Power Supply Amperage Help!

I am designing a printer and I have purchased a RAMPS kit and four stepper motors.
Each stepper motors is 1.5 Amps
I will also need a heated bed plate (plugged into D8)
(This is all going to run at 12V by the way)

Now for my questions: Can my heated bed plate be more than 11A if I buy a power supply capable of giving that much power? Is anything else using power from that half of the power terminal?

For the other half of the power terminal, I guess that I will need at least 6 Amps for the stepper motors plus some power for the Arduino and fans etc. or am I overcomplicating things because I will theoretically never have all four motors at full speed at once?

Also, how do I know how much power goes into each half of the terminal when purchasing a power supply? I found: http://www.makergeeks.com/5012siatxpos.html which has a nice tutorial on how to install it. It shows this on the bottom of the page. Can someone help with interpretation?

DC Output Specifications:
+3.3V, 22A
+5V, 30A
+12V, 22A
-12V, 0.8A
+5VSB, 3A

Thanks a ton in advance!
~Evan

Another limitation is what your controller supports. Sanguinololu is known to be quite limited, RAMPS is known to support some 10 A max unless you replace the MOSFET or use a SevenSwitch. Other controllers like Gen7 are known to be good for 15A.

Also, don't forget the extruder heater, which can draw between 2 and 4A, too.

ATX PSUs typically require some 20 - 30% margin to run flawlessly, so multiply your calculation with 1.3.

Other than that, yes, you can run such a setup if your PSU delivers this current.

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So if I have a power supply with these specifications, how do I figure out the total amperage that goes into each side of the RAMPS board?

DC Output Specifications:
+3.3V, 22A
+5V, 30A
+12V, 22A
-12V, 0.8A
+5VSB, 3A

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Evmanw
Now for my questions: Can my heated bed plate be more than 11A if I buy a power supply capable of giving that much power? Is anything else using power from that half of the power terminal?

Your heated plate will draw as much current as it can get up to V/R amps where V = your voltage and R is the resistance of the heated plate. Depending on how your plate was designed, that may be limited to around 11 amps. Or it could be more, or it could be less. If it wants right at 11 amps or more, then expect the polyfuse to trip. If your bed specs are out of whack from what's typical, or you want to use a more powerful heated bed, you'll need to use a relay (solid state or mechanical) that is controlled via D8 that is rated for the current that you are needing and hooked up to a power source that can handle the load.

D8 draws it's power directly from the 11A power terminal and is the only thing on that circuit. The other power terminal is 5A and powers everything else in a typical setup..

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For the other half of the power terminal, I guess that I will need at least 6 Amps for the stepper motors plus some power for the Arduino and fans etc. or am I overcomplicating things because I will theoretically never have all four motors at full speed at once?

You're not overthinking, but it's not just add up all the current ratings and that's what you need. It's more complicated than thatSteppers due to how they operate have a constantly changing current requirement as each winding energizes and de-energizes for each step (or microstep). ~5A usually is all that is needed for the extruder, electronics, and motors.

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Also, how do I know how much power goes into each half of the terminal when purchasing a power supply? I found: http://www.makergeeks.com/5012siatxpos.html which has a nice tutorial on how to install it. It shows this on the bottom of the page. Can someone help with interpretation?

DC Output Specifications:
+3.3V, 22A
+5V, 30A
+12V, 22A
-12V, 0.8A
+5VSB, 3A

You don't decide how much power goes in. Electronics, whether they are a micro controller, stepper motor, or heated bed, draw current...the current isn't pushed to it. You just need to hook up the wires, usually 2 or 3 are sufficient to each connector. The ATX specification says that the yellow wires are +12V and the black wires are ground. Those are the only two that you need aside from the typically green PS_ON lead. All the others can be taped off or snipped as you won't need them. The one exception to this is that some power supplies need a load on the +5V rail in order to have full power on the 12v rail. When selecting a power supply, don't look at the overall power rating of the power supply (typically 300-500 watts or more). The only thing you are concerned about is the rating of the 12v rail. In the Maker Geeks case, that is 22 amps which is sufficient. If they list a wattage instead of a current rating, divide the wattage by 12.

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cdru
You don't decide how much power goes in. Electronics, whether they are a micro controller, stepper motor, or heated bed, draw current...the current isn't pushed to it. You just need to hook up the wires, usually 2 or 3 are sufficient to each connector. The ATX specification says that the yellow wires are +12V and the black wires are ground. Those are the only two that you need aside from the typically green PS_ON lead. All the others can be taped off or snipped as you won't need them. The one exception to this is that some power supplies need a load on the +5V rail in order to have full power on the 12v rail. When selecting a power supply, don't look at the overall power rating of the power supply (typically 300-500 watts or more). The only thing you are concerned about is the rating of the 12v rail. In the Maker Geeks case, that is 22 amps which is sufficient. If they list a wattage instead of a current rating, divide the wattage by 12.

Ok, so I am not hooking up "two different sides" (an 11A and a 5A) for the RAMPS board. Instead, the electronics (heaters, motors, etc.) are all drawing current from the 22 amps which is available?

Thanks a ton! This information is really helping me out.

Just of note:

The reason these connectors are separate is two-fold:

1. It's easy to connect a different voltage up for your heatbed (eg: you want to run it at 15 volts).
2. The connectors on the boards are rated for a max of about 12A usually (you can get higher spec connectors, but they're not as easy to get hold of). As such, there was no easy way to run more than 12A into the board on one pair of connectors.

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Evmanw
Ok, so I am not hooking up "two different sides" (an 11A and a 5A) for the RAMPS board. Instead, the electronics (heaters, motors, etc.) are all drawing current from the 22 amps which is available?

Correct. One other thing to look out for is if the supply has just a single 12V rail or two (or more). Sometimes the power regulation is split up to use multiple lower-power circuits than a single higher-power one. You can still use such supply (presuming each rail meets the necessary power requirements) but you just need to make sure you combine wires from the same rail. Obviously a single rail design is easiest as it's a non-issue then.

Ok, that makes sense.

Now, I purchased a heater and I just realized that it runs at 10.3V. Is there any way while still using the same 22A 12V power supply from MakerGeeks (link above) to be able to run this heater?

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Evmanw
Now, I purchased a heater and I just realized that it runs at 10.3V. Is there any way while still using the same 22A 12V power supply from MakerGeeks (link above) to be able to run this heater?

Do you have a link to a product page? 10.3V sounds like a really strange voltage. Is the heater specifically made for 3d printing? Or is it for something else that that you are repurposing it?

It's possible that you can run your heater at a higher voltage, but probably not likely unless it was seriously over-engineered/under-speced. P=V*I and V = I*R, so rearranging P = V^2/R. Say your heater calls for 10.3 volts and runs at 1 ohm giving you 10.3 volts ^2 / 1 ohm = 106.09 watt heater. If you ran it at 12 volts, that would be 12 volts ^ 2 / 1 ohm = 144 watts. Increasing the voltage 1.7 volts gave you nearly 35% more watts of power, which depending on what us being used as the resistance, may result in a shorter lifespan, releasing it's magic smoke very quickly, or could be acceptable.

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Evmanw
Now, I purchased a heater and I just realized that it runs at 10.3V.

Sounds like you need thicker wires from PSU to controller and from controller to heated bed. 2.5 mm² is a good choice.

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