Im powering 4 of 5 meter led strips, a 20 meter setup, and im using 2 different 5v power adaptors with 20a and 40a power. can these adaptors connect on their grounds and 5v?
I created a power rail that runs through all the strips, there is a power adapter at the beginning and at the end. and this power rail connects to each 5m strip at its beginning and end, can I connect ground and power of both adaptors to this power rail? Is it absolutely necessary to keep +V of adaptors isolated from each other?
In general, that's not a problem. However, if I were doing it, I'd make sure the 5v power adaptors have the output isolated from the input so as to not create a shock hazard, then I'd check the ground voltage differential between the two PS's to ensure there isn't something funny going on. Lastly, I'd review the specs on the PS's to avoid an extremely unlikely possibility that their regulation could create a ridiculous phase load issue. I assume you're dealing with 5V DC that's loosely regulated, if at all. If that's the case and they have enough impedance to split the load, then it shouldn't be a problem. I'd only isolate the V+ if the load doesn't balance properly or if one of the PS's presents a load when it's powered off which would create an unnecessary load and possibly damage one or both of the PS's. Also, keep in mind that doubling the current on a branch could create a fire hazard if you have a short because the overload protection circuits may not be aware that the branch has twice the load. This is not an issue if you keep the V+ from each PS isolated.
The concern commonly expressed about using two power supplies is that they will "fight" each other to regulate.
This is of course, not a meaningful concept. The actual concern is that they may not be properly stabilised under such an unusual condition and will oscillate.
That is a possibility if their design is less than adequate. What should happen is simply that the one with the slightly higher voltage setting will dominate at low loads and the other will shut down until the first is sufficiently loaded to drop its voltage. But that might not happen until the first has exceeded its nominal current rating. Connecting the supplies to opposite ends of the combined strip (or rather the supply cable running alongside the strip to bridge in power every 50 or so LEDs) will generally avoid this.
So it is safer to separate the positive rails.
Now your big problem is to remember to keep all supply and ground wires tightly together all the way from supply to strip. This also applies to data and ground from one part to another, so data and ground must run as a pair from the Arduino controller to the very start of the first strip. Now you want to power the Arduino from your 5 V supply, so the 5 V must also run back with the data and ground from the first strip to the Arduino "5V" pin (except if and while you are unfortunately using a UNO or Mega 2560 plugged into a PC via USB).
