Re: Proper size of capacitors for transient suppression?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "washcomp" <jeff@w...> wrote:

> I've decided to "beef up" the transient suppresion on my cabling.
> I'm trying to pick the proper size capacitor to use between

the "hot"

> leg and ground. I've seen people use .1 mf and Gecko uses 470 mf
> across their terminals. Thats a hell of a range. Also Gecko
> recommends using if the cable is longer than 18 inches (.5 meter).
> They show the cactior next to the active equipment. My original NC
> controller had capacitors in series with resistors for this funcion
> on many of the longer lines.
>
> ANY RECOMMENDATIONS OF HOW TO SORT THIS OUT WOULD BE APPRECIATED
>
> Jeff

Jeff: It all depends on the type signal you are trying to suppress
and the function of the line. Power lines (DC) of any length over
the listed 18 inches, should have larger electrolytics on the far
end. The larger the cap value the lower the reactance (ac
resistance) to a given frequency. A cap from the line to ground acts
as a "shunt" to decouple frequencies off the line. The larger the cap
value the lower the frequency it will shunt to ground. The large
caps tend to shunt higher frequencies down to a "roll off" point.
(Most larger electrolytics have charactersitics that cause them to
have high frequency roll-off as well which is sometimes you will see
smaller caps in parallel with the big ones). On a DC line all you
want is pure DC so the large caps react to everything down to even
60HZ. A large value cap on a signal line shunts the signal itself to
ground (bad thing!). The trick is to pick a value that gets rid of
noise above the base signal but has little effect on the signal
frequency itself. You will see values of .01 to .1 depending on the
signal. RF caps are a factor lower in value and can be in the
picofarad range.
Combining a series resistor with a cap to ground creates an RC filter
that has different characteristics than just a cap and the
transmission line impedence. If you use to large a value for the
resistor the signal level will be effected (depending on the source
and destination impedence). Most logic level stuff uses 100 ohms or
less.

Decoupling caps on your logic at the pendant end are important.
*Each* chip needs a .1 MFD from the + pin VCC to a close ground.
This decouples chip switching noise from getting into the local low
voltage supply and effecting the other chips.

Another good suppression technique is the use of ferrite cores.
Typically the larger the core the lower the frequency it will block.
The magnetic material of the ferrite "absorbs" the frequencies and
has a better roll off characteristic than simple caps. For power
lines the bigger donut cores are good.

There is a whole section of electronics involved with RC and LC
filters. You can using various configurations build Low pass filters
(pass everything BELOW the roll off frequency), bandpass filters that
pass a given range of frequencies and reject lower and higher
frequencies, and high pass filters that reject lower frequencies.
There are entire books devoted to the design and use of filter
networks. One of the issues with dealing with logic level signals is
that they run a pretty wide range of signals all the way from a
single pulse per second up to several thousand in the IO circuits.
You have to be able to pass the highest control signal but suppress
the noise. Noise most often takes the form of higher frequecy
impulse spikes that can be suppressed with smaller values of caps.

Beware that caps conduct noise in either direction so if the noise is
actually on the ground it can inject it into the circuitry and make
things worse!

Are you having actual noise problems or is this just a level of
paranoia? Put the DC electrolytics on the power lines and use the
small bypass caps at the logic chips. Only start adding RC to signal
lines if you actually need it. Noise will most often manifest itself
as random problems...things that are hard to predict or duplicate.
It can be difficult to troubleshoot under those circumstances.

BTW transients are ususally fast rising edge spikes of voltage
created from a component or switch being turned off and the stored
energy in a circuit trying to disssipate as rapidly as possible.
Large inductive loads generate nasty transients that can exceeded the
ratings of semiconductors by many times...never mind they only last
for a few nanoseconds. Snubbers (typically a resistor in series with
a cap) gives it a path to ground. I only add this because you have a
switching PS in your remote controller and could be a source for some
noise. Make sure you have an AC input filter on the AC of the
switching PS.