Ok to groud LED's with shielding ?

[Jackal66]

I am connecting some external LED's to my PMDX board.

Power on, outputs enabled, E-stop, & Step activity.

The power to these is 6ma.

My shielded wire that I want to use has (4) conductors. I need a 5th wire for ground.

Would it be ok to use the shield for grounding the LED's back to the B_O_B terminal?

Or, should I run another single wire for a dedicated ground?

Thanks,

JAckal

[Al_The_Man]

By ground do you mean an earth ground or the PS common?
If E.G., do you need this on LED's?
Otherwise if you have the PS common, then all you need do is earth ground your shield.
But it would defeat the object if you used the shield as a P.S. common. (run a separate conductor).
Al.

[doorknob]

The way that I interpreted this (which could be incorrect) was that the OP wants to mount 4 LEDs some distance away from the BOB, and has a 4-conductor shielded cable that he intends to use for the purpose.

So, I believe that the OP's proposal is to use the 4 wires such that each wire connects to the anode of a different LED, with all of the LED cathodes tied together to the shield, in order to return the current to the GND terminal of the BOB.

While the OP has not stated the length of cable that he is contemplating using, my thought is that it should work just fine.

While the current carried through the shield will vary depending on how many of the LEDs are turned on at any given instant, the expected low resistance of the shield return should mean that the variation in the potential where the cathodes join together (due to I2R losses in the shield) should be so little as to be considered to be negligible. You're talking about at most 24 mA through probably no more than a fraction of an ohm, which if I've done my math correctly, is a millivolt or less...

[Jackal66]

Sorry for the confusion, Al.

I'm not real good with terminology.

Here is a pic:



I'm needing to just ground the LED's to the PC power supply.

My wire is 4 conductor and, since it doesn't need to be shielded, I was gonna cheat and use the shield for the ground from the LED's to the break out board.

Thanks for the help,

JAckal

[Jackal66]

Doorknob,

You have it right. I was typing & drawing while you answered.

According to the manual on this PMDX 126- B_O_B board, it has 6 mA output and they recommend using high intensity LED's.

There isn't a limit listed on length. My wires will probably be around 6 foot long.

If this is a bad idea, let me know and a separate wire can be run for the ground.

Thanks for the help,

JAckal

[doorknob]

I expect that it will work just fine the way that you are planning to do it.

[Al_The_Man]

You most likely can get away with it, although the concern I would have is what type shielding is it?
There are basically four types, foil only, foil with drain wire, braided, and braided with drain wire.
The optimum and easiest would be the pair with the drain, and make sure the shield does not contact earth ground, otherwise you may end up with using the earth ground as a conductor which is not good.
Especially as you mentioned using the PC power supply which is at earth ground potential already.
Al.

[Jackal66]

This wire has the tough foil that is blue colored inside the foil.

Instead of a braid ( like a co-axial cable), it has just one wire running down the side between the foil and the sheath.

Would this be the drain wire type?

My thought was to use very small shrink wrap on the shield/drain wire for insulation on both ends of the cable.

Then use a larger shrink wrap to cover the foil & where the wires exit the cable, up to about 4" from the cable end.

Maybe this would keep the wire from touching anything it shouldn't. It would only be exposed where it goes into the connectors.

It might just be easier to run a dedicated ground though.

NOTE: WE NEED A SMILEY ICON FOR BRAIN FART!!!!!!

JAckal

[Al_The_Man]

That is the drain wire, it should work for what you want the way you have described.
As a rule the conductors in a shielded cable are twisted pairs, this tends to cancel any signal that may be induced into the conductors and acts together with the O.A. shield to prevent spurious signals from being picked up, the drain is usually a straight untwisted conductor.
Try it and see if it works in your case.
Al.