Best cable for 5v power...

[MuntyScruntfundle]

Hi folks.

I have a rather bonkers need to power 128 raspberry pis. (small pc boards)

I have decent quality power supplies which will provide plenty of watts, the signal will be nice and stable.

I need to supply power to the GPIO on the board which uses standard dupont connections.

What AWG would you folks suggest?
Am I better using a larger diameter to reduce voltage drop?
Are there any ceramic cable types that would suit to further reduce voltage loss?

The maximum cable length would be around 1 meter so I'm overly worried about power drop, but I need this to be as stable and clean as possible.

Many thanks.

 

[Spoon]

I need to supply power to the GPIO on the board which uses standard dupont connections.

What is the max size of cable the connector can take?
Are you thinking of single cables?
For your project, won't the signal quality depend on how the cable is run and the connections to the connector and power supply?

 

[Shoei]

Needs more info. There are different models of Raspberry Pi which take different amounts of power at idle, in use they'll take more power, maybe much more if they're doing something intensive.

For example if they're model 2B they'll take 200mA at idle, for 128 that's 25.6A before they do any work, which is already a serious 5V supply.

When you know the maximum power requirement you can work out wire sizes to limit the voltage drop.

FAQs - Raspberry Pi Documentation - https://www.raspberrypi.org/documentation/faqs/#pi-power

 

[Spoon]

Welcome to the forum @Shoei
Bung a small post in the 'New Members' section and tell us a little about yourself.

 

[Bobster]

How long a run is this cable going to be?

 

[MuntyScruntfundle]

Thanks for getting back to me folks.

The Pis are the 3B+. The documentation states their official PSU provides 5.1v at 2.5A, which will cater for low and high load even when clocked a little.

Max length of cable would be 1m, some will be less.

Cable runs will be parallel from PSU to each pi in stacks of 16.

Cable diameter- I don't know, I need advice, I'm using standard 2.54 Dupont connections.

------

And, I've just found out, under full load the PSUs I'm using will NOT handle load spikes from the number of pi's I'm running. I have a temporary solution, add more PSUs and split less, but I need this to be better. So...

Can anyone suggest a power supply that will supply 16 x 5v 2.5a safely without any spikes or brown spots?

Thanks again.

 

[netblindpaul]

I'd probably go for something like this:
S8VM-30005C | S8VM Switch Mode DIN Rail Panel Mount Power Supply, 300W, 5V dc/ 60A | RS Components - https://uk.rs-online.com/web/p/din-rail-panel-mount-power-supplies/0110429/

 

[Spoon]

I'm using standard 2.54 Dupont connections.

According to Amazon, They take 18-26 AWG wire.

 

[davesparks]

Voltage drop over 1m won't be too much of an issue as long as the cable is sized correctly fir the current. You do seriously need to consider the overcurrent protection of each unit though, you need to provide fuse protection for each unit. Having power supplied with 'plenty of watts' could rapidly result in 'plenty of flames' if not correctly fused.

 

[Shoei]

You're right about the bonkers part. The 3B uses 400mA at idle so a little over 51A just to switch 128 of them on. Presumably you want them to do some work which will have them take more power.

The comments about fuse protection are well made, that kind of power needs to be treated with care, even if you don't set fire to anything it would be a sad day if a transient wiped out all 128 boards.

I'd be inclined to keep it manageable by using multiple supplies for groups of boards. You need to know the peak current of your application to attempt a reasonable design. The recommended supply for a single 3B is 2.5A with 1.2A available for peripherals which suggests the board can take up to 1.3A. That leads you to a 167A supply which is more than just bonkers.

Work out what power your application needs, maybe using a small number of boards and then you'll be able to specify the supply and wiring properly.

Without giving away any secrets can you say anything about the application? It's hard to imagine any systems design question where the answer is 128 Raspberry Pi's.

 

[Wilko]

Agree with Shoei, perhaps distributed supplies and appropriate flexes? Likely you'll find 5V supplies with 30A output that might each support 10 devices (say).

 

[littlespark]

Reduce the cable size by using a ring circuit configuration?

