Is there any way to lower my Prusa MK3S+'s wattage?

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Hi all,

I have a Prusa i3 MK3S+ 3D printer that I wanted to operate at a stall at the beach. For this I bought a Ryobi power inverter (link) to supply it. The inverter can supply 220 watts which I thought would be enough to power the machine given it maxes at 150 watts during printing. However it appears that on start up it uses 300 watts, as a result the printer does not turn on and the inverter flashes.

Is there any way I can lower the wattage on start up? It can be either hardware or software. I think I can get away without a heated bed so maybe I could unplug that?

Thanks in advance! Any help would be greatly appreciated!
Ezra

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Hi Ezra
You might get away with a “soft start”. It is a technique where the initial brush current is limited by resistors which are shorted out after a predetermined time to allow full voltage be applied. There may be commercial devices around but I don’t know for sure. The only time I have had any dealings with this the system has been built in. Silicon Chip magazine had some projects along these lines some time ago but I don’t remember at what power level.
Cheers Bob

Where did that 220W come from. The link clearly states 150W with a PEAK of 225 W. NOT continuous. That PEAK is for a very short time to accommodate just what you are experiencing, INRUSH current. But apparently your inrush is 300W. You have virtually no margin anyway at a usage of continuous 150W. That is about as skinny as I have seen for awhile.

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Hi Bob,

Thanks heaps for the reply.

You’re right, I accidentally wrote 220W instead of 225W - sorry 'bout that.

While the soft start is probably a valid solution, it is not really something that I am familiar with. I have been advised on a separate forum that I may be able to get away with connecting a large enough capacitor to support the output during the surge. Would you be able to elaborate on the intricacies of this, and if it would work at all?

I appreciate your help and expertise!
Ezra

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Hi Ezra
I would not take any notice of that. Might help a DC supply (but that is arguable) but all it will do with an AC supply is increase the load and you have not got enough capability now.
I think your device is too light for the job.
Cheers Bob

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Hi Bob,

Thanks for the reply. I did think it was slightly too good to be true for a capacitor to allow the printer to handle the inrush.

My final idea was to employ the use of an Inrush Current Limiter, potentially inline with the printer and the inverter. Would this be a feasible option, and if so, do you have any recommendations as to one that would work.

Thanks again for your help!
Ezra

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Hi Ezra

That is what soft start is. As for options refer above.

It is not so much the printer but the usual switch mode power suppliy for the printer which have an enormous inrush charging up the capacitors. For instance I have a power supply which only uses well less than 1A (@ 240V) normal running but the inrush surge is quoted as up to 40A. A huge difference.
Cheers Bob

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It’s not a capacitor - it’s a thermistor. See, for isntance:
PTC Thermistors For Inrush Current Limiting | Ametherm

Whether or not that would be suitable in your instance would require good examination of the device startup characteristics, and careful matching to the correct component. The advantage of this solution is that it might be possible to install it inside the device.

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Hi Jeff, Ezra
The “normal” device of this type is a NTC thermistor.
I have used PTC thermistors where you don’t want 1 amplifier say to take out a whole rack full because of a power supply shut down. The resistance goes up very quickly and dramatically when overcurrent causes them to heat up. They reset when the cause is removed. We used to call them Polyfuses, Jaycar call them Polyswitches. They are primarily triggered by a fault and I have never seen them used for inrush control.

I have seen NTC devices used for inrush control where there is not a lot of power involved. They are not any old thermistor and are designed for the job. Down side is they can get very hot and stay hot under normal use. I have come across a couple of 2 legged strange blobs in series with A and N of switch mode supply inputs and I strongly suspect these are inrush control NTC devices

Jeff, That link is a bit misleading. The example circuit demonstrating the use of PTC devices actually bypasses the NTC until inrush settles then becomes an overcurrent or fault monitor. Due to being bypassed they can co nothing about controlling inrush.
Cheers Bob

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No. The link is to the NTC thermistor example, not the PTC. PTC thermistor limiting is discussed elswhere in the context of specific scenarios which do not apply in this case.

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Hi Jeff
It is the heading
“PTC Thermistors For Inrush Current Limiting”
That is misleading. Nothing you said.
Cheers Bob

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Hi Ezra,

If your goal is to print remotely why not just use a battery directly?
Some other printing enthusiasts have used car batteries to power up their Prusa Mini:
https://www.reddit.com/r/prusa3d/comments/oo0rx7/prusa_mini_on_camp/
It doesnt look like they have a regulator between but that might be worth adding for some of the more sensitive components.

It most likely voids the warranty but is super neat seeing that it works!

Another good discussion here: https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-general-discussion-announcements-and-releases/ups-for-mk3-full-day-of-printing-on-battery/

Liam

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Hi everybody,

Thank you very much for all of your suggestions and expertise.

Liam’s suggestion of a car battery seems the most viable for me, though I have noted every other suggestion, as they are definitely helpful nonetheless.

Thanks again.
Ezra

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