24V build with R3 bed officially underway...

[0110-m-p]

Finally scored a deal on a 750W 24-volt Meanwell power supply (SP-750-24). Offered the guy on ebay $50 (plus the $22 shipping) for this "excellent condition with little to no signs of use" and he took it . Beats paying $200 for a new one.

http://www.ebay.com/itm/361177739904?_t ... EBIDX%3AIT

Also went ahead and picked up a Rev. 3 heated bed from SeeMeCNC for $39 and a new glass build plate (since I did this over the weekend).

The plan is to eliminate the stock PSU completely and replace it with this power supply, 40A SSR/heatsink between PSU and Bed, and a 24V to 12V step down converter between the PSU and RAMBo.

I'm excited that I will finally be able to print with the bed above 90C and it won't take 20+ minutes to get there.

[JFettig]

Why not run 24v to the rambo and run a 24v heater on the hot end? Steppers should have more torque at high speeds with 24v vs 12v.

[BenTheRighteous]

since I did this over the weekend

Seems like that's been happening a fair bit lately...

[Jimustanguitar]

Why not run 24v to the rambo and run a 24v heater on the hot end? Steppers should have more torque at high speeds with 24v vs 12v.

Fans, LED's, and other misc accessories is why I'm doing the 12v step down to mine.

[0110-m-p]

Why not run 24v to the rambo and run a 24v heater on the hot end? Steppers should have more torque at high speeds with 24v vs 12v.

Fans, LED's, and other misc accessories is why I'm doing the 12v step down to mine.

Same for me. I would rather keep the rest of the machine wired as is and just put 24V to the bed...but I don't want two power supplies to do this like I've seen others do.

Also, I don't think the steppers in the Rostock Max need any more torque since none of them are really moving much mass...especially on a machine like mine with significantly weight reduced moving parts.

[Mac The Knife]

The rambo board uses seperate inputs for hotend, logic/motors and bed. The main reason I've seen posted for using a SSR is the concern that the current requirements of the bed will overload the rambo board.

[0110-m-p]

The rambo board uses seperate inputs for hotend, logic/motors and bed. The main reason I've seen posted for using a SSR is the concern that the current requirements of the bed will overload the rambo board.

Yes, the SSR is being used because I don't want all that current going through the Rambo (too expensive of a component to risk). I forgot that the power input for the motors was separate from the rest of the logic/heaters/fans though. That means I could technically run the heated bed off 24V through the SSR, the motors off 24V directly from the PSU, and the heaters/fans off 12V with a 24V-12V buck converter.

More research needed...

[Jimustanguitar]

I'd be curious about the current settings in the firmware if you did that. Does the Rambo actually control the current of the output, or is it just a digital trim pot? Would you actually need to change anything in the FW if you switched the motors to 24v?

*edit* I'm talking about setting motor current in the firmware. The things hooked up directly to the source voltage (heaters, LED's, etc) would definitely change according to Ohm's law. I'm not trying to break physics

[jdurand]

Simple math tells you why you don't run a 24V heater through the Rambo. The bed is about 1 ohm, therefore:

At 12V the bed draws 12 Amps which is right at the 80% point of the 15A fuse. You don't want to exceed 80% long term because fuses have a +/- tolerance and they also age (they will blow sooner when they get older...ask the guys who designed the Solar Max satellite).

At 24V the bed draws 24 Amps which is way over the fuse level.

Just for info, a Wattage comparison:

At 12V the bed is drawing 144 Watts.
At 24V it's drawing 576 Watts (about half what a small burner on an electric cooktop uses).

Math used:
Amps = Volts / Resistance
Watts = Volts * Amps
Watts also = Amps * Amps * Resistance
Watts also = Volts * Volts / Resistance

[Jimustanguitar]

I spoke poorly, I'll edit the original post... I was asking about setting motor currents in the firmware at 24v. I'm wondering if the Rambo is actually a current limiting motor driver that would be voltage agnostic.

[JFettig]

If you're replying to my comment - yes I know, you'd still run the SSR on the bed, but the motors would benefit from the higher voltage is all I'm saying.

I do think you could still run 24v straight through the rambo, in the control for the bed and hot end there are values for max current(voltage maybe?).

