Custom size mk3 heatbed


#261

Thanks hoh61 and hsoubry for looking seriously at this issue. There’s indeed a lot going on regarding to the heatbed.
The main issue for me is that I want to make the heatbed on a way that’s safe and solid, but it seems that everyone has a different opinion about this, so it’s rather confusion. But you convinced me not to use a SSR, that’s clear.

I took notice of the configuration Danf is using but it is to technical for me to understand. In a earlier stage I made, with the help of Marlark and some posts from others about this subject, the paper ‘Heatbed for dummies’ (post oct 6th) to show the not-technicians how to install a heatbed with the knowledge of that moment. At that time I would never have thought that it ran out of date that fast. The PSU seems to be to weak and a Power Expander to tricky…
But dummies still can be skilful in copying the knowledge of technicians…

So if Danf or someone else could try to make things clear for dummies (images, simple connection scheme, ordernumbers, etc.), a lot of K8400 users I think, including myself, would be very grateful. And Marlark has still some heatbeds in stock, so new users would benefit too.

Thanks again.


#262

I received my heated bed today, thanks Marlak!
Just waiting on a part from china and I’m ready to build it. :slight_smile:


#263

svdv,

The heat-bed has two power options; 12 and 24 V. The main benefits of using the 24 V option with an extra 24 V supply as opposed to 15 V:

  • Less than half the current, but slightly reduced heating time (24 V @ P=24^2/2.6 = 222 W vs 15 V @ P=15^2/.65 = 346 W)
  • You can safely use the power expander (20 A rated continuous), even with screw terminals mounted (likely rated ~10 A continuous)
  • You can use thinner wires, weaker connectors (max 9.2 A vs. max 23 A)

#264

Thanks danf, I understand.
But I’ve got a 15V PSU 320W already (MeanWell RSP-320-15 with rated current 21,4A). I understand I need 346W so my PSU is 26W to weak, but what will happen if I use this only for the heatbed? And with the Power Expander (or even 2 in parallel)? And with the use of 14AWG silicone wiring.

Or should I go for 24V anyhow to make it safe and solid without risk of overheating the PE?
The smell of a burned PE is disgusting and stay’s around the printer for several weeks. Got it twice and I don’t want a third one :slight_smile:

So, I am looking forward to your answer and than it’s time to decide what to do…


#265

[quote=“svdv”]Thanks danf, I understand.
But I’ve got a 15V PSU 320W already (MeanWell RSP-320-15 with rated current 21,4A). I understand I need 346W so my PSU is 26W to weak, but what will happen if I use this only for the heatbed? And with the Power Expander (or even 2 in parallel)? And with the use of 14AWG silicone wiring.

Or should I go for 24V anyhow to make it safe and solid without risk of overheating the PE?
The smell of a burned PE is disgusting and stay’s around the printer for several weeks. Got it twice and I don’t want a third one :slight_smile:

So, I am looking forward to your answer and than it’s time to decide what to do…[/quote]

The RSP-320-15 is adjustable between 13.5 and 18 V. If you’re only using it for the heat-bed, then lower the voltage to its lowest. Verify using a volt meter. Your max current draw will then be 13.5/.65 = 21 A consuming 280 W (it will drop as the bed heats up). This is over the power expander continuous rating, but you might get away with it. Have air blowing over it, like Jansinger in this thread. Might be wise to check with reprap.me. I wouldn’t use the screw terminals however, but solder directly like the fourth picture on http://reprap.me/power-expander.html.

Your previous power expander cards, how did they fail/burn more specifically? Concentrated to any specific component?


#266

They burned the same way, on the power out side.

RepRap.me advises a heatsink ‘on top of the FET or on the backside of the PCB, underneath the FET’.

This encourages me strongly to buy a 24V PSU…

Edit: I think the MeanWell RSP-320-24 is allright to add as PSU for the heatbed. And a new power expander…


#267

Make one yourself and go with the scheme raby has on his wiki page.
Make sure to add a piece of aluminium or some sort of possibility to dissipate the heat of the FET. And it will work perfectly with 15 volts.

This is what I have going on successfully for quite some time

Kind Regards
JeAfKe


#268

Hi Jeafke,

Thanks for showing me, but I don’t think for me it’s a wise thing to do. Shure, I like making things but electronics is not my cup of tea.
And Danf have convinced me with his resistance calculations and the heat that the power expander has to manage. While printing I don’t want to be alert all the time because something could burn. I know, some other parts could burn as well, but lowering the risk-element feels good. So I am going to add the 24V PSU.
Another reason is that design is my core business and I want to get to work again without this time consuming distraction of a printer. I made my printer XL, made some improvements and now, finally, it has to print!

Kind regards


#269

I’m using a cheap 40A SSR (3-32 DC in - 5-60V out) and it works well. I’ve screwed it on the alu case of the PSU to use it as a heat sink. The wiring consist of 4 x 2.5 flexible wires (one pair is heating a bit so you’re loosing power in the wiring instead of the bed).


#270

Yeah @sdvd I just think that buying a different psu is a bit crazy if there are other cheaper ways to get a decent working heatbed. Like raby says a different cheap ssr might be easier instead of buying a new psu… It wouldn’t work neither with the ssr u’r having allready because it only switches ac loads.


#271

I appreciate your (and everybody else) thinking along, but after a serious consideration I purchased a 24V PSU already.
I think Danf has made my problems perfectly clear. I did even understand it :slight_smile: The power expander can manage 20A. The 15V PSU with 12V heatbed connection rates 23,1A. So it’s getting too hot. As a second best option Danf suggests to lower the Voltage, but to be honest, I am fed up with this and want a safe solution.
Also ordered a new PE, which has to manage only 9,2A now. I won’t use the screw terminals but solder everything and if needed, put a fan besides.
It is a pricey experience but I learned a lot.


#272

Lowering the voltage would raise the current, withdrawn by the heatbed. Anyway, have fun with the new PSU!

Kind Regards!
JeAfKe


#273

No.

The heat-bed is just a physically large small valued resistor, not a constant power load.


#274

@danf yes you are right. Basic ohms law, but it was a test to see if anyone would respond :wink:
[size=85](tbh idk where I was with my mind writing such bs) [/size]

Kind regards
JeAfKe


#275

Well, I finally finished my heatbed install (thanks, Marlak). I elected to install a second PSU (MeanWell 24V, 350W), and with the RepRap power expander it works quite well. The power supply is mounted underneath beside the 15V one and partly covers the triangle hole. The cork insulation I have is 4 mm thick and with the glass/BuildTak on top the total bed thickness is 20mm. Of course I had to adjust the Z-stop screw alot, and I had to make new clips. I bent some metal for the clips, as I don’t trust printed ones to stay tight on a hot bed. Overall I am very happy with this.


#276

@Bert490

I think you made the right choice. This is exactly what I am going to do. But it did cost me several months to discover :slight_smile:
I am wondering, what kind of wires (to the heatbed) do you use and how warm do they get?


#277

The wires I use for now are 1.02mm dia / 18AWG stranded wires, twisted loosely together, soldered to the heatbed and in the screw terminals of the power expander and the power supply. They do not seem to get warm (so far). This is a bit too small for the part between the expander and power supply due to the other wires close by (wire rating is lower if it is surrounded and can’t cool easily). I plan to change to 16 AWG. I tied the top of the moving wires to the Aluminum bed supports and made a curve to the bottom so it rolls evenly as the bed lowers.


#278

Hi all,
Yesterday I started with the upgrade to a heated bed. However I do still have some questions, I talked with Raby about it and bought all the stuff he recommended, therefor I also