The printer draws large amounts (20+ Amps) of current at 12V and these supply exactly that - nothing more, nothing less!
The power supply will certainly deliver less current if the load demands less. If the 3D printer draws 1A, the supply will supply 1A. Your statement that the power supply delivers a certain power level - no more, no less - is incorrect.
I'm looking for a more stable power supply for my 3d printer.
You haven't mentioned stability in your posting. You did say that at high loads, the output voltage sags. The stiffness of a power supply is related to how well the voltage regulates at high load. The stability of a power supply is related to how well the voltage regulates when the output is subjected to a rapidly-changing dynamic load.
There could be two things going on causing the voltage sag:
1) The voltage sensing point is close to the power supply; it is regulating the voltage at that point and the loss you see at the load is due to resistive losses between the sense point and your measurement point
2) The power supply is entering a protection state and is limiting the voltage to limit the output power and keep the power supply thermally safe.
I just have a few concerns about the safety of using one of these devices.
A 'safe' power supply, in industry parlance, means it has been evaluated by a regulatory authority and found to comply with certain national / international safety standards for the application in which it was intended to be used. A single abnormal should not cause a safety hazard (shock / fire / shrapnel). The unit should bear one or more well-recognized safety marks (UL, CSA, TUV, etc.)
My main concern is that I noticed there is nowhere to simply plug a mains cable as an input to the power supply.
As Olin pointed out, this is a unit meant to be permanently installed into some other piece of equipment, not something that a user would be expected to swap-in or swap-out often.
However, isn't there a fair likelihood that someone may just pick it up by putting their fingers and short the live and neutral together? The exposed screw contacts look awfully prone to accidental contact with not just fingers, but nearby metallic objects. A built-in fuse won't exactly help here would it?
Notice in the photo that there's a clear insulating shield over the terminal block. That shield is part of the inherent safety of the unit and should prevent against mains shock from casual contact with the unit. If someone wants to hot-screw a powered mains cord onto this UUT, well, they deserve what they get. Not trying to be facetious, but these sorts of power supplies are meant to be installed by 'qualified' personnel who have some basic knowledge.
It's quite common for power supplies like this (meant for use inside other equipment) to get fed from a feed that has a fuse or breaker in it; there's the possibility that the internal wiring may make contact with the equipment itself. That doesn't mean there isn't a fuse in the power supply though (there should be!)
The other concern I have is whether it will be extremely dangerous if I accidentally reverse the live and neutral wires?
Yes, but only if there's a fault. The power supply will 'work' with reversed L and N. However, the power supply internal fuse is in series with the terminal marked L (line). Blowing the fuse means the neutral is now floating, which is a big no-no (netural must never be interrupted) and a big risk that your chassis can become a shock hazard.
At the moment, I am using a typical 320W PSU for a desktop computer to handle the power workload.
PC power supplies have minimum load requirements on various rails and are rarely a good choice for industrial applications like a 3D printer. Spend the money and get a single-output 12V supply that can deliver the power plus has remote sensing capabilities so that you get the best regulation possible.
Best Answer
You'll most likely have to limit the current yourself. It's under that condition that the voltage is specified. Going higher than 200\$\mu\$A will probably cause the voltage to sag below 2.4V, which is the minimum for a high level in TTL. (I've also seen 2.7V as the minimum, I guess it depends on the TTL subfamily.)
Note that the same table says that "signal type = +5V TTL compatible", and that a low level is specified as < 0.4V. That output level is 0.4V less than the maximum TTL input level for a logic
0
. And the 2.4V is 0.4V higher than the minimum for a high input level. This gives a 0.4V noise margin.If you want to control a MOSFET with the PWR_OK signal, like OP, you'll need a logic level MOSFET, which draws enough current at a low \$V_{GS}\$. The BSG103 may be a good choice; it has an \$I_D\$ of 750mA at a \$V_{GS}\$ as low as 1.5V.
edit
On second thought the BSH103 may be too good. It has a \$V_{GSth}\$ of 0.4V, which means that worst case you'll have a drain current of 1mA with PWR_OK low. Even FETs with a \$V_{GSth}\$ of 1V typical indicate 0.4V as a minimum value. Can be fixed by using a resistor divider to lower the output voltage from PWR_OK. A 15k\$\Omega\$ + 25k\$\Omega\$ gives you a minimum gate voltage of 1.5V, while the current is maximum 125\$\mu\$A.