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Calculating amp usage on >12v setup

Posted by miro87043
 Calculating amp usage on >12v setup May 02, 2012 07:57PM Registered: 12 years ago Posts: 77
I'll start with the question then add my back story.
Is there an easy way to calculate maximum amp draw for 5 steppers and a j-head while printing at various voltages? With steppers at the same pot settings does amps x volts = watts directly apply? i.e. if 5 amps at 12v is sufficient to power 5 steppers and my j-head, would I need 10 amps at 24v?

I've recently started using >12v to supply my steppers as I've learned higher voltages are needed to maintain torque at higher speeds. I started by hooking up a universal laptop adapter to my ramps 1.4. I actually have 2 of these adapters hooked to my ramps. I already had one powering the 11a spot for my heat bed. I set the adapter to 20v and the printer ran great. I'd like to turn it to 24v (max it offers), but I'm afraid the motors and the hot will require more than the 120w available.

What about running hotends at >12v? It seems to me that a hotend should work fine at higher voltages as it is already temp limited. Of course it would require twice as much power at 24v which would need to be taken into acount when selecting a power supply.

Anything else I should keep in mind while increasing voltage to my steppers?

Thanks!!!!
 Re: Calculating amp usage on >12v setup May 03, 2012 05:09AM Registered: 13 years ago Posts: 1,797
double voltage double current requirements seems ok,but be aware that power is voltage*current so power requirements need to be looked at as well. torque is only part of the equation.steps per second is what you are really looking for along with torque, a real stepper driver such as a geiko will do wonders. the quality of the driver is important, but about any driver will work better with more voltage.

keep in mind this is for drivers with a chopper circuit, that is one that has control for motor voltage and for current. if you use a current only driver,, it will die at the higher voltages.

what i would do is run 2 supplies. 1 at 12v for bed heater and extruder heater, the have a separate laptop 6amp 19v supply for all the steppers and electronics for the steppers. this would be 1amp per motor and 2amps for logic .

ground the grounds wires on the supplies into each other, and make sure i/o signals to the stepper drivers are around 5v

if you choose optical isolated parts, be aware that most are setup to expect 15v on input from parallel port, and that the max frequency response is 5-15khz. so firmware will need to make sure that pulse widths last about 200us.

as for the motors, it really only requires a large amount of power when motor is from stall point. when the coil is energized its resistance actually increases. so the requirements should be double the voltage and keep the current the same. higher voltages are required to overcome the resistance at the higher speeds. keep in mind that above 12v you should use heat sinks on all stepper drivers. and max rating is really the max rating. back emf can ripple voltage on supply lines up to several volts, so if max states 36v, then don't go above 30v for example.

it is ok for motors to get too hot to be touched, just not hot enough to melt the plastic holders. 90c is the specification that many motors are spec to perform at. keeping motors cool does however reduce resistance that will increases rpm.

also do you have high rmp motors? they could make a difference as well!

hope this helps. good luck!
 Re: Calculating amp usage on >12v setup May 03, 2012 05:05PM Admin Registered: 17 years ago Posts: 7,881
If you increase the supply voltage to the stepper drivers the current will go down. So at 24V the supply current will be about half as much. That is because they are constant current drives so the power in the motor stays the same and so the supply power stays roughly the same.

[www.hydraraptor.blogspot.com]
 Re: Calculating amp usage on >12v setup May 03, 2012 08:05PM Registered: 12 years ago Posts: 77
-------------------------------------------------------
> If you increase the supply voltage to the stepper
> drivers the current will go down. So at 24V the
> supply current will be about half as much. That is
> because they are constant current drives so the
> power in the motor stays the same and so the
> supply power stays roughly the same.

I like this answer best, because it is the cheaper. So what your saying is wattage stays when changing voltage to the steppers. If that is so and 5 amps is sufficent for 12 v then I'd only need 2.5 amps at 24 v. This would make finding a power supply cheaper.

How do you determine optimal supply voltage for a given motor?

jamesdanielv Wrote:
-------------------------------------------------------
> double voltage double current requirements seems
> ok,but be aware that power is voltage*current so
> power requirements need to be looked at as well.
> torque is only part of the equation.steps per
> second is what you are really looking for along
> with torque, a real stepper driver such as a geiko
> will do wonders. the quality of the driver is
> important, but about any driver will work better
> with more voltage.
>
> keep in mind this is for drivers with a chopper
> circuit, that is one that has control for motor
> voltage and for current. if you use a current only
> driver,, it will die at the higher voltages.

Polulus-I'm not sure the specifics of their circuit, but I know they're rated for 36 v.

> what i would do is run 2 supplies. 1 at 12v for
> bed heater and extruder heater, the have a
> separate laptop 6amp 19v supply for all the
> steppers and electronics for the steppers.

Unfortunetly on Ramps 1.4 the steppers and hotend are powered together.
My j-head works good at 20 v. Heat up takes about a minute

> this
> would be 1amp per motor and 2amps for logic .

Is this because your motors are rated at 1 amp?

>
> ground the grounds wires on the supplies into each
> other, and make sure i/o signals to the stepper
> drivers are around 5v

Ramps uses usb to power controlers=5v

> if you choose optical isolated parts, be aware
> that most are setup to expect 15v on input from
> parallel port, and that the max frequency response
> is 5-15khz. so firmware will need to make sure
> that pulse widths last about 200us.
>
> as for the motors, it really only requires a large
> amount of power when motor is from stall point.
> when the coil is energized its resistance actually
> increases. so the requirements should be double
> the voltage and keep the current the same. higher
> voltages are required to overcome the resistance
> at the higher speeds. keep in mind that above 12v
> you should use heat sinks on all stepper drivers.
> and max rating is really the max rating. back emf
> can ripple voltage on supply lines up to several
> volts, so if max states 36v, then don't go above
> 30v for example.
>
> it is ok for motors to get too hot to be touched,
> just not hot enough to melt the plastic holders.
> 90c is the specification that many motors are spec
> to perform at. keeping motors cool does however
> reduce resistance that will increases rpm.
>
>
> also do you have high rmp motors? they could make
> a difference as well!

Not sure. These are the motors [www.kysanelectronics.com]

> hope this helps. good luck!
 Re: Calculating amp usage on >12v setup May 04, 2012 04:17AM Admin Registered: 17 years ago Posts: 7,881
miro87043 Wrote:
-------------------------------------------------------

> I like this answer best, because it is the
> cheaper. So what your saying is wattage stays when
> changing voltage to the steppers. If that is so
> and 5 amps is sufficent for 12 v then I'd only
> need 2.5 amps at 24 v. This would make finding a
> power supply cheaper.

Yes but remember the heaters do not behave this way. The current will double and the wattage will increase with the square of the voltage. You may find the extruder heater resistor does not last long at four times the power. You should increase it to 27R for 24V.

>
> How do you determine optimal supply voltage for a
> given motor?

Higher voltage will allow higher top speeds but may reduce the microstepping accuracy, see [hydraraptor.blogspot.co.uk].

Also as James says, 35V is absolute max rating of the chip, so you should never run it that high. You need some margin for supply transients as the motors generate back EMF which can raise the supply rail.

You will probably need about 1A to drive the motors, but that is the motor current and with a resistance of 2.8R the power is 2.8W. At 12V that would only take 0.23A plus a bit more for switching losses. To work out the exact current you can use the maths I show here: [hydraraptor.blogspot.co.uk] but if you allow say 0.3A per motor @12V you shouldn't be far wrong and half that at 24V.

Edited 1 time(s). Last edit at 05/04/2012 04:32AM by nophead.

[www.hydraraptor.blogspot.com]
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