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### Topic: calculating my power needs (Read 3554 times)previous topic - next topic

#### tgrwilde

##### Mar 03, 2013, 07:27 am
here's my issue

i'm looking to make a thriftshop 3d printer, after salvaging 6-8 printers i have an assortment of steppers and other motors and various other components bits and bobs for my project.

i have on hand:
1 m42sp-6te -24v 400ma has 10? on the sticker   (x axis)
1 m42sp-6nka - 24v 400ma has 6.3? on the sticker   (y axis)
1 m42sp-6te - 24v 517ma has 20? on the sticker   (z axis)
1 m35sp-7t - 6v 807ma 9? on the sticker   (feeder)

i plan to run it something similar to this

arduino uno r3 controls 2 atmega328s each of which controls 2 L293ds and 2 sensors (x+y 2 sensors are momentary switches for endstops) (z endstop momentary and extruder on). my extruder system (heating and temp control) are separate from the stepper controlling circuit

basically i'm trying to find out what size power source i need to handle; those 4 steppers, 2 atmega328s, 4 l293ds, 3 pushbuttons and 1 photoresistor(extruder on)

i know there has to be a formula for it but i'm a bit lost on it if someone could point me in the right direction of the formula and how to enter the information i would be grateful

#### Shpaget

#1
##### Mar 03, 2013, 08:00 am
400 + 400 + 517 + 807 = 2124 mA
Add some margin and you get up to 4A 24V.
Don't forget to lower the voltage for the feeder.

#### tgrwilde

#2
##### Mar 03, 2013, 08:44 am
thank you i wasn't sure if you added the amps together or the voltage or resistance or some combination thereof

ok next question is there an easy way to lower voltage while increasing amprage? i have a few printer powersupplies

2 32v 375ma / 16v 500ma
1 30v 0.4a
i have a few more somewhere in my "oooooo save it box" but can't locate them this min

#### Grumpy_Mike

#3
##### Mar 03, 2013, 09:05 am
Quote
is there an easy way to lower voltage while increasing amprage? i have a few printer powersupplies

Not an easy way, it is called a DC to DC converter or a switching regulator. These can be tricky to build for high currents or expensive to buy ready made.

#### tgrwilde

#4
##### Mar 03, 2013, 11:46 am
first thank you shpaget and grumpy_mikefor the help so far

ok found in my box o' goodies

K2750 - N CHANNEL MOS TYPE (HIGH SPEED, HIGH VOLTAGE SWITCHING, CHOPPER REGULATOR, DC-DC CONVERTER AND MOTOR DRIVE APPLICATIONS) - Toshiba Semiconductor

basic problem i have... having issues understanding the datasheet
http://pdf1.alldatasheet.com/datasheet-pdf/view/30681/TOSHIBA/K2750.html

i'm assuming it is similar to the 7805 mosfet

in  ground out

from looking at the datasheet i can't quite understand if i put 32v 940ma (yup found another adapter) in how do i find out what will come out {without the obvious try it and volt/amp meter it} it appears to be quite a few formulas on there mainly i'm just asking for little help putting it in layman's terms

#### JT007

#5
##### Mar 03, 2013, 12:04 pm
I have built my 3D printer and opted for a PC laptop power supply
Its rated for 8A output.

you might want to look at the Reprap wiki again as you may find that the 24v is the stepper motor rating and not what is actually needed.

when playing around with steppers i found that you get a choice between torque or speed, increasing the voltage provides more torque but doesnt seem to make the motor perform any faster.  i aslo ended up at a point where if i increased the voltage it made no difference apart from loading the current sensor circuit in my bench PSU.  I got no where near 24v, the motors im using i think are 12v but running with the pololu ref voltage set for 3.5v ish

http://reprap.org/wiki/Sanguinololu

in the end i bought all of my kit (sing-along-with-lulu board, 4 motors and the pololu stepper drivers) from think3dprint3d under the illusion that it would be plug and play.
to be fair it was pretty straight forward - the problems i had were more to do with setting up the mechanical accuracy of the axis.

As this is the arduino forum the Gen7 board is an excellent choice as it is built around our beloved chip.

anyways dont listen to me - Grumpy Mikes the dude

#### Shpaget

#6
##### Mar 03, 2013, 12:48 pm
increasing the voltage provides more torque but doesnt seem to make the motor perform any faster.  i aslo ended up at a point where if i increased the voltage it made no difference apart from loading the current sensor circuit in my bench PSU.

