AU specific - Bunnings Extension leads for DC Power feed

[AussiePhil]

A question was raised in chat could the seemingly always on special, outdoor extension cords (AC) be used to run 24v around a display before dropping the voltage using DC-DC bricks at different props.
Questions raised:

• Was it feasible?
• how much voltage drop was there?
• How many DC-DC bricks could be run?
• Related then, how many pixels?

Things to consider if you do this:

• Pixels are constant current devices so power used is constant at 100% settings
• Current always leads to voltage drop.
• Using 24v and above for distribution is sensible but may not always be the best option.
• Voltage drop per metre per amp for cable is completely calculatable based on wire resistance.
• Voltage drop resulting from dc-dc bricks along a run results in non linear voltage drop, (just like pixel strings)
• Four convertors, one every 5M is completely different to 4 at the end of the run.
• It is essential you draw this up and even do some basic maths to see if it is suitable.

Is it feasible?
Absolutely, Bunnings currently show these available in 25M lengths for $22. The have 3 x 1mm2 copper conductors. cut the molded plugs off, pair neutral/earth wires for ground and leave the brown active for positive.
using two wires for ground reduces the voltage drop (rise) on the ground wires which is better.
Compared to other options often available in AU this is reasonable cost effective.
4mm2 solar cable would be significantly better for this role though at a higher price.

How much Voltage drop?
Here's the drop measurements using a electronic load at the end of a 10M run to get the baseline numbers all other numbers are calculated.



13A was the highest current measured as that was the limit of the 27v PSU I had on hand.
The common 12/24v DC-DC convertor handles 27v without issues under no load.


How many DC-DC bricks could be run?
This is a "depends answer" and for this we will largely use watts in the next lot of dot points.

• Every AC-DC or DC-DC convertor has an efficiency rating and this needs to be factored in.
• Modern DC-DC bricks can approach 97% but lets use 94% as a more likely average
• A common 350w PSU should deliver 320w all day everyday at 27v
• So a 100w (20A 5v) brick would need 106w input
• Three of these will be 318w required maxing out the PSU
• Place all of these at the end of the power run at 35M it will still work but is getting marginal.
• Place these at 10, 20 35 and the numbers get a lot better. subject of an update.
• Use a higher wattage PSU then voltage drop will become the prevalent issue and you will need to calculate it out.

Related then, how many pixels?
This is entirely based on the pixels power but at 100% white 5v pixels properly power injected will need around 4.4A/100 around 400 per 20A brick. so around 1200 per cable run.
runs then at lower brightness etc will increase this, often dramatically.


Want to do some further calculations based on the above cable then this is the magic constant as measured.

V Drop /Metre /Amp measured average

0.033071

How does it work.
Scenario, 10m run with 10A
10m * 10a * 0.033071 = 3.3v drop
25m * 12a * 0.033071 = 9.9v drop

Cheers
Phil

 

[Skymaster]

This is very relevant to me, thanks for the write up Phil. Certainly looks like a feasible option.
In still tossing up between the extension cable idea and the solar cable.... Namely for aesthetic reasons, black cable vs red and yellow would like wildly different. That said, against a white house, both don't look great.

 

[sabo]

Many thanks Phill. Exactly what I was after and those bunnings leads are the ones I use. Think I might head down this route. PSU at each end, like should be possible for me should make my PI a lot simpler.

 

[AussiePhil]

This is very relevant to me, thanks for the write up Phil. Certainly looks like a feasible option.
In still tossing up between the extension cable idea and the solar cable.... Namely for aesthetic reasons, black cable vs red and yellow would like wildly different. That said, against a white house, both don't look great.

The colour and aesthetics is something i didn't touch on. The public perception may also be a issue as everyone in AU instantly recognises them as an AC 240v extension lead.
Personally for any lead that is in public view I would use Solar cable as my own preference... black will disappear better at night than yellow/red.
The purple ones are good for that though

Many thanks Phill. Exactly what I was after and those bunnings leads are the ones I use. Think I might head down this route. PSU at each end, like should be possible for me should make my PI a lot simpler.

I would be testing the dual supply setup and the cabling "on the bench" well before committing this to the display just in case, at least the supply at each end and tapped of DC-DC bricks.

Cheers
Phil

 

[i13]

PSU at each end

Remember to cut the positive wire halfway between the two power supplies.

 

[AussiePhil]

Remember to cut the positive wire halfway between the two power supplies.

OK i'll address this via testing and i13 i invite you around as a local to help out, but by the time we have 20m + of cable this immediately falls into the completely valid passive current sharing model for power balancing between supplies.

Cheers
Phil

 

[i13]

Invitation accepted, thanks

I would have expected that the power supplies would fight each other. The DC-DC bricks' outputs might stay stable though.

 

[battle79]

If you connected the supply outputs together and then ran to props, yes you would end up with unhappy supplies. The resistance over a long length of cable, especially if it's only 1mm, will help avoid this problem.
That said, I would always consider it bad advice as people with less electrical knowledge might not see the need for a large resistance of wire between the supplies and run into problems.

 

[AussiePhil]

Pardon my very rough whiteboard skills with a mouse, This is the layout that is being discussed and is not about joining two supplies together in the same box.
So for extra clarity..... DO NOT DIRECTLY PARALLEL CONNECT SMPS's together. you must use some form of load sharing setup. this can include passive load sharing via ballast resistors or cable or OR'ing diodes

In this scenario the distribution cable is acting as the passive load sharing device.

This drawing lets me also address a major concern with people doing this, not just with DC-DC bricks but with 12v pixels
In the diagram I have specifically said each 5v point is 150w whilst the end supplies can provide up to 300w
So the outcomes are as follows:

• PSU A only with power, 450w attempted to be drawn by 1,2,3 ... PSU A should go into Over Current Protection (OCP) as the power run doesn't have enough overall resistance to limit current. Fusing may blow.
• PSU B only, same as above but in the opposite direction.
• PSU A & B being used, if "2" is in the middle then A should supply @225W and B will do the same. This will go up and down based on the current drawn by 1.2.3.
• The current sharing will vary based on loads for each of the DC5 point but will be balanced by the resistance of the power run and hence the voltage drop.
• PSU A & B on with no load. A & B must be adjusted so they have effectively matching voltages. The 10 metres plus of cable provides the passive load share setup

The last dot point is what we will test in conjunction with I13 and in the event that it all goes bang then we can update all the conclusions.

The Major concern is that people will intentionally or accidentally overload a single supply resulting in issues. The bigger, thicker, better your Power Run wiring is the easier this will happen.

IDEALLY you should use OR'ing diodes on the output of each power supply to prevent any DC back feeding to each PSU

 

[Dreamin]

I've used plenty of the colourful 25m bunnings extension cord. Works great, nice and thick outside for 5v and 12v. Voltage drop is minimal as the wire is good quailty. The only issue I had was my 12V 10W spot lights which needed a boost to the data signal as the run was 10m, the power was fine at that length. I think running 24V just complicates it a bit too much.