Best practice for protecting individual devices hooked up in parallel to PSU - Page 1
Forum section: electronics / beginners

author: bob21
author_rank: Regular Contributor
timestamp: June 23, 2020, 09:03:31 pm
content: Hi again allI'm looking for some advice to do with separating and protecting a few of 5V devices that are hooked up to a very large 5V power supply. The power supply in question is a 5V 30A PWM PSU typically used for strings of LEDs and I recently got some help from @Ian.M hooking up these LEDs (thanks btw, the LEDs themselves are still working well!). The LEDs are part of a larger project, Hyperion, which requires several other devices to simulate a Philips Ambilight.The problem that I have recently experienced is to do with the other devices hooked up to the PSU. All devices are connected in parallel, and I  managed to hook one of them up in reverse.As you can guess, this didn't end well.It got me into thinking about how to isolate each of these devices in case a short, or  user error occurs.My first thought was to look at power distribution boards like this:https://www.ebay.co.uk/i/254417101703or those used for drones like this:https://www.hobbyrc.co.uk/matek-fchub-6s-with-184a-current-sensor-pdbbut they don't seem to have protection circuitry. I also found this youtube video which talks about protecting circuits:https://youtu.be/IrB-FPcv1DcI was hoping that I would find a little board on AliExpress or the like which I could place just before each 5V device or a power distribution board/block which contains such circuitry.. something in a similarformatto this:https://tinyurl.com/y9s39735i.e. PSU > PCB > device.So my question is: How do I protect these individual devices, what are the best practices? Is there a PCB like the above which I can place before each device? If not, how can I make one?Thanks as always for humoring my electronic-inept-ness

author: Ian.M
author_rank: Super Contributor
timestamp: June 24, 2020, 12:06:56 am
content: Connecting something electronic backwards *will* cause damage unless it has built-in reverse polarity protection. Many ICs 'look' like a series pair of diodes if you power them with reverse polarity, and MOSFET body diodes will also be forward biassedso expect very large currents to flow. Tracks burning up, boards catching on fire, wires melting, or even ICs and MOSFETs desoldering themselves or exploding is expected when you make dumb-ass mistakes with a high wattage PSU.External reverse polarity protection usually doesn't make sense - it just gives you an opportunity to get it backwards after the protection!However the use of a fuse box to distribute your 5V supply will vastly reduce the drama, as you can then fuse the branch circuits at a low enough current to prevent the high energy excitement, athough it won't do much to save reverse connected devices, as semiconductors typically blow at least an order of magnitude faster than fuses!Low voltage fuse boxes are commonly sold retail by marine chandlers and RV/Caravan specialists,and of course on EBAY. If they take blade fuses, check you can get ones with a low enough current rating in the specific size the box takes. The higher quality fuse boxes have terminal screws to take ring terminals.Anther option would be to build your own power distribution board. You can do that on a PCB but beef up the main power bus bar tracks by soldering copper strip or heavy bare copper wire along them. Either use PCB mount holders for 20mm glass fuses, or usepolyfuses, which self-reset when the load is removed, but are notorious for the high variability of their trip current with temperature.There are 'electronic fuse circuits' that interrupt the power and and latch off when a particular current threshold is exceeded, but the simple cheap ones tend to have excessive voltage drop, and ones with minimal voltage drop tend to be rather complex and relatively expensive.Please draw us a block diagram with the max. supply current needed by each block marked, and give us an idea of how compact the power distribution solution needs to be so we can give you more useful suggestions.

author: james_s
author_rank: Super Contributor
timestamp: June 24, 2020, 02:05:20 am
content: Do it exactly the same way your house handles dividing up a 200A (or whatever) main service to individual branch circuits, use separate protection devices. Circuit breakers would work but fuses are typically a lot more economical for this sort of thing.

author: eblc1388
author_rank: Frequent Contributor
timestamp: June 24, 2020, 02:10:56 am
content: Never have that ever happened after I religiously using red cable for positive and black for negative.

author: bob21
author_rank: Regular Contributor
timestamp: June 24, 2020, 08:19:02 am
content: Quote from: eblc1388 on June 24, 2020, 02:10:56 amYes, I couldn't agree more. Most of the wiring is indeed black and red. The problem is that one of the tidiest ways I could find to take the connections from the PSU enclosure to the main enclosure was to use a length of 2.5mm 4 core flex, the connection from the main enclosure to the TV is similar 3 core flex. Obviously the core colours are not red, red, black, black and a stupid error occurredHere are some diagrams:As always, please excuse their awfulness.

author: eblc1388
author_rank: Frequent Contributor
timestamp: June 24, 2020, 08:55:45 am
content: One has to admire how Murphy works, at the only bit of cable which is not red/black in your case.I understand the needs to use multicore 3-core or 4-core cable to carry DC to various circuits. However, the core color of these cable varies quite widely. On the few inches of both end of the cable where the outer sleeve is removed, I would slip red/black heat shrinkable tubing on the cores so I will never mistake them for AC power later, which is brown/blue.I know all these measures would keep me safe and eliminate all associated dangers later on.

