting to the regulator follower by an RC or LC
filter. This capacitor should be more like 10 or 100 uf in parallel
with a .1 uf film or ceramic cpacitor.

And make sure yout 1 second low pass filter is referenced to exactly
the same node as the reference voltage it is compared against.

--
John Popelish

 

[Ziguy]

Before the regulator, I put in series a diode and a resistor, and in
parallel a zener, a 100uF and a 0.33uF tantalium capacitors.

Later,

"John Popelish" <jpopelish@rica.net> a écrit dans le message de
news:3F201018.D1A69098@rica.net...

Ziguy wrote:

Hi,

I currently design an small electronic circuit. The circuit is mainly a
5V
regulator and an accelerometer connected to a comparator (LM393) that
drive
a MOSFET switch. Do I need decoupling capacitors for the accelerometer
and
the comparator? I thought that maybe just one 0.1 uF capacitor after the
regulator would do the job... By the way, this is a automotive
application,
the output of the accelerometer is filtered to 1Hz and the board has
about 1
square inch.

Thanks

If there is no other sources of noise being powered from this
regulator, there is little need for lots of capacitance on the 5 volt
bus. Your idea of .1 uf sounds pretty good. The bigger problem will
be noise from the 12 volt side that gets past the regulator. I would
worry more about what you do about the + or - 50 volt spikes that will
occasionally appear there. Perhaps, a diode to prevent reverse
voltages from getting to the regulator follower by an RC or LC
filter. This capacitor should be more like 10 or 100 uf in parallel
with a .1 uf film or ceramic cpacitor.

And make sure yout 1 second low pass filter is referenced to exactly
the same node as the reference voltage it is compared against.

--
John Popelish

 

[John Larkin]

On Thu, 24 Jul 2003 14:41:35 -0400, "Ziguy"
<robertlatour54@hotmail.NOSPAM.com> wrote:

Before the regulator, I put in series a diode and a resistor, and in
parallel a zener, a 100uF and a 0.33uF tantalium capacitors.

I'd advise against the tantalum. They're expensive and have a bad
habit of exploding when used across supply rails. In a car, where
serious overvoltage spikes are likely, tantalums will likely die. A
100 uF aluminum cap should be enough.

I recently did a strain-gauge signal conditioner with eight excitation
supplies, and foolishly bypassed each regulator output with tantalums.
Even there, it had enough peak current to blow the caps occasionally.
We had to replace them with aluminums, and fortunately found some that
fit the tantalum surface-mount footprint.

John

 

[Ziguy]

Do you suggest me to forget the 0.33uF cap at all? Not even a 0.33uF ceramic
cap! Are they not needed for high frequency noise?

Thanks again,

"John Larkin" <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> a écrit dans
le message de news:j1d0ivob32qhi3ssb6cm3snoc58fd6u1k4@4ax.com...

On Thu, 24 Jul 2003 14:41:35 -0400, "Ziguy"
robertlatour54@hotmail.NOSPAM.com> wrote:

Before the regulator, I put in series a diode and a resistor, and in
parallel a zener, a 100uF and a 0.33uF tantalium capacitors.



I'd advise against the tantalum. They're expensive and have a bad
habit of exploding when used across supply rails. In a car, where
serious overvoltage spikes are likely, tantalums will likely die. A
100 uF aluminum cap should be enough.

I recently did a strain-gauge signal conditioner with eight excitation
supplies, and foolishly bypassed each regulator output with tantalums.
Even there, it had enough peak current to blow the caps occasionally.
We had to replace them with aluminums, and fortunately found some that
fit the tantalum surface-mount footprint.

John

 

[Dan Fraser]

No matter what, put a .01 ceramic disk cap across every IC, as close to
the pins as you can get. I have seen too many strange things happen
without them that it is cheap insurance against power line induced
noise.

I'll spend the extra 5 cents per chip so I don't get hassled a year from
now and have to retrofit a pile of units in the field.

--
Dan Fraser

From Costa Mesa in sunny California
949-631-7535 Cell 714-420-7535

Check out my electronic schematics site at:
http://www.schematicsforfree.com
If you are into cars check out www.roadsters.com

 

[Cees Keyer]

Dan Fraser wrote:

No matter what, put a .01 ceramic disk cap across every IC, as close to
the pins as you can get. I have seen too many strange things happen
without them that it is cheap insurance against power line induced
noise.

I'll spend the extra 5 cents per chip so I don't get hassled a year from
now and have to retrofit a pile of units in the field.

--
Dan Fraser

From Costa Mesa in sunny California
949-631-7535 Cell 714-420-7535

Check out my electronic schematics site at:
http://www.schematicsforfree.com
If you are into cars check out www.roadsters.com


The small cap mentioned above is a inrush C, the switching action

(charging mos transistors) in the i.c. needs for a short time a lot of
charge (current, due to the impedance of the power supply lines on the
board you'll see a dip in the voltage, which might result in a reset of
a piece of the circuitry. The small ceramic C will solve this problem.

The problem with 78 or 79 series regulators is that they are quite noisy
over a large piece of the spectrum up to 30MHz or even higher.
A small 10nF ceramic capicitor close to the regulator, i.e. on the
"legs" of it will solve this.
It's good engineering practice to put small ceramic 10nF C close to non
switching regulators both on input and output.

In a car a load dump might occur which takes the voltage on the 12 V
supply upwards to 80 volts or more for a few seconds.
And the spikes from the ignotion are anoying as well.

Cees.

--
# Javascript is a powerful language, except it mutates faster than a
# fruit fly in a X-ray machine (Andrew TanenBaum)
# Cees Keyer, HVA dept I&E, Weesperzijde 190, 1097DZ Amsterdam, Holland
# c.h.a.keijer@ie.hva.nl Voice: (+31)20-5951639, Fax: (+31)20-5951620