Can I connect a PSU with a higher current rating than RAMPS 1.4 can handle?

Question

I have a 12 V/30 A power supply. Will a RAMPS 1.4 shield burn if I use that power supply with it?

Answer 1

Voltage and current behave differently and it is important to remember two essential points:

• PSU voltage specifications should never be exceeded, else the circuit will (most likely) be damaged, and;
• PSU current specifications can be exceeded, without any risk to the circuit¹.

So, if your circuit requires 12 V and 3 A, then the PSU must supply a maximum of 12 V, but current can be 5 A, 10 A or whatever.

It may help to think of it this way:

• Voltage will force its way into a circuit, and if you exceed the specified requirement of that circuit, then the circuit will be damaged, for sure;
• Current passively sits there and is is only taken by the circuit as it is required - if your power supply can provide 1000 A, but the circuit only needs 11 A, then the circuit will only take 11 A. The remaining 989 A will be ignored.

Note: There is one caveat, where the mismatching is reversed: If a PSU can only supply 5A, but the circuit requires 10 A, then the PSU may be damaged as the circuit attempts to draw more current than the PSU can provide.

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The water analogy

You can think of electricity as similar to water in a water tank, and a tap as being the outlet/socket/circuit:

• The higher the water tank is above the tap, the higher the water pressure - this water pressure is analogous to voltage. If the water pressure is excessively high, then the tap could fail, in the same way that a circuit will blow,
• The larger the tank, the more water there is that can be provided - This capacity of the tank is analogous to the current. You don't have to use, or drink, all of the water in the tank, but (often) it's nice to have an excess available, even though it might be wasteful or not particularly cost effective.

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Footnote

¹ The only major downside to having an over rated PSU with respect to current, is that should the circuit that the PSU is supplying have a issue, such as a short circuit, then a lot more current will be drawn, than if the PSU wasn't so over rated. This could be dangerous, if, for example, you are touching part of that circuit. This is why the current rating of a PSU should closely match the current requirement of the circuit which the PSU is supplying.

Other considerations for using an over rated PSU could (arguably) include a longer life or increased reliability. This would be due to less stress being placed upon the PSU, if the current rating of the PSU is, for example 25 - 50 % greater than the current requirement of the circuit.

Answer 2

Yes you can connect that PSU to the RAMPS 1.4 shield, and no, it will not burn the shield under normal operation (too much and/or too high current draining peripherals attached or shorting of circuits).

It is not the PSU that determines how much current is being drawn from the PSU, it is the peripherals that determine this. The RAMPS 1.4 shield has (poly) fuses for 11 A and 5 A, so the maximum you can draw through the board is 16 A. You cannot draw more than 11 A for the heated bed, and 5 A for the hot end heater and the steppers combined. If your heated bed uses over 11 A or the rest over 5 A, the fuses will burn through cutting the circuit. These fuses are meant to protect your board from draining too much current.

Connecting a PSU with a higher current specification is perfectly fine as long as you do not exceed the board specifications with your peripherals.

In case your heated bed (which takes the majority of the power) requires over 11 A (which requires thick cables!), you need to use an external MOSFET; this doesn't add a safety, it merely allows to use more current from your PSU to power the high demanding peripherals.

Answer 3

No, it won't burn if you use a power supply that put out 12 Volts and tolerates up to 30 Amps.