# question about steppers and power supply current rating

• I have calculated that I need 100 watts for my system. I am upgrading from 12V to 24V but that also drops the rated output current, but my steppers will still draw the same amount (of current) as before. The total expected current of my three steppers is more than the rated output current of the 24V. I know of things like peak vs continuous, etc but I need a rule of thumb here.
BTW, if anybody here can recommend a 24 v supply in the power range that is SMALL, please give me a link. All the 100+ units I can find are like 8" long, except for the DIN-mounted expensive versions.
thanks

• thanks Phaedrux. I already read the first but no mention of trade-off of current wrt 12 and 24 supplies. The others are new to me but still don't address my question. Maybe I don't understand the details enough but here my working calculations:
-my steppers are rated at 2 amp, so 85% of three of those is 5.1 amps.
-but the rated voltage on these is only 3.5V. That means 3.5*5.1 = 18.85 *1.5 for overhead = about 27 watts.

• A 27 watt, 24V supply will only source about 1 amp, WAY below the 5.1 amps above. To get the required current means a way oversized power supply of 125W. Since the stepper drivers are not switching power supplies, you can't trade voltage for current to source high-current to the steppers and still use a low-power supply. This is my conundrum.

• I'm also trying to find out why I would chose a small dense sealed LED supply over a cage-enclosed type of traditional switcher. What's the catch? It's not price.

• I believe the question regarding voltage is summed up in the 2nd link (above), in the Resistance and rated voltage section: These [resistance & rated voltage] are unimportant, except that the rated voltage should be well below the power supply voltage to the stepper drivers, and in the How to work out the power supply voltage section, item 6: Preferably, the driver supply voltage should be at least the sum of these two back emfs, plus a few more volts.

In essence, they're saying steppers are not typically driven at their rated voltage, but rather at a much higher voltage (which can cover a wide range). When calculating power supply size, only worry about their rated current.

Regarding your last question of why choose a small LED supply - I don't understand your question. From what you asked in your first post, you yourself have chosen to go that route based on size. I recently built a new delta with a 300mm diameter build plate, and had ample room to use a 24V 150W Meanwell cage-type supply in the base (the Duet is mounted in the top). Not only was it only \$18 from Mouser, I just prefer the greater heat dissipation ability of the cage-style.

• I'm also trying to find out why I would chose a small dense sealed LED supply over a cage-enclosed type of traditional switcher. What's the catch? It's not price.

I chose that one because you said you were looking for a small unit.

I guess I don't understand what it is you're asking though. I don't think amps works the way you describe. If you follow the method of adding up the total system draw in watts and choosing a supply with enough current in amps to meet your single largest draw item you'll be ok.

• @phaedrux I'm going to chose to understand the power supply body styles, it's not a huge deal and the sealed unit is still quite long. I'm used to the caged type in general.

I'm still confused about the power supply specs lets just skip the voltage part.
-Motors are 2 amps, derated to 1.7 each. Three motors makes that 5.1 amps drawn from the supply, regardless of the supply voltage.
To keep this simple lets just ignore the other loads and overhead.
Do I just choose a 24V module with at least 5.1 amps? (which is a 122W supply).
Or do I chose a supply based on the documentation, which is 2A^2*1.4ohm in my case? For three motors that is only about 18W.
Which is right? One of my assumptions is vastly wrong.

• -Motors are 2 amps, derated to 1.7 each. Three motors makes that 5.1 amps drawn from the supply, regardless of the supply voltage.

No, it's very much less than 5.1A, and less at 24V than at 12V. The stepper drivers act like buck regulators, reducing the voltage and increasing the current. See https://duet3d.dozuki.com/Wiki/Choosing_the_power_supply#Section_Total_power_needed for how to work out the power needed.

• apologies if I already replied but I don't see my post here.
Anyway, thanks Dave, that makes total sense. I figured I might be oversimplifying the stepper loading.
-Since I don't want to change everything over to 24V, I plan to wire 24 into the Duet but wire my old 12 into the middle pin of the V_fan select header. Both supplies would merge their negative terminals. Sound ok?

• Since I don't want to change everything over to 24V, I plan to wire 24 into the Duet but wire my old 12 into the middle pin of the V_fan select header. Both supplies would merge their negative terminals. Sound ok?

Yes that's OK.