Dual power supply mains question?

Hi!

I'm planning on using two power supplies, a 24v for electronics and another 12v for the bed only.

Has anyone connected both the main inputs together? I'm wanting to use one switch and power cord for simplicity, so is this safe, or should I go with a independent switch and power cord for each power supply?

I've looked around and haven't found an answer to this, since it's probably a simple answer.

Thanks everyone!

You should be able to do it assuming the line side of your power supplies current draw doesn't exceed the rating of your wire, plug, switch, etc...

So you need to know what your line side current draw is for both your power supplies, add them together and make sure your wire, plugs, switches, wall outlets are rated for that much combined current.

Best practice is to fuse at 80% of the max cable rating. Make sure fuse level is above the maximum expected loading with a margin. There is a chance of a nuisance trip if the fuse is too close to expected load. If you decide to have more than one fuse in a line (eg to enable the use of smaller csa wire for a run with lower draw) then to ensure propper fuse discrimination the maximum rating of a branch fuse is half that of the major, unless you don't care that a fault on the sub branch may take out the major fuse.

I've got three power supplies running after a switched, fused mains inlet. Two 24V PSUs (steppers and heaters on one, fans and safties on the other), and a 5V PSU for the duet power supply. All running fine on a 5A fuse with a 300W heater and steppers running at 800mA and 1300mA on the extruder.

The recommended fuse levels for PSUs tend to be above the expected draw. Probably a safety margin for inrush or tolerable faults. Adding up the recommended fuse level on each PSU will probably be overkill. The start up of my 24V PSUs is naturally staggered as one will only power up after M80, which closes a relay that cuts the mains supply to the stepper/heater PSU.

Quote
WesBrooks
The recommended fuse levels for PSUs tend to be above the expected draw. Probably a safety margin for inrush or tolerable faults. Adding up the recommended fuse level on each PSU will probably be overkill. The start up of my 24V PSUs is naturally staggered as one will only power up after M80, which closes a relay that cuts the mains supply to the stepper/heater PSU.

The recommended fuse level is generally determined to protect the wire and not the device. Your fuses and breakers should trip before your wire would overheat and catch fire. In most industrial applications, the fuses and breakers are sized against the wire size and not the device loads as its much easier to replace the device than it is to replace all of the wiring.

Quote
PDBeal
The recommended fuse level is generally determined to protect the wire and not the device. Your fuses and breakers should trip before your wire would overheat and catch fire. In most industrial applications, the fuses and breakers are sized against the wire size and not the device loads as its much easier to replace the device than it is to replace all of the wiring.

Agreed, hence my previous post about best practice being to fuse at a maximum of about 80% of the cable rating.

Design process should be something along the line of:
* What is the current draw of the device under normal circumstances and expected use.
* Pick a fuse level above this rating.
* Select wire with a rating of at least (fuse rating)/0.8. Size up is necessary.
* Check voltage drop at expected loading isn't too great. 10% voltage drop is usually classed excessive. Only likely to be an an issue on big printers, ones with high current beds, or long runs to 3.3V devices.

My comment about the PSUs was badly worded. I was referring to the rated current draw rather than the recommended fuse level. The rated draw is often notably higher than the draw in normal use. If you simply sum all the rated draw the final rating is likely to be significantly higher than necessary.

There is no problem to have multiple users behind one power chord.
About fuses, the cable protection is the MAXIMUM rating of a fuse, but it may be safer to use a lower rate near the effectively used power. You just shall take into account that power supplies have a significant inrush current at startup and that using slow blow fuse may be needed. Another point is that a power switch cannot be trusted and may stay shorted (it occured to me) and adding a real emergency stop (inline of live wire, with appropriate rating) will add safety.
BTW, this particular printer had two power supplies (19 and 12V) and one halogen lamp on the unique power chord and on/off switch.

Also, other fuses may also be installed downstream the power supplies, with separate fuses for electronic, bed heater and hotend heater. Unfortunately, few boards separate electronic supply from heaters, so in this case you can protect heaters with inline fuses but not the electronic. Resettable fuses as found on ramps boards cannot be trusted. Automotive (blade) type are easy to find and probably the best choice.

Edited 3 time(s). Last edit at 12/23/2017 11:42AM by PRZ.

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Pierre

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