Instrument Sub-bus?

[jsharkey]

I'd like to wire all the instrument power and backlight +12V and ground leads to a simple bus attached to the back of the panel so that I can remove, add or replace instruments more easily. I've looked at terminal blocks from Stein and Home Depot and have considered making my own from copper bus bar but it al seems big and clumsy.

Does anyone have a neat and tidy solution?

Jim Sharkey
RV6 - Wiring

Just realized the instrument power and back lights should be independent circuits - unless I want the lights on all the time. Also any reason not to daisy chain the +12V and GRND from one instrument to the next in parallel and then have one lead to the breaker and one to ground?

[az_gila]

Jim... the daisy chain you mention should be fine.

What type of terminals and/or screws exist on the back of the instruments in question?

gil A

[jsharkey]

#6 screws.

Just done it and it looks neat. Also connected back light ground leads to each individual instrument ground lug. That now leaves me with the back light live leads to deal with so the problem is about 25% of what it was before.

Not sure about the security of the 22G "red" lugs crimped on the very small back light wires though. They test OK - mechanical tug and electrical resistance. Do you get smaller lugs for the really tiny wires?

Jim Sharkey

[az_gila]

Originally Posted by jsharkey

#6 screws.

Just done it and it looks neat. Also connected back light ground leads to each individual instrument ground lug. That now leaves me with the back light live leads to deal with so the problem is about 25% of what it was before.

Not sure about the security of the 22G "red" lugs crimped on the very small back light wires though. They test OK - mechanical tug and electrical resistance. Do you get smaller lugs for the really tiny wires?

Jim Sharkey

The red crimp terminals are rated down to 22 g - they should be OK.

I presume you are using the good ones tha also crimp down on the insulation? AMP PIDG is one brand...

Stein has them...

http://www.steinair.com/terminals.htm

Aeroelectric Bob says that you can use two small wires in one terminal... this might neaten up your wiring...

http://www.aeroelectric.com/articles...iplewires.html

gil A

[elippse]

Only "Daisy-Chain" if you don't mind stuff from one instrument appearing on the others. What a daisy-chain does is gives you what is called common-mode coupling. What that means is that current from the return line of the later instruments on the return going back to the system ground will cause a voltage drop which then appears on the next instruments toward the system ground. Even though wires appear to have zero resistance when checked with an ohm-meter, they can and do offer substantial impedance to transient currents. Any time you depart from a single point ground you may have problems. The operative word is "may"! It is much easier to do the wiring correctly during the construction phase than having to go in and correct a problem that shows up down-stream, or in deciding to live with an unsatisfactory condition because it would take too much re-work to fix it. Why do a construction job on your airframe a professional would be envious of, but do a less-than amateurish job on the electricals. The same can be said for your choice of wiring materials; don't spend lots of bucks on avionics then go cheap on your choice of wires and coax. Use the best from the start and you'll have no regrets later.

[az_gila]

Ellipse (Paul) is correct on the grounds.

I was really addressing the instrument lighting circuits you mentioned.

Daisy chaining is no problem for the light circuits

Daisy chaining is OK for the Instrument 12 volt source (quite typical for wiring)

This is where it can get into a possible problem....

Daisy chaining grounds is OK if the instrument senders are low value resistive sources - such as resistive fuel senders and resistive oil and fuel senders - which is most of the VDO (and old SW) type senders.

It can (and probably will......) cause a problem with high impedance voltage sources/senders - such as EGT/CHT thermocouples.

If in doubt, then the separate ground wires to a common grounding point is the best solution.

Also, do yourself a favor for future possible fault finding and make all of your ground lines black... tracing will be much easier....

gil A

[jsharkey]

Probably good idea to go with single point grounds then.
Thanks guys
Jim