Low voltage DC power distribution (small distances) 1

[grockj]

I am currently laying the ground work for the mechanical design of a new product for which I am in charge of many electromechanical interfaces, which additionally means I will be specifying our wiring harnesses.

Given:
1) Various 5VDC sensors on a regulated line
2) Sensors have a ratiometric output so I definitely care about the supply voltage they receive
3) Wiring has to pass from main PCB through a wall which acts as a gas barrier
4) I am limited on space on said wall so I am trying to pass as few wires through as possible (I would like to pass through 2 for the device power for these if at all possible)
5) The opposite side of the wall (which contains the sensors) should be considered "dirty" in so far that an open terminal block may not be the best idea due to corrosion/lifetime contact resistance, but industry consultants inform me un-sealed tin plated terminal type connectors will certainly suffice beyond the expected life of my product
6) Current is sufficiently low and external effects on my 5VDC regulated output can be dismissed

Question: What is the preferred method to distribute the 5 VDC power to my array of sensors if I only want to pass 2 wires through the wall?

Goal: I am designing a wiring harness scheme which I will farm out to a harness shop.

- Terminal blocks are open for contact resistance issues from corrosion & contamination, or are there available methods to mitigate this? What about vibration?
- Could splicing inside the harness itself be sufficient? How many times can a wire be split out assuming current is not an issue before the harness manufacturing becomes infeasible? Significant cost driver here?
- Is it generally infeasible to only pass through 2 wires for the 5VDC "rail"?
- Other concerns?

Cheers,

grockj

 

[VEBill]

The 'gold plated' approach (if it fits within the budget) is to pass raw DC power to the other side of the barrier, and (in the opposite direction) pass the information by means of a serial link. With a few simple tricks, the DC power and the serial signal can pass along the same two conductors. This approach assumes that you can afford to have another electronics module on the far side. The module cleans up and regulates the DC power, and then processes the signals for transmission back along the same wires. This sort of approach is sometimes used for deep-sea ROVs where the one-and-only cable sets the entire design approach.

 

[grockj]

Bill,

Thanks for the input.

We have considered an approach using conditioning electronics before we go through the wall (possibly multiplexing the sensors to even reduce the signal wires) but nixed it due to corrosion concerns/ignition protection concerns with PCBs on the "dirty" side. I can see how this would be handy for a single-cable approach, but you hit the nail on the head with the cost issues prohibiting this.

One thing I am looking for specifically is some advice on wiring harness splices and what is considered good or poor practice; if I am feeding 6 sensors 5VDC+ can I come through with 1x 18AWG wire and then, somehow, break out my 6x 22AWG wires from a central point? Then on the return do the same? Naturally the signal wires will be independent (based on my details above).


What exactly would be the right way to make this harness?

 

[waross]

How about automotive type sensor connectors and daisy chain the wiring from connector to connector?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[grockj]

Waross,

That sounds wonderful provided there are no significant supply voltage drops at each chain link; if I understand your recommendation then it should be alright as it would just be the (resistance of the wire+connections)*(n legs) for the worst case of the final link. I want to avoid significant voltage differences from sensor to sensor as their output is ratiometric to the supply.

For daisy chaining the wiring, would the method be pairing the wires in the terminal? Or do certain connector systems provide another method for this?

Please excuse my ignorance... I am a fluidics/mechanical systems guy who is now doing more top level system design and I am not very well versed in the best practices of electrical assembly.

Cheers

 

[OperaHouse]

I've always found that wire size was based on the installer. Some people can't handel anything below 18 ( really want to say 14) ga. What about using gel filled crimp connectors. Just leave enough pigtail for replacement.

 

[VEBill]

You haven't stated the current draw of the sensors. Also, if the current is perfectly steady then the fixed voltage drop could presumably be calibrated out perfectly. It should be quite easy to estimate the parameters (current, current variation, voltage drops, voltage drop variations, impact of these variations on the sensors' measurements). Hopefully a quick review would indicate that these are all low enough that they can be subsequenctly ignored (*after* the one-page analysis). It's common to perform a formal Voltage Drop Analysis that formally documents that it's a non-issue.

The choice between splicing (solder?, crimp?) and terminal blocks depends on requirements and details. If it's a dirty environment, then you'd want a sealed solution.