[Solved] How to get +19V for Melzi board?

I power my RAMPS 1.4 board from a computer ATX power supply, hacked by following some online instructions, gives
+12 V at up to 22A, so no problem.

I've now acquired a Melzi Ardentissimo board, according to the wiki it needs +19V

Melzi wiring

So what's the straightforward / resonabie price way to get +19V input ?

Thanks in advance

Mike

Edited 1 time(s). Last edit at 07/24/2012 04:39PM by 3D2B.

A laptop power supply is pretty much ideal for this, which is why it's spec'd that way.

Thanks Annirak.

I did a gogle search before posting and saw laptop PSUs, but the Amps seemed low compared with the 22A of my hacked ATX supply.

Is there a minimum amp rating I should be looking for ?
(Would like to be able to run heated bed as well as the usual steppers & hot end).


Mike

3D2B Wrote:
-------------------------------------------------------
> Thanks Annirak.
>
> I did a gogle search before posting and saw laptop
> PSUs, but the Amps seemed low compared with the
> 22A of my hacked ATX supply.
>
> Is there a minimum amp rating I should be looking
> for ?
> (Would like to be able to run heated bed as well
> as the usual steppers & hot end).
>
>
> Mike


Well, first off, there's no reason that you must use a 19V PSU. All the parts on there run fine from 12. Second, your steppers are probably rated for what, 1A per phase? Well a microstepping driver will drive an absolute worst case of sqrt(2)A per motor, since the sum of both currents is always the maximum vector sum phase current. In addition to this, a microstepping driver will limit the current; it's very unlikely that you have a stepper motor which has a coil resistance of 12 ohms.

To shorten this down, here's a an approximation for you:

P_avg = 2 * (I_phase_max^2) * R_phase * #motors + V_in^2 * ((1/R_hot_end) * D_hot_end_max + (1/R_hot_bed) * D_hot_bed_max)

P_avg is the worst-case DC power consumption (everything you can't filter down with capacitors)
I_phase_max is the current rating of a stepper motor phase
R_phase is the resistance of a stepper motor phase
#motors is the number of motors you're driving
V_in is the supply voltage (19V for a laptop supply or 12V for a PC supply)
R_hot_end is the resistance of the hot end heater
D_hot_end_max is the maximum duty cycle of the hot end (0 to 1. Choose 1 if you're not sure)
R_hot_bed is the resistance of the hot bed heater
D_hot_end_max is the maximum duty cycle of the hot bed (0 to 1. Choose 1 if you're not sure)

If you're using a fan, add it in a similar way to the hot end and hot bed.

Finally,
I_avg = P_avg/V_in

Edited 1 time(s). Last edit at 07/24/2012 04:27PM by Annirak.

Melzi works fine from 12V. The wiki instructions are for a Huxley with RepRapPro's heated bed. A laptop PSU has enough power for that, but for a Prusa heated bed you need 12V and a lot more Amps.

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[www.hydraraptor.blogspot.com]

Ok thanks,

I can see it now in:

Meliz big pic
at full resolution theres a section saying 12-30V
My board has a label covering that section.

Mike