That's because the speed of steppers is not controlled by the voltage you provide them with, but rather the frequency of pulses you send to it.

#### tgrwilde

#7
##### Mar 04, 2013, 02:28 am
JT since i'm trying to do it cheaply/upcycled from old printers bought at goodwill i am kinda stuck with 24v steppers

since only the pin 8s on the L293ds are 24v and the rest being 5v

my basic question now is since i have multiple power supplies available and can use lm317t's to reduce the 32v to 24v

below is an example of possible pcb layout, i plan to use 2 of these  (x+y 2 sensors are momentary switches for endstops) and  (z endstop momentary and extruder on) controlling these 2 "modules" with the uno (design not finalized yet i figure i might need a few more pins to communicate between the uno and the onboard atmel, right now it is designed with 2 for comm and the other 2 lead to sensor)

just say this board controls X-Y and that i use the 2 24v 400ma motors if i understand right to run this module i need 24v 800ma minimum but 1.6a would be better. without having the lm317ts yet (don't want to waste 4 bucks a pop until i'm sure it if feasible) to my understanding the lm317 has max 1.5a output, if i use power each module with a 32v 940ma stepped down to 24v with a l317t will it cover the current needs, and do i need resistors to lower the current to 400ma prior to loading each pin 8 or will they "self regulate"?

#### Grumpy_Mike

#8
##### Mar 04, 2013, 08:55 am
Quote
i'm assuming it is similar to the 7805 mosfet

No it is not. Basically convert the voltage and current into power by multiply them together. Then take 0.8 of that value for switching loss. Then divide by the output voltage to get the output current. But as I said they are not easy, they require a good PCB layout to make them stable and many an experienced engineere can't get it right first time.

The L293 is not a regulating driver, you need one with those motors because the current rating will be exceeded if you put them on 24V. You can tell this because of the resistance and ohms law.

#### tgrwilde

#9
##### Mar 04, 2013, 11:05 pm
ok if i understand right that means

(32v * .940) - .8 = output
output/24v = current

(30.08) - .8 = 29.28
29.28/24 = 1.22

thank you again grumpy_mike

#### Grumpy_Mike

#10
##### Mar 05, 2013, 09:43 am
With a supply of 32v @ 375ma it has a total power of 32 * 0.375 = 12 Watts
Assuming 80% efficient you have 12 * 0.8 = 9.6W of transformable power.
So if you drop this down to 24V then 24 * I = 9.6
giving I = 9.6 / 24 = 0.4A or 400mA
So as you can see such a small drop in voltage is not giving you very much increase in current because the switching loss absorbs most of it.

#### MarkT

#11
##### Mar 05, 2013, 04:08 pm
Firstly we need to find out about these motors, the datasheets are not easily found(!)

I found M42SP-7 but not M42SP-7T.  The -7 has at least two possible winding variants anyway.
I found M42SP-6NK but not  M42SP-6NKA.  The -6NK also has multiple variants.
M42SP-6TE I couldn't find.

What is clear is that these are all bipolar motors and the nominal voltage figure is irrelevant
as is usual in bipolars (well you don't want to exceed the inter-winding voltage breakdown).

The current figure seems to be a peak (not RMS) value.  Check the resistance values with
a multimeter - hopefully these agree with the label.

You want some decent performance out of these motors so you absolutely do not want to
build a step-down switching supply, you want to go with the highest voltage supply you have
and use a bipolar chopper mode driver (such as the Pololu A4988 stepper drivers).

Given the info you have the the total power needed to energize one winding in each motor is
about 14 watts.  Assuming 80% efficiency this means about 17.5W needed which would be
about 0.55A at 32V - that 32V 0.94A supply sounds upto the job (for driving the motors slowly
at least).  For faster speeds more power is drawn and I've read somewhere you want a supply
capable of about 50% or the motor winding peak value (summed over the motors).  That would
mean more like 1.2A.  Depends how many of the motors you drive fast simultaneously.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### tgrwilde

#12
##### Mar 05, 2013, 11:14 pm
i decided to do it with 2  steppers per module one being x & y the other being z & feed so that i could with a few module swaps change it from a 3d printer to a laser cutter, or a cnc mill

thats why decided to power each module seperately

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