With great power comes great responsibility - A PSU Discussion.

[jaguarking11]

5v Rail is nearly useless on reprap based printers. Some run as high as 24v, 5v rail is unused in most DIY ramps based printers.

[Mjolinor]

5v Rail is nearly useless on reprap based printers. Some run as high as 24v, 5v rail is unused in most DIY ramps based printers.

Why?

I use only 5 volt on my CNC machine because PS3 supplies are so cheap and small. I don;t see why you couldn't run repraps on 5 volt, my steppers are all at least nema 24, and I thought repraps used nema 17 generally but like I say, I've never really looked at them.

[jaguarking11]

They are nema 17's. However what you don't have on your cnc is a hot end or a heated bed. Which both use normally 12v modules to keep the cables at a reasonable level. It also stands to reason the same amount of work done at 12V @ 10amps you need 24AMPS from a 5V psu while increasing the cable size dramatically. Lets say the heated bed needs 100W to operate, at 5V it needs 20AMPS, while all fine and dandy, however the cable size becomes a larger issue, now you need a 5 Gauge cable, vs at 12v you need a much thinner 12 gauge cable. Assuming 1M length. ~3feet

Your argument may be so what? Let me tell you that most of these printers need at least one cable that is constantly flexing while being moved, and it add load to the machine if your moving battery jumper cable sized wires. That's 5mm cable vs a 16mm cable..... I am generalizing here as there are other factors in cable sizing, however higher voltage is a nice thing in these things, also most of the accessorizes are designed to run on 12v.

I Put this together as no one in the community bothers talking about power requirements, yes I went overboard on my power domain, and yes it may cost a bit more than the ps3 power brick. The fact of the matter is that I got my server spec psu for absolutely nill, and my needs were for 12V power.

[printbus]

You are right that for the same power the current draw will be far less at 230V than it is at 12V. I was unaware you could get heat beds that ran on 120V or 240V. The ones that run on 120V or 240V likely use resistance wire; suggested by the wirewound reference in the resistance description. From a thermal standpoint it should work. Most of us would just prefer to deal with low voltage at a higher current than mains voltage at a lower current.

[printbus]

Ignoring the safety concerns, you'd of course need a heat bed designed to operate off the mains voltage. A 120V or 230V heat bed would be tougher to fabricate than one that operates on 12V.

We can calculate the required heater resistance using R = (V * V)/P, where V is the voltage applied and P is the desired power dissipated as heat. For a nominal 130 watt 12V heat bed, this comes up with R equal to about 1.1 ohms. Such a heat bed is readily achievable in a circuit board layout by applying the input voltage across a trace designed with a particular copper thickness, trace width, and trace length.

For 230V mains and the same 130W heat dissipation, the same formula shows R would be around 400 ohms. This would be extremely hard to achieve with just a circuit board trace, meaning the bed would have to be formed from a resistive material or multiple resistive components.

Those interested can play with the necessary trace characteristics using any of several online circuit trace resistance calculators.