2 12v 12x12 Heated Beds

Greetings,

I'm sooo close!

Currently I'm building a CoreXY printer which is doing mock prints successfully, everything is running great on a Ramps 1.4. Repetier Firmware, Cyclops hot end, etc.

BDP - Big Damn Printer

Only problem i'm having (or I think I have) is powering 2 Makerfarm 12x12 heated beds.

My original plan was to switch a 24v circuit using a DC/DC relay (tested working), and wire the heated beds in series. See attached drawing.

On testing with a 24v/480w/20a PS directly to both heated beds wired in series, the PS immediately blew.

Is there something I'm missing? Have I done my math wrong? Or did I get a bum power supply?

Anyone successfully accomplished this?

The relay is tested working activated by the ramps, so that's not going to be an issue.

Thanks in advance.

- Z

Edited 1 time(s). Last edit at 03/04/2016 09:49PM by Zzyzxx71.

The wiring diagram looks OK.
What does the multimeter tell you about the bed's resistance?

You never tested the PSU without load? Adjusted the V_out , before using?

Many PSUs have a switch to select between 110V and 220V AC, did you check that?

Edited 1 time(s). Last edit at 03/05/2016 03:11AM by o_lampe.

Resistance looks good, checked that after blowing up the PS.

Tested the PS with no load, meter showed 24v, not sure what else I was to look for.

PS was set to 110, learned that lesson years back.

It looks like a nice printer! You will probably need a heatsink and maybe a fan on the SSR. Where is the thermistor that monitors bed temperature mounted?

How is the Z axis implemented? All I can see are two screws and no guides/bearings.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Quote
the_digital_dentist
It looks like a nice printer! You will probably need a heatsink and maybe a fan on the SSR. Where is the thermistor that monitors bed temperature mounted?

How is the Z axis implemented? All I can see are two screws and no guides/bearings.

Thanks! I alreaady have a heat sink for the SSR, and plan on a fan to supply airflow for both the ramps and the SSR, which will be installed with the power supplies in a "cradle" to be installed in the lower section of the frame.

As far as Z axis, the heated bed platform is supported entirely by the M8 lead screws on either end, they're mounted to the support carriage via these Openbuilds Anti Backlash Nut Block.

Initially I was worried about stability, but there is no deflection without the application of significant pressure. So far during mock prints there is no vibration and no visible deflection of the bed.

Ok well I'll get another power supply ordered.

Any thoughts on what guage wire I should be using for this?

Resistance through the heated beds in series is 1.4ohms, I'm not really sure what to do with that information.

Is that significant?

Quote from Makerfarm website:

"New high quality Super Sized 12" x 12" Heatbed PCB , 12VDC input,
WARNING! The large Heatbed draws more current than a standard size Heatbed, approximately 20+ amps at startup for a few minutes."

So i think the 20 Amps PS is not enough.

Quote
Zzyzxx71
As far as Z axis, the heated bed platform is supported entirely by the M8 lead screws on either end, they're mounted to the support carriage via these Openbuilds Anti Backlash Nut Block.

Initially I was worried about stability, but there is no deflection without the application of significant pressure. So far during mock prints there is no vibration and no visible deflection of the bed.

I hope those screws are straight. You're relying entirely on their straightness to maintain the print layer registration. The slightest bend in either screw will cause wobble in the print surface because the bed has no choice but to follow the bends. That's why there is usually some sort of guide rail/bearing arrangement used along with the screws.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I'm starting to get the idea a 480w power supply isn't going to cut it. I've now burned up my second one, although the numbers indicate 480w should be enough.

It's generally not a good idea to operate things at the limits of their performance. If the heater is going to take 450W, I'd probably get a 600W power supply. If you're switching 20A, don't use a 20A relay- get a 30 or 40A relay. The cost difference is minimal and reliability will increase dramatically.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Hard to believe, the 480W PS didn't survive. They usually come with inbuilt overcurrent/overtemp protection.
The SSR in series would add another resistance in series with the beds, so you should be on the safe side.

When you're buying cheap power supplies from China, is it rational to expect them to meet even their most basic specs?

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

An 8 Hz crowbar across a power supply is a pretty tough test. I'm still somewhat surprised that my old ATX supply is holding up.

DD's advice is good, look for a significant factor above your maximum expected power draw when rating the supply. 1.5x to 2x should be good, (but 2x may be getting quite expensive!).