Mount Hub PSU - Measuring Output V&A over multiple rails

[SnakeyJ]

Just wiring up a new Mount power hub for my much neglected observatory - I have a 12V 30A switched power supply that outputs 3 x 12V rails, that I want to power; Celestron CGE, startech USB3 hub, SGL focuser, Astrozap dual channel dew controller and probably a small primary mirror fan.

Can I assume the PSU provides approx 10A to each rail?

Is there a clever way I could wire in a single DC Volt/Ammeter, so I could either see the combined output over all three rails, or perhaps have some form of switch so I could monitor one at a time? 

I could of course buy three, but lack the space to mount multiple meters on the front of my box and keen to keep things as dark as possible.

[saac]

Jake any chance you can post a wiring diagram - the distribution of the current to each rail depends on how you have arranged the wiring but it is unlikely to be 10A to each rail unless the resistance on each rail (including their loads) is 1/3 the total resistance.  If you have wired it in parallel , which is most likely, then the current will divide as you've suggested but not necessarily 1/3 to each rail. The current will try to meet the demand of the rail.  I think a wiring diagram would definitely be useful.

Jim 

[SnakeyJ]

11 hours ago, saac said:

Jake any chance you can post a wiring diagram - the distribution of the current to each rail depends on how you have arranged the wiring but it is unlikely to be 10A to each rail unless the resistance on each rail (including their loads) is 1/3 the total resistance.  If you have wired it in parallel , which is most likely, then the current will divide as you've suggested but not necessarily 1/3 to each rail. The current will try to meet the demand of the rail.  I think a wiring diagram would definitely be useful.

Jim 

Thanks Jim - I guess it was a little light on pictures!     The PSU is mounted in parallel, with three DC outputs connected independently to; 1-Astrozap, 2 and 3 go to car type four way fuse blocks which will supply the mount and other output loads.    I've drawn a very rough diagram with my daughter's felt tip pens:

I'm trying to balance the loads across the three 12V output rails, but ideally would like to be able to monitor the combined Voltage and load across all three rails or potentially be able to switch this so I can see each output in turn.

 

Still a work in progress, but trying to shoehorn this into an IP rated box that came with the mount.   This will be fixed to the pier with the Dew Controller and VA meter mounted to the hinged front, opening up with input mains supply to bottom.

 

 

Edit - Not a great angle/view, but note that I have doubled up the 12V output wiring between the PSU and the chocolate block, where it will connect to the fuse blocks and Astrozap.

[Gina]

The three -ve terminals are simply connected together and three +ve terminals are also connected together on those PSUs.  I know because I have several of different voltage voltages and current ratings so it isn't necessary to balance the loads as far as the PSU is concerned.  May be a good idea from the wiring point of view but as long as you make sure the wires can carry the current it really doesn't matter.

[SnakeyJ]

2 hours ago, Gina said:

The three -ve terminals are simply connected together and three +ve terminals are also connected together on those PSUs.  I know because I have several of different voltage voltages and current ratings so it isn't necessary to balance the loads as far as the PSU is concerned.  May be a good idea from the wiring point of view but as long as you make sure the wires can carry the current it really doesn't matter.

Thanks Gina - I had wrongly assumed that the PSU had three distinct 12v rails , but this makes things a good deal simpler and I will cross bond the -ve cables and install the V/A meter in series - simples!

Much appreciated - Jake

[Gina]

The terminals are connected together on the PCB so it might be a good idea to connect the terminals externally as well if you want to use just one thick wire to each set of three.  That would put less current through each solder joint and PCB than using just one terminal of each.  This is assuming you will be using the full current capacity of the PSU like I do with my 3D printers.  I have used just one or two terminals myself but when advising others I like to err on the safe side.

[saac]

5 hours ago, SnakeyJ said:

Thanks Gina - I had wrongly assumed that the PSU had three distinct 12v rails , but this makes things a good deal simpler and I will cross bond the -ve cables and install the V/A meter in series - simples!

Much appreciated - Jake

Jake looks like you have your solution there , I'm going to defer to Gina on this.  I like your distribution box, neat design and something I need to get around to doing soon.  :)

Jim 

[hughgilhespie]

Just for interest, if you did have three separate power rails I think that a DC current clamp meter fitted around the three leads would work to show the total combined current.I don't know this for certain but I guess the magnetic fields would add up in a scalar fashion and so the meter output would be that of the total current.

Regards, Hugh

[SnakeyJ]

Thanks to all - I did follow Gina's advice and bond the three output channels - works a treat and has made a much neater solution than my original solution that was mounted inside the polystyrene cool box (ex vaccine transfer).     The box is still quite useful and makes a decent laptop table, but I will eventually remove this and go for a headless pc to remotely control from indoors.     If I can shoe horn my newly acquired VX14 in to the observatory there is going to be very little room for me or the laptop.