35V RAMPS? April 11, 2013 01:59PM Registered: 11 years ago Posts: 809
Any reason we don't do this?

Assuming of course that you run DRV8825 and appropriately-valued components and no D1 onboard RAMPS.

Running RAMPS with this would be nice.

Currently running @24V with no issues (and no PTC fuses). I can't see any problems, aside from driving PIDMAX value for hotend down.

Anyone done this before or am I entering uncharted territory?

- akhlut

Just remember - Iterate, Iterate, Iterate!

[myhomelessmind.blogspot.com]
 Re: 35V RAMPS? April 11, 2013 04:57PM Registered: 10 years ago Posts: 544
ive been wondering my self, i know for the steppers you could run them up to 3 times faster, i have these crapy little motors that can only to 60mm/s on 12v, they are such pissing off. i think 36v psu are readily atainable, and for the same price as the 12v led psu's. you can tweak the voltage down to 35, and also the pololus will take up to 35v so its all good there. also if you have both a 24 and 12v psu can you look inside to see how different they are component wise, it might be as simple as changing out a resistor and cap or two.
 Re: 35V RAMPS? April 12, 2013 01:13PM Registered: 10 years ago Posts: 57
Remember your bed watts are going to increase drastically as you increase your voltage. I think that power supply would be under-rated for the bed.

Your bed resistance is set and can't be changed (unless of course, you change the bed). I will use my bed for example, which is the TrintiyLabs 24v 200w heater, which means it draws 8.3a @ 24v.

V*A = W
24*8.3 = 200

Resistance (as given by Ohm's law is)

24/8.3 = 2.89 ohms

So my bed should be around 2.89 ohms. If we change the voltage to 35v, this means the current will also increase:

35/2.3 = 15 Amps

Now you have a 530 watt bed, which would heat up super fast. Down side is you are now pulling 15 amps so you're going to need a bigger power supply.

Edited 1 time(s). Last edit at 04/12/2013 01:14PM by mitchese.
 Re: 35V RAMPS? April 12, 2013 01:51PM Registered: 11 years ago Posts: 809
Now you have exposed a chink in the armor... Not really the end of the world though. I've switched all of my heated beds over to 110V AC and control them via SSR's. As a consequence I'm drawing very little from the DC PSU's. This works very nicely for me.

BUT...

I didn't think about this, but the SSR control voltage is limited to 32V. So I'll plug a 12V source into the 11A port of the power strip and tie the grounds together. Hooray! Fried SSR averted!

Thank you for making me think this through a bit more! Nothing like hitting an unexpected roadblock just as you 'fire' something up.

Edited 1 time(s). Last edit at 04/12/2013 04:06PM by akhlut.

- akhlut

Just remember - Iterate, Iterate, Iterate!

[myhomelessmind.blogspot.com]
 Re: 35V RAMPS? May 24, 2013 05:31PM Registered: 11 years ago Posts: 7
I think running those DRV8825's at 35V is likely to seriously screw up your micro-stepping thanks to the higher voltage and their rather long minimum on-time. It's similar to the issue Nophead describes with the A4988 here:
http://hydraraptor.blogspot.com/2012/04/stepstuck.html

The DRV8825 has a minimum on-time of 3.75μs with an effective off-time of around 30μs (It's fixed frequency, not fixed off-time). Since you'd be running at 3x the voltage and that's over 3x the minimum on-time, the problem is going to be 9x worse. Unless you're running 18V steppers or something, you're likely going to be skipping ahead several steps when the driver tries to step from 0% current to whatever small % you choose.

I'm actually not sure the DRV8825s are a good idea even at 12V, since they have such a long minimum on-time.
 Re: 35V RAMPS? May 24, 2013 06:55PM Registered: 11 years ago Posts: 809
DRV8825's have been remarkably consistent at 24V so far.

Actually consistent is an understatement. More like rock solid.

They have outperformed A4988's in every way.

But maybe it's my setup that has lent to the successful use of the drivers - only 1/8th microstepping.

I haven't gotten around to pushing 35V through my setup yet - I've been...distracted.

- akhlut

Just remember - Iterate, Iterate, Iterate!

[myhomelessmind.blogspot.com]
 Re: 35V RAMPS? May 27, 2013 07:37PM Registered: 10 years ago Posts: 369
hmmm i dun quite get the correlation of the voltage to this DRV8825s. i do have 3 of this chips in my little "to do" box (waiting to build up my PCB mill, then a tester DRV8825 ... maybe soon hopefully lol).

so what i gather from what you guys are saying is that, DRV8825 have more problems at microstepping? unless you inject higher "Vmot" so to speak?

KSevcik Wrote:
-------------------------------------------------------
> I think running those DRV8825's at 35V is likely
> to seriously screw up your micro-stepping thanks
> to the higher voltage and their rather long
> minimum on-time. It's similar to the issue
> Nophead describes with the A4988 here:
> [hydraraptor.blogspot.com].
> html
>
> The DRV8825 has a minimum on-time of 3.75μs with
> an effective off-time of around 30μs (It's fixed
> frequency, not fixed off-time). Since you'd be
> running at 3x the voltage and that's over 3x the
> minimum on-time, the problem is going to be 9x
> worse. Unless you're running 18V steppers or
> something, you're likely going to be skipping
> ahead several steps when the driver tries to step
> from 0% current to whatever small % you choose.
>
> I'm actually not sure the DRV8825s are a good idea
> even at 12V, since they have such a long minimum
> on-time.

