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Help needed calculating power supply

Posted by LukeTansell 
Help needed calculating power supply
January 17, 2017 09:28AM
G'day everyone,

I'm trying to figure out what power supply I need, but cannot for the life of me figure out how to calculate it. I have a 24v 350W power supply at home, which should run 14.5a.

Am i right in thinking that It'll power this silicon heat bed: [www.ebay.com.au]
and nothing else?

I'll also be running up to two hexagon hotends (https://www.bilby3d.com.au/DispProd.asp?CatID=&SubCatID=131&ProdID=ptHotEndHex175)

There's also the option of just getting a 240v silicon heater and running it with an SSR, but I'm going to try running a 24v heatbed through my Duetwifi first off.

Cheers in advance!
Re: Help needed calculating power supply
January 17, 2017 09:54AM
It won't even power that since the heater consumes 50W more than the maximum power output of the PSU.
In general you should stay within a reserve of 10-20% of a PSU, so for your 350W PSU you should stay below 300W.
Every heated hotend will add roughly 40W, the motors can use up to 30W per piece (in worst case, usually <20W) plus some power needed to run the controller board, lights etc.
I would go for the 240V heater with SSR. In that case you can continue to use the PSU.

Re: Help needed calculating power supply
January 17, 2017 10:14AM
It depends if you want to bang-bang heat the hotbed or PWM it with P, I, D values.

If you PWM it, you will be limited to 7/8*100/1 = 87.5% duty cycle. Bang-Bang heating, putting full drain on a PSU, is called bang-bang for multiple reasons - it can make your PSU go bang (more of phut, really), and it's a term for digital / on off switch control, as opposed to proportional analogue control.

So you could use it, but it would heat less quickly than a unit fully capable of running at 100% permanently on Bang-Bang, but if using smoother PWM, it is just limited to how hard the PWM is. 50% is the regular value on Marlin, you can up that for quicker heating but usually less controlability.

Bang bang gives you least control, especially with a slow heat measurement device (themistor, thermocouple). You want fast feedback from the sensor, ideally. Sometimes you have to go that way and do bang bang on a hotbed, because PWM isn't an option. That's when you are glad that your PSU is full capacity, 100% duty cycle, and at least it won't be going "phut" or "bang".
Re: Help needed calculating power supply
January 17, 2017 12:42PM
There is guidance on working out how much power you need on the Duet3D wiki, at [duet3d.com].

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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