Re: [CAD_CAM_EDM_DRO] Limit Switch Connectors

Hi Peter;

There are many connector choices available, and RCA jacks may work, but
they aren't a connector I ever selected for test fixtures. I'm not a
CNC guy (yet), but designed quite a bit of factory test gear in the
80's.

Several choices that come to mind include:

DB style connectors. Easy to get in 9, 15, and 25 pin configurations,
with 50+ pin versions aviailable. Easy to get hoods so your cables look
good and provide strain relief. Available with crimp on insertable pins
or fixed solder pot style. Screw jacks included, so you don't have to
worry about things coming loose at embarrassing times. If you don't
have to plug and unplug a lot, and only have to handle an Amp or two per
pin, these are really quite good if you buy a good grade. As you get
plenty of pins, you can keep shields, signal grounds, and signal lines
sorted out to meet you needs. You can get a pretty good hole in a
chassis for the chassis mounted connector via chain drilling and a bit
of file work if you don't want to spend the money for DB series chassis
punches - they aren't cheap new!

BNC: More work to install on the cable, and require coax and connector
body to match in size to be done right. Secure connections and easy to
plug/unplug. The good BNC connectors require care to install correctly,
see the ARRL handbook or some other book on RF construction for complete
instructions. You only have one signal pin with standard BNC, so the
shield and signal ground have to be joined at one end if you use a two
conductor shielded cable rather than coax.

Cannon style barrel connectors : Expensive, but a good way to get high
pin density in a small package with a gasketed bayonet locking ferrule.
Very strong and reliable connectors, used extensively on airborne
electronics.

Molex connectors: Sort of cheesy, the common style most folks have seen
is the 4 pin connector used for floppy/hard disk power connections in
PC. Cheap, easy to make up, chassis mount styles usually use springy
nylon ears to lock the connector to the chassis that don't always hold
securely.

DIN connectors: As used on older PC keyboards, the 5 pin connector that
is around 5/8 inch in diameter. Easy to get, not the greatest, but easy
to wire and mount. Hoods tend to be a flexible plastic shell with a
tight grommet that is just an extension of the hood. Mini styles as
used on newer keyboards, or the similar S-VHS connector seem hard to
find locally, I have no experience with them, as extension cables are
cheap to buy made up.

Jones plugs : The old (usually black) style connector often seen in OLD
radio chassis for power supply connections. The male is a number of
flat blades, the female chassis mount is a matching socket. Pretty good
for semi-permanent setups, although the pins are tinned rather than gold
plated. Most styles don't have jack screws, so connector body retention
is provided by the electrical connectors themselves. Hoods are either
supplied or available, so you can provide strain relief and a good
looking installation. As said above though, the final strain relief is
via connector holding force in most cases, rather than via the connector
body.

Point to Point soldering: Cheap, reliable if done with care to provide
strain relief. A pain to deal with when you want to change something
though!

Crimp on individual lugs in male/female pairs, sometimes called barrel
connectors or butt splice connectors. Easy to install, can be sealed
with self vulcanizing tape or heat shrink. Easy to get at most auto
parts stores in the electrical section. Try to find AMP or some other
name brand, the "Made in one or more of the following countries..."
parts vary from good to made of such poor materials that the metal
cracks when crimped. You can find these in two pin connector styles,
the ones I've seen are very much like a two pin molex style connector.

You have probably figured out by now that RCA jacks come in low on my
list. They have several defects, unless you purchase rather expensive
examples. Poor dielectric materials in easy to find units, poor
vibration resistance, open construction that makes contamination a
problem, weak physical design, contacts loose their grip resulting in
poor connections if plugged/unplugged repeatedly. The center conductor
on the female socket doesn't provide a good gas tight self wiping
connection in most versions as the gold "plate" (if any) is really a
flash for show rather than the 35 to 50 microinch plating thickness
needed for a gas tight self wiping contact interface. The nut that
secures the female jack also grounds the outer shell to the chassis,
mounting them on dielectric material to avoid this usually results in
the connector loosening over time. Just barely OK in the intended
application, and even then a common upgrade point in audio gear for good
reason. There are exceptions, but be ready to spend 10 to 15 dollars
per jack or socket to get good units, and that's if you luck into a
deal. The 6 for $2 RCA connectors at the local chain "electronics"
outlet are none too good. Really good ones are machined from solid
stock (usually brass), with a 50 microinch gold plate over nickel (I
think - it's been years), have teflon dielectric, well designed center
contacts, and cost the earth! 15 years ago you could drop $35 or more
per piece on male plugs. But they were really nice connectors!

If you want a cheap and easy approach without much work, you can always
use barrier strips. The twin screw style common these days (usually
white plastic with all parts contained rather than the old screw
terminal on bakelite style) allows you to mount the connector to the
outside of the chassis, pass the wires from one side of the strip into
the chassis through a grommet, and connect the outside world easily. I
would add one or more cable clamps to the chassis and run your incoming
cables through it/them to provide strain relief so thing don't get
pulled loose should you snag a cable. Radio Shack even stocks these
little gizmos, they aren't bad in the right situation.

Binding posts are still around too.

Another option might be 1/4 inch stereo or mono jacks as used on
unbalanced microphones, headphones, and electric guitars. The stereo
style will allow two signal lines and a ground. Avoid the really cheap
jacks and plugs, they don't have good vibration resistance or strong
retention of the plug. You could even use guitar patch cords to get
color coded cables with the connector already installed on shielded
cable. A good guitar or music store will have these, the ones Fender
makes are quite good. Since I play a Strat I might be a tad biased on
this :-) The jack on my guitar takes about 10 lbs of pull to
disconnect. Some of the jacks have a switch mechanism built in, perhaps
this could be wired to build in a failsafe, so if a limit switch isn't
connected the unit won't run? This switch mechanism is what disconnects
the speaker(s) when you plug in headphones on many amps. At least the
jacks would be easy to mount, just drill a hole! I never used these on
test equipment, but I've never had good examples come apart while
playing. They used to be used for the test leads on many vacuum tube
voltmeters (VTVMs), the predecessors to todays multimeter.

Hope this helps,
Stan

Peter Homann wrote:

>
> Hi,
>
> I am after advice on what sort of connectors do people use to connect the
> wires from limit switches to the controller unit.
>
> I am thinking of using RCA jacks. They are the ones used on a lot of audio
> equipment.
>
> Any comments.
>
> Cheers,
>
> Peter Homann
> <mailto:peter.homann@...>
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