Welcome! Log In Create A New Profile

Advanced

24V with Ramps 1.4 questions

Posted by AVRkire 
Re: 24V with Ramps 1.4 questions
July 16, 2017 10:56AM
@legarasta:

Short answer: Set the voltage to 0.75V

If you wish to take out the guesswork and calculate the current yourself, download and read the datasheet of the driver IC:
DRV8825 pdf
On page 12 you can see how the chopping current is calculated:

Ichop = V(xref) / ( 5 x Risense )

Ichop = 0.75 / ( 5 x 0.1 ) = 1.5A (so Vref x 2 is correct in this case)

Take a peek at your step stick or motor driver board (looking glass/microscope wherever needed) and find the Rsense resistors and read/measure the value (connected to pins 6 and 9). Now you can be certain what it should be set at. Sometimes the Rsense may be different to that of the schematics.

The driver has a voltage regulator running at 3.3V, which means it doesn't matter what voltage you're using, the reference voltage doesn't scale with the motor input voltage.

So somehow your potmeter was changed, maybe you bumped it during the refit of the new PSU? It should be 0.75V.


--
Kind regards
Imqqmi

New blog:
[makercentral.net]
Re: 24V with Ramps 1.4 questions
July 30, 2017 11:10AM
Hello all,

I've just wired up my RAMPS 1.4, removed the diode and replaced the fuses with 'normal' ones. I have no stepper motors hooked up yet, the drivers are in place. There is no hot-end or any sensors attached. The MEGA is of course and the LCD with memory card reader is. BTW, I have two off board larger drivers, attached via extension boards. I configured and uploaded the latest Marlin to the MEGA this afternoon.

If I understand correctly removing the diode means the MEGA does not get any power from the RAMPS anymore, so I hooked up a converter from the 24V to 5V and soldered it to the 5V pin of the MEGA. I powered up the lot without the fuses (so the MEGA didn't get any power) and the converter blew. This is only a 1 dollar item and it didn't look all that sturdy to begin with, so I soldered up an LM7805 regulator and replaced the converter with it. I restarted the lot, this time with the fuses in place. The LCD lit up but only gave little blocks. I touched the LM7805 which was way too hot to touch after only a few seconds and decided to unplug the power... The MEGA is OK, hooked up to the PC via USB (without the RAMPS attached), it works fine. When I test the LM7805 regulator without load it is fine, but I bet the voltage was lower when under load. It was really hot in a few seconds.

So, what to do? I am going to move the lot to my computer so I can plug the MEGA into the PC via USB and power the RAMPS with the 24V source. That should get me going. But what do I need to power the MEGA from the 24V source? I would think it cannot ask for much current, but it seems it does. Is that normal?

Cheers,

Hugo
Re: 24V with Ramps 1.4 questions
July 30, 2017 03:56PM
There are two diodes

D1 and D2

D1 is under the X and Y pololu modules and feeds the mega
D2 is next the the fuses and is reverse polarity protection

I suspect you removed the wrong diode
Re: 24V with Ramps 1.4 questions
July 31, 2017 12:00AM
'Unfortunately' I removed the right one, the one under the stepper motor driver. The one next to the fuses is still in place. If I loaded the Marlin firmware correctly and power everything up, should the LCD read something?

Cheers,

Hugo
Re: 24V with Ramps 1.4 questions
July 31, 2017 01:00AM
mega normally uses very little current, on its own with no lcd mine uses 0.077amps from USB.

Do you have endstop plugged in? are they in the correct way? (a 3 pin endstop plugged in backwards shorts 5v to gnd)
Re: 24V with Ramps 1.4 questions
July 31, 2017 05:15AM
No endstops, yet. Only the thermosistor for the hotend, as I would like to read the temperature to check the settings before firing up anything else.

Cheers,

Hugo
Re: 24V with Ramps 1.4 questions
July 31, 2017 03:33PM
Please take a look at the schematic of the arduino atmega2560. There's a 5V regulator MC33269ST-5.0T3 or MC33269D-5.0 (I suspect the same regulator but a different package).
If you put 5V on one of the 5V output you put 5V on the output of the regulator. Since the output of any regulator is low impedance, effectively giving it a short circuit. Hopefully the thermal protection kicks in and it still works. You'll need to unsolder the regulator as well but I suggest you use the pwrin (adapter power input) input instead. Just use a buck converter from 24 to 7 to 12V and all should be ok.

Another thing you can try if you're feeling adventurous, you could try replacing the MC33269 regulator to one that can take 24V. That would eliminate the need for a step down converter and simplify the design.

Edited 2 time(s). Last edit at 07/31/2017 03:37PM by imqqmi.

--
Kind regards
Imqqmi

New blog:
[makercentral.net]
Re: 24V with Ramps 1.4 questions
August 01, 2017 12:44AM
Great, thanks, that makes a lot of sense. I'll check the item and see what I can do. If the regulator is a replaceble item for me that sounds like a good option. I'll solder the diode back onto the RAMPS if that works.

Cheers,

Hugo
Re: 24V with Ramps 1.4 questions
August 01, 2017 05:56AM


The 3-legged part just above the auxillary power socket, right?

Hugo
Re: 24V with Ramps 1.4 questions
August 01, 2017 02:01PM
Yep, that's the one.


--
Kind regards
Imqqmi

New blog:
[makercentral.net]
Re: 24V with Ramps 1.4 questions
August 10, 2017 07:40AM
Just a thought. I have tossed the diode in the bin, but now I want to re-instate it because I have replaced the voltage regulator. Would I really need a diode? The only use I see for it is to drop some voltage across it, which is not required now I have an LM7805 in place (can take 35V in).

Hugo
Re: 24V with Ramps 1.4 questions
August 11, 2017 02:13AM
You'll have to check a few things to make sure it'll work. I think the protection diode is for protecting against connecting the input voltage to ramps and arduino power connector. If both are connected and not at the same potential, a short circuit can occur without the diode.

Also check the power dissipation of the voltage regulator at the input voltage you're using it. If it needs to drop a large amount of voltage, it may heat up. You can calculate based on the information in the datasheet how much that is, or just feel if it's still cool to the touch, if you can't keep your finger on it, it's >60 degrees C, shutdown is around 125 degrees C but at that point it will start smelling like burned plastic. You'll need to add cooling as appropriate or use a low drop regulator.


--
Kind regards
Imqqmi

New blog:
[makercentral.net]
Re: 24V with Ramps 1.4 questions
August 18, 2017 01:41AM
My recommendation will be to use a buck converter and an external MOSFET. Use 24V power supply and use buck converter to step down to 12V and power the Ramps. After that use external Mosfet to power the print bed. The easiest solution.
Re: 24V with Ramps 1.4 questions
August 18, 2017 02:51AM
External mosfets are a fad and quite unneeded, just replace current mosfet with a IRLB 8743
Sorry, only registered users may post in this forum.

Click here to login