Can I use 2 standard MaxV2 Pwr supplies in series for 24V

[boarddesigner]

Question,
Why couldn't I use 2 of the standard rostock max v2 power supplies in series to create my 24V supply for heated bed?
Does this supply not allow a series interface?

Would this supply enough current for both the Rambo and bed? I am guessing not.

What is the total current load for a stock machine?
(this is most likely in the documentation but I seem to have missed it)

thanks!

[Eric]

If you have PC-style power supply that used to come with V2's, don't even think about it.

If you have the single-voltage supply currently shipped with the Max, then it's possible, but probably not without modifications. The chassis frame has to be isolated from the DC ground before it's safe to do so. Unless the manufacturer has done it for you (probably not), it's up to you. This isn't horribly difficult if you know how, but you'd be taking your power supplies apart and voiding warrantys. End result is double the voltage, same current limits.

However, realize that when you double the voltage for the heat bed, you also double the current requirements. One version of Ohm's Law: I = V/R, where R is the resistance of the bed, V is the voltage, and I is the resulting current. So when you add in other stuff, you risk not having enough to run everything.

You'd probably be better off getting a second 24V supply instead just for the bed. You can still use your existing supply for everything else at 12V.

[boarddesigner]

Yes, it looks like it will be pushing the limits a bit.
1.1ohm bed @ 24v appears to be about 22A
All other devices combined not including anything extra appear to be about another 10A.
I would have liked to shoe horn everything into the lower chassis but maybe this is not such a great idea.
Thanks for the input.

[Xenocrates]

You have some interesting points here.

Firstly, no you can't use standard power supplies for this, as the DC ground is connected to the actual ground. I am told that style of power supply can be modified to leave the DC ground floating, which might allow for this to work. They are designed to disallow series configurations, but can be modified to make it work.

Second question. Probably not. Two supplies is series will have the same amperage. As up to 10 of those amps are drawn off by the motors/hotend/fans/logic off of one of the supplies, that leaves 19 amps (Dedicated) for the other bits. At 24V, the onyx will want to draw between 20-24A. If you dialed back the second supply, it would probably be enough. Same thing if you largely don't use layer fans, and use lower amperage cartridge heaters (Such as using a 24V cartridge. a 40W 24V cartridge will take less amperage than a 30W 12V, and about as much as a 20W 12V. Yay electronics.). I still wouldn't suggest doing it like that. The margins are small here.

Total current load is dependent on what is installed, however the fuses installed cap the current draw at 25A (5 motor, 5 extruder/fan/logic, 15 bed). This leaves plenty of headroom on the stock supply (A whopping 4A). Let's look at say, a printer that is nominally stock.

It uses a pair of 6.8 ohm resistors in parallel. Nominally, this is ~42W. The bed draws ~130W. the hotend fan ~1.2W. Part fans ~1.08 each. Motors seem to be happy ~18W apiece (1.5A per phase as the manufacturer documentation specs, 1 phase energized). This means that you have a total draw of 246.28W, which at 12V is 20.5A.). This leaves out the Rambo's logic draw, as well as that for the Rambo cooling fan, and in newer units, the external cooling fan, as I'm not sure on good numbers for those.

[boarddesigner]

Eric, Xeno,
Thank you for the excellent feedback.
I am abandoning the idea of squeezing power supplies into chassis and will simply add a 24V 30A externally.
(yes, 30A is a bit overkill but the price was almost the same as 25A so why not)

Question:
I see other 24v upgrades on forum and they tend to be using a 100A SSR. Is there a reason for this? The only reason I can think of is greater thermal dissipation possibly.

[Tincho85]

I would be one of those
No particular reason, I used a 100a Fotek just to be sure it won't overheat.
The Crydom D1D40 seems to be the standard for this upgrade. Go with that one if you can.

[boarddesigner]

Tincho,
Are you not using a heatsink on your SSR? Have you by any chance calculated what the wattage is for SSR?
I selected a 60A SSR from amazon. Not sure of the quality. I should probably put this on a heatsink to be safe. What are your thoughts on this?

[Tincho85]

Uhmm... I did some research before doing the upgrade, but I can't recall what was the min wattage required. With a good 30a SSR I think it would be fine. Though, someone with electrical skills should pop up here and leave some feedback.

Yes, I have mine mounted on a heatsink but it could be removed. It doesn't even get warm when the bed is heating.
One recommendation, for the first couple of days/weeks/months put the SSR outside the machine so you can monitor it. I've read about some SSR knockoffs burning out and sometimes start a fire.

[Xenocrates]

Eric, Xeno,
Thank you for the excellent feedback.
I am abandoning the idea of squeezing power supplies into chassis and will simply add a 24V 30A externally.
(yes, 30A is a bit overkill but the price was almost the same as 25A so why not)

Question:
I see other 24v upgrades on forum and they tend to be using a 100A SSR. Is there a reason for this? The only reason I can think of is greater thermal dissipation possibly.

The reason we use the 100A Auberton is simple. It's cheap, and direct from the manufacturer, while having low voltage drop (And thus lower heat generation). 20$ US is a really good price. I looked at a 25A 24Vdc specific SSR. 70$~. I'll take higher amperage and voltage capacity for a lower price, especially with better performance.

Also, with 30A, you could move the motors onto it (If you use a Rambo), and thus have higher step rates and more torque available.

[gchristopher]

Can someone tell me why we aren't looking at the switching frequency ratings of the SSRs? That seemed to be the main difference between an $80 Crydom SSR and the less expensive ones.

I know you can turn the PWM switching frequency down to 15 in the firmware, or go to bang-bang control for the bed. (Isn't there a way to limit switching speed even lower then?)

That just seems to me like something important to report about SSRs in addition to amperage, if we're trying to keep everything within safely derated values?