Or

Use the boards own power supplies and a heck of a lot of 13A 4 way adaptors?


That’s the best you get from me at half 7 in the morning.

 

[marconi]

128 Raspberry Pis - sounds to me like you are making a cluster computer.

If it was me I would be distributing the 5V dc using busbars and having taps off them for each Raspberry Pi. I am undecided on whether it is best to use one power supply or 2 to 4 - but for economy one is cheapest. I have not (yet) researched suitable low voltage semi-conductor fuses - here again I am minded to go for a number of fused busbars depending on the layout of your cluster.

Regarding cable connections between the busbars and power supply - I would use one short 10mm2 to one end of the busbar or (preferred) 3 x short equal length 4mm2 with connections to the ends and middle of the busbar.

Have you considered cooling fans and the air flow through the cluster so that there are no hot spots? What you don't want is warm air from sets of Rasperry Pi boards flowing over other RP boards so the latter are warmer then the former. For what happens on a bigger scale in data centres - but the principles are the same -take a look at:

https://assets.tripplite.com/white-...heat-related-problems-white-paper-230v-en.pdf

I thought you might use one of these 5V circa 300W psus and the n bus and wago connectors for it indicated by the last two links:

SunPower SPS 320P-05 320W Enclosed Power Supply 5VDC 50A - https://www.rapidonline.com/sunpower-sps-320p-05-320w-enclosed-power-supply-5vdc-50a-58-5834

TDK-Lambda RWS-300B-5 Enclosed Power Supply 5VDC - https://www.rapidonline.com/tdk-lambda-rws-300b-5-enclosed-power-supply-5vdc-63-9689

WAGO 210-281 Connector for Busbar with Blur Cover - https://www.rapidonline.com/wago-210-281-connector-for-busbar-with-blur-cover-50-7124

WAGO 210-133 N-Bus Rail, Zinc-Plated, 1000mm - https://www.rapidonline.com/Catalogue/Search?Query=n%20bus%20rail

Could we see a picture of the finished cluster please?

(Your problem reminds me of the Ferranti FM1600B computer with its heavy 5V circa 100A cabinet power supply and busbars for the TTL logic. Mains supply was three phase 200V 400Hz from a Westinghouse static frequency changer). There was a big warning sign to remove rings from fingers and to be careful with tools near the busbars - you need the same signage.)

 

[marconi]

Further to my #13: In view of my postscript above and the risk of a short circuit between the busbars I think if it was me putting it together I'd have the busbars for each pole placed left and right or top and bottom so that say all the + go up or right 5V and the 0V down or left.

I also like the idea of turning the busbar into a ring/regular triangle arrangement with the three feeds to the 0, 120, 240 degree points/vertices.

I'd make the feeds to each RP the same length and distribute the tap offs evenly around the ring/edges.

 

[static zap]

Reduce the cable size by using a ring circuit configuration?

Or

Use the boards own power supplies and a heck of a lot of 13A 4 way adaptors?


That’s the best you get from me at half 7 in the morning.

Like above due to
a) Naturally safe fold back current limiting still built in !
b) Multiple redundancy , naturally !
( Cost + Energy efficiency , probably can be bettered )

Thin wires and 30A supplies always scared me without some form of star-point fuse box. (and a chunky cable feeding -fuse box)

 

[marconi]

Use three of these to protect each busbar feed - suggest 25 or 30A.

( I assumed 5V and 300W so total current is 60A. Each feed then conducts 20A. If one feed fails then current divides between remaining two to 30A each which is sensible safe limit for 4mm2. If only one feed then 60A through 4mm2 is excessive and current trip operates. This type of trip has visual indication of overcurrent event - push button is out. They also have low contact resistance 0.006 and 0.004 Ohms respectively so volts drop at 20A is circa 20 x 0.005 = 0.1V.)

Anyway - some ideas for you to mull over. There will be other schemes one can think of but this is mine.

http://www.farnell.com/datasheets/5...VPjTF3_QxYzo_V5PNaSPK2XNI44u5mIBoCW-gQAvD_BwE