[jdurand]

The motor drivers are constant current. The way motors benefit from increased voltage is the rate at which the current can climb in the motor winding. At low RPM a low voltage is fine as the motor windings have plenty of time to get to full current before the next step. But at higher speeds the winding may not reach the programmed current before the next step and so you lose power just when you need it.

So, two things about stepping motor drive:

Higher voltage allows for faster RPM but may or may not lead to more heat in the motor driver chip (good ones shouldn't really care).

When idle, the software should toggle the low current pin or pins to drop the motor current. High current isn't needed when the motor isn't turning and will just cause the motor to run hot. Typical motor controllers have two pins allowing 4 levels of power so at idle you use the lowest power and at higher levels at faster RPM. This is separate from the Digi-Pot which sets the max current.

[0110-m-p]

Simple math tells you why you don't run a 24V heater through the Rambo. The bed is about 1 ohm, therefore:

At 12V the bed draws 12 Amps which is right at the 80% point of the 15A fuse. You don't want to exceed 80% long term because fuses have a +/- tolerance and they also age (they will blow sooner when they get older...ask the guys who designed the Solar Max satellite).

At 24V the bed draws 24 Amps which is way over the fuse level.

Just for info, a Wattage comparison:

At 12V the bed is drawing 144 Watts.
At 24V it's drawing 576 Watts (about half what a small burner on an electric cooktop uses).

Math used:
Amps = Volts / Resistance
Watts = Volts * Amps
Watts also = Amps * Amps * Resistance
Watts also = Volts * Volts / Resistance

R3 bed is actually a bit higher resistance which is why they are selling it as 24V only. Apparently they are more like 1.2ohm, so 20A and 480W at 24V. The PSU is also voltage adjustable from 22V-26.4V, so I plan on turning the voltage down to its lower limit of 22V...making the bed see 18.3A and 403W.

The only think I'm unsure of about this is whether or not a 24V-12V buck converter will cut the voltage in half on make the output 11V or if it is specifically targeting 12V whether the input is 22V or 24V. Maybe someone with a bit more knowledge of voltage converters can comment.

[jdurand]

The buck converter SHOULD be regulated. Only the low power cheap ones that I use on some PC boards are unregulated. Not a problem with a known regulated input.

[0110-m-p]

That is what I thought and I think I just confirmed it after finding this one...

http://www.amazon.com/DROK-Current-Volt ... +converter

Input Voltage: DC 24V (DC 17-35V)
Output Voltage: DC 12V
Output Current: 20A, 240W

Any thoughts on this SSR? 40A DC-DC w/ standard heatsink for $23.....don't want to get stuck with a fake Fotek or something sketchy.

http://www.ebay.com/itm/USA-Hi-Q-SSR-25 ... 53ebf0be71

[jdurand]

I notice they don't have the RU (UL listing for components) mark. If you REALLY want to make sure you have real ones, you'll have to buy from a registered distributor for the product.

http://www.mouser.com
http://www.digikey.com
http://www.automationdirect.com
etc.

[0110-m-p]

It does seem a bit sketchy. I'm bidding on a used Crydom D1D40 with heatsink on Ebay. I want quality, but really don't want to spend $80 on a SSR.

[Jimustanguitar]

I bought a Crydom on Amazon for $20 http://www.amazon.com/gp/product/B005T8 ... UTF8&psc=1

The product description says new, but that's not the case, they have dielectric grease and heatsink compound on them and screwdriver marks on the clamps... Definitely used, but a decent deal regardless.

I'll have to look for the RU mark. I'm not sure.

[0110-m-p]

I bought a Crydom on Amazon for $20 http://www.amazon.com/gp/product/B005T8 ... UTF8&psc=1

The product description says new, but that's not the case, they have dielectric grease and heatsink compound on them and screwdriver marks on the clamps... Definitely used, but a decent deal regardless.

I'll have to look for the RU mark. I'm not sure.

That is a good deal. I have submitted an offer on this ebay item for $30+shipping. Figure it is worth it since it comes with the heatsink and looks to be in good condition.

http://www.ebay.com/itm/Crydom-D1D40-So ... OC:US:1123