Re: Proper size of capacitors for transient suppression?

Tom:

A thought just occured to me. If a DC line ends at a relay and I put
a capacitor across the coil, won't that introduce a time delay which
could cause either lack of relay coordination and (also) increased
deterioration of contacts by arcing when the relay opens?

Jeff

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <thom@t...> wrote:
> > of the following types (ground is by a seperate #6 AWG Cu. bus):
> >
> > DC voltmeter leads
> > Connections to remote Enable relay coil
> > +5, +12, +24 volts DC lines
> > E-Stop contacts
> > MPG encoder leads
>
> Dc voltmeter use a 10mfd cap.
> Connections to remote enable relay if it's just driving a relay
coil
> don't work about it.
>
> All DC lines need 100mfd to 1000mfd caps a the end fartherist from
> the source.
>
> E-stop contacts. Hopefully this is back through an opto to give
you
> some noise immunity. Probably won't be a problem but a .1mfd cap
> would not hurt.
>
> MPG Encoder leads....now this one could be a problem. It's the
wires
> that should be shielded and isolated if possible (our good ole
friend
> the optoisolator. I would work to try and keep the grounds of the
PC
> and the input signals out of a loop condition.
>
> Your grounding approach should work (big honkin' ground lead to a
> common point ground). It is accepted practice to have signal
grounds
> and power grounds and to send voltage and signals down twisted
pairs
> or wires where one is a ground. Twisting wire tends to cancel out
> noise since the wires act like a winding on a transformer wire with
> bucking coils. The more twists per inch the better the noise
> rejection.
> >
> > Except for the MPG leads (not yet installed), the longest
distance
> of
> > any line is less than 10 feet (3 meters). I used #18 wire for
> > everything to keep the resistance to a minimum.
>
> Wire is funny stuff. At DC it looks like a resistor and resistance
> is an issue. #18 for your power runs and power grounds are good.
> For low voltage logic levels they create termination problems, and
> the bigger wire has different characteristics, like higher
> capacitance (yes, wires have capacitance, inductance, and
> resistance.) By far the biggest problem with getting low level
logic
> signals down a pair of wires is the capacitance. Since larger wire
> has more surface area (like a plate on a capacitor) bigger wires
> running alongside other wires tend to have more crosstalk (I say
tend
> because the spacing of the wires has to do with it as well as the
> frequency and rise times on the signals). In short You don't buy
> yourself anything by running low current, low voltage signals in
> bigger wire. Voltage loss is a factor of the raw DC resistance and
> the amount of current involved. At a few milliamps it takes a lot
of
> wire to have enough resisitance to matter. Take #26 wire. It has
> 43.6 ohms of resistance per 1000 ft (.044 ohms/ft). At 20ma the DC
> voltage drop would be .088 volts at 100ft. Think about your
phone.
> The actual signal on the wires is small and you can run that
miles.
> They use #20 or smaller for most runs. The run a "balanced pair"
> meaning that the pairs are twisted and not ground referenced. Any
> noise (and there can be volts of noise) is on both conductors and
> cancels out. If either of the wires get ground referenced you will
> get a loud "hum" on the line.
> >
> > The pendant also has the keyboard emulator embedded. I have
heard
> of
> > people connecting each terminal through a capacitor to ground (56
> > terminals? Ee Gads!)
>
> Treat the keyboard signals like its an extended keyboard to the
> computer. Shielded wires. If you have false triggering then
> consider making sure the DC power source for the keyboard is well
> filtered. The problem here is that the PC ground and keyboard
ground
> offer a ground loop opportunity. Since you already have a low
> impedence ground via your star grounding scheme the ground from the
> keyboard if connected to the PC ground might setup a ground loop
and
> be a noise conduit. Hopefully there is enough debounce in the
> signals to reject noise. If noise becomes an issue you will
probably
> have to buffer the signals back to the computer. I don't think the
> individual keys will need caps back into the chip as long as the
chip
> is close to the keys.
>
> Glad to see you are close to switching things on. We all await
your
> results!
>
> It sometimes counterproductive to try and determine where noise is
> going to occur. Take care of the obvious and deal with the
> circumstances as they arrise.
> >