author: Ian.M
author_rank: Super Contributor
timestamp: June 24, 2020, 09:16:41 am
content: Obtain an 8 way automotive fuse box with screws for ring terminals, similar tothis one, but probably a clone as Blue Sea are <expletive> expensive and you don't need the water resistance or as good quality. Also get an eight way bus bar (+ main supply terminal, so eight screws + a big one) for the negative return.Parallel the power in wires in pairs, crimp on ring terminals and either colour code them with coloured heatshrink, or label them at both ends. Fuse all the loads individually, the LEDs at 20A, and everything else with the next fuse up from its rated current. Crimp ring terminals on each wire to each load. For all except the LEDs, use red & black zip cord to let you colour code the wires and keep them in pairs.Check the polarity between the fuse box main terminal and the bus bar *BEFORE* inserting any of the fuses. Tape a little bag of spare fuses, one of each value used, and a map of which fuse goes where inside the main unit case, to discourage future bodgery like replacement with the wrong value fuses!

author: bob21
author_rank: Regular Contributor
timestamp: June 24, 2020, 09:56:44 am
content: Quote from: eblc1388 on June 24, 2020, 08:55:45 amI know right?! I really kicked myself. Shrink wrapping is actually the first thing I did afterwards: and a lesson learned. The 4 core is grey, black, brown, earth, I think it's boiler cable.Quote from: Ian.M on June 24, 2020, 09:16:41 amThanks Ian, I think I have found just the thing:https://www.ebay.co.uk/itm/8-Way-Blade-Fuse-Box-Fuse-Block-Holder-with-Fuses-for-12V-24V-Car-Marine/153977001102This seems to be the cheapest suitable block for return:https://www.ebay.co.uk/itm/12-Way-Screw-Bus-Bar-with-Cover-150A-48VDC-Marine-Tinned-Boat-Caravan-Motorhome/254399020203they seem to jump from 6 to 12.I think I have everything else.Sorry, dumb question: what do you mean by parallel the wires in pairs? Won't that have 2 loads on one fuse?

author: Ian.M
author_rank: Super Contributor
timestamp: June 24, 2020, 10:22:58 am
content: The fusebox only has ONE power in stud, and you've got four wires (two positive, two negative) coming from the PSU, so you have to parallel those wires in pairs. (I'm assuming your PSU has a single 5V 30A output.)You could get away with a six way negative bus-bar: the twelve way one you linked has a stud at each end, use one for negative power in and the other for the LED negative. If the six way bus-bar has only one stud, put both power in and LED negatives on it. That cuts down the number of remaining connections to six.

author: bob21
author_rank: Regular Contributor
timestamp: June 25, 2020, 06:00:50 pm
content: Sorry for the delay, it's been busy on this end. I haven't ordered anything yet, but I will grab the 6 way bus bar.The PSU has 2 x ground and 2 x +5V. I used 4 core to separate the load. 2 cores have just the LEDs and the other 2 have everything else. I might just grab an inline blade fuse holder for the positive used for the LEDs and use the fuse box for the others.

author: Ian.M
author_rank: Super Contributor
timestamp: June 25, 2020, 07:59:57 pm
content: How many outputs does the supply have (not how many positive terminals)? If its a single 30A output,you *should* wire the two positive terminals in parallel, (and of course also parallel the two negative terminals). Wiring them separately with such a heavy load on one pair of wires is asking for trouble with ground and power bounce, The fusebox and bus bar should be used to provide a single star point (each) for distributing power and ground.

author: bob21
author_rank: Regular Contributor
timestamp: June 25, 2020, 11:27:21 pm
content: It's this:https://www.amazon.co.uk/Switching-Power-Supply-adapter-Transformer/dp/B07Q2VPPL1

author: Ian.M
author_rank: Super Contributor
timestamp: June 26, 2020, 01:27:16 am
content: OK. Fifth image down is fairly clear the two +V terminals are internally connected together, as are the -V terminals, inside the PSU. Your diagram above shows 18A max. current draw on one pair of wires (+V,-V) in the four core cable from the PSU, and only 8A max. on the other pair. If you parallel the pairs, +V at the fuse box positive input stud, and -V at the busbar negative stud, you'll reduce the voltage drop in the four core cable by 28% and dramatically reduce transient differences between ground as seen by the Pi and as seen by the LEDs and their controller MCU. There's no need to add external links to parallel them at the PSU terminal strip as they are already linked internally, so simply put each wire on its own terminal.TLDR: Crimp individual ring terminals on each end of each wire going to a plain screw or stud terminal, but put both +V ring terminals from the four core cable on the same stud at the fuse box, and both -V on the same stud at the busbar.

author: bob21
author_rank: Regular Contributor
timestamp: June 26, 2020, 08:34:40 am
content: Thank you <3 parts ordered. Will let you know how I get on!

author: SilverSolder
author_rank: Super Contributor
timestamp: June 26, 2020, 12:42:18 pm
content: Quote from: eblc1388 on June 24, 2020, 08:55:45 am+1 for heatshrinking some coloured tubing on wiring as you strip the cable. It is nice to be able to thank your past self when, years later, you revisit something and don't have to mess around remembering what you did...

author: bob21
author_rank: Regular Contributor
timestamp: September 06, 2020, 10:16:10 am
content: Well thank you for all the input on this thread and the pointers with heat shrinking wires. After having waited an age for the bus bar, I had to order again as it went missing somewhere is Atbasar! But I am up and running now and all it working well!