Amp Rating

I am building a Prusa Mendel with Ramps 1.4 and a heat bed.

I want to use my switched 12vDC 25A 300W power supply box.

The site I purchased my pre-assembled Ramps from (ultimachine.com) has the following note:
"• Min 12V 5A DC power supply required (up to 16A total required if heatbed is used)"

I imagine a 25 amp PSU is not going to be a problem/cause damage - do I need a different 12vDC PSU that has a lower amp rating?

I know the proper operating voltage is critical for not harming the electronics (e.g., 12vDC) and amps indicate current draw (need at least 16 amps because I have a heat bed per the Ramps reprap wiki entry), but I just want to double-check re saftey. Sorry if this is a stupid question re too much amp put out by PSU, but I always play it crazy safe with any project that involves drawing power from a wall plug.

Thanks.

Should be fine with what you have. If you're super worried someone a bit more electrical savvy than I will hopefully post here soon :-)

A lot people seem to use much higher than 16A, I've seen people commenting that they use up to 30A supplies. Sounds like you have a proper box but if you've hacked a computer PSU it is my understanding that you need a dummy load on the 5v rail for the 12v to reach its stated output (you'd have to look at the details a bit more if it is a comp psu, if not you're all good).

For the record I use a 10A supply for the electronics and run the heat bed seperately. Have had no power issues with this setup.

Good luck!

The amp rating can be as high as you like as the machine will only draw the current it requires , ie if you have a 40A power supply and your load only requires 2 A that is all it will take from the power supply.

If it is an ATX pc power supply it is good practice to fit a load resistor on the 5 V line of about 5 ohm. This helps keep the 12V line up.

5 Volts into 5 ohms draws 1 amp, right ? And dissipates 5 Watts ?

Since 5 ohms is not a standard resistance value, how about a 4R7 ohm 10 watt 5% wire wound resistor ($1.85 at Jaycar). That would give a current of between 1 and 1.12 amps, depending on the accuracy of the resistor, and power dissipation of between 5.0 and 6.3 watts.

Can I just connect this between a red and a black wire on one of the molex pins on the ATX wiring harness?

Andrew Roberts
brazenartifice@gmail.com
brazenartifice.wordpress.com

The value is not that important but 5R can be found here 10W Arcol Aluminium Clad Wirewound Resistor 5R at spiratronics in the UK or ebay.

A good read here [reprap.org]. It is important to connect a group of the yellow cables together and also a group of black cables to enable to get a bigger current rating, rather than just one yellow and one black.

OK, I've just hooked up a 4R7 ohm 10 watt resistor to red/black on a spare molex connector. My 12v line has gone from 11.65V to 12.5V unloaded. I haven't tried it fully loaded yet (motors + hot end + heatbed), because I need to rewind a whole spool of filament first

The resistor gets very hot, as it warns in that useful link you gave. Maybe I'll put two 10 ohm resistors in parallel in place of the 4R7, to spread the heat. Not that I've noticed any problems without the load resistor. (Don't fix it if it ain't broke?)

So thanks for giving me another detail to tinker with in the search for printing perfection

Andrew Roberts
brazenartifice@gmail.com
brazenartifice.wordpress.com

I use a 5R bolted to the inside of the ATX. The case will then dissipate some heat from it. You won't notice a difference until you use a heated bed that will draw about 12A But the ATX will be more stable..

Good luck with your build.