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 Re: 35V RAMPS? May 28, 2013 07:41AM Admin Registered: 16 years ago Posts: 7,871
Higher voltage makes it harder to get the low current micro steps, especially for 1/32 as the on time needs to be very short if the off time is kept short enough to be ultrasonic.

[www.hydraraptor.blogspot.com]
 Re: 35V RAMPS? May 28, 2013 08:00AM Registered: 10 years ago Posts: 369
ok i understand the problem now ... [plus.google.com]

or partially lol ...

hmmmm quite a handful there ... for A4988, when i use it with low current steppers, the whining disappears (opposite when you use high current steppers)

so for the drv8825 ... it is going the opposite way it seems. i wonder how could 1 mod a DRV8825 to supercede the A4988 ... or it can never be done?

Edited 1 time(s). Last edit at 05/28/2013 08:05AM by redreprap.

______________________________________
__my mixed bag blog || aka --> [http] || ___ so 3D printing is everywhere ... dont worry, hospitals can now 3Dprint body parts, they will charge you \$1million excluding surgical fees ... you will die paying your debts. thats their aim ___ if every patent expires tomorrow, everybody will surely get a 3dprinter and make EVERYTHING ! ____ there is a "DIY-DTG" t shirt printing forum, you can mod an EPSON printer to PRINT like a pro. ___ CNCzone? overly commercialized it seems ___ my country? they will be taxing you for every cm of road you use and track you to your grave using GPS and its government authorized, now they will fire all the traffic wardens instead.___ EEVBLOG? there is only 1 way to do things --> take it apart like a pro
 Re: 35V RAMPS? May 28, 2013 10:11AM Registered: 11 years ago Posts: 7
Your observation on the A4988 whining makes plenty of sense when you start looking at how it works. It's a constant off-time driver, so the set off-time determines the maximum switching frequency. When it's putting out a higher current, the driver has to stay on longer, which lowers the switching frequency. Plus, the A and B sides of the driver are nearly guaranteed to be switching at different frequencies, which means they'll be creating a beat frequency that could be in the audible range.

The 8825 uses fixed frequency switching which means there won't be any beat frequencies and you'll likely never hear switching noise from it.

I think the only way to get an 8825 to accurately microstep would be to use fast decay all the time, or use rather high voltage, high current steppers on it. High enough that the 3.75μs minimum on-time doesn't make you miss your first microstep. Although thinking about it, I'm not entirely sure that's actually possible.

I'm now somewhat interested in getting my hands on one of those and hooking an oscilloscope up to it to see how it actually works, because it's currently looking like there no way it could ever make accurate microsteps smaller than 1/8.
 Re: 35V RAMPS? May 28, 2013 10:17AM Admin Registered: 16 years ago Posts: 7,871
I think 1/32 is only achievable when the motor voltage is close to the supply voltage due to the minimum on time being much too long and the off time fixed. But with a low voltage supply speed is severely limited. The later Allegro chips have a fixed minimum on time that is to long and this is even worse. And it doesn't have the low current micro stepping mode we need.

I don't know what experience the people who design these chips have, but none of them seem ideally suited to driving real stepper motors!

Fixed off time doesn't seem to be the best idea. I saw an audio amplifier design where it compares the speaker coil current with the target and switches on when too low and off when too high with some hysteresis to stop it oscillating too fast. I have often thought that would be a much better way to drive stepper motors.

[www.hydraraptor.blogspot.com]
 Re: 35V RAMPS? May 28, 2013 11:43AM Registered: 10 years ago Posts: 369
hmmm i guess the blokes who made this 8825 chip doesnt play with motors alot LOL ... so it seems it need a very perfectly "aligned" motor spec for it to achieve max specs or something ...

i can understand what u r referering to re the audio drive ... so far the best audio drive peeps are from germany. at about 90+ % efficiency, they PWM up the incoming AC into DC, and PWM this DC into audio ... pure evil power! the sheer accuracy of power control is just amazing.

______________________________________
__my mixed bag blog || aka --> [http] || ___ so 3D printing is everywhere ... dont worry, hospitals can now 3Dprint body parts, they will charge you \$1million excluding surgical fees ... you will die paying your debts. thats their aim ___ if every patent expires tomorrow, everybody will surely get a 3dprinter and make EVERYTHING ! ____ there is a "DIY-DTG" t shirt printing forum, you can mod an EPSON printer to PRINT like a pro. ___ CNCzone? overly commercialized it seems ___ my country? they will be taxing you for every cm of road you use and track you to your grave using GPS and its government authorized, now they will fire all the traffic wardens instead.___ EEVBLOG? there is only 1 way to do things --> take it apart like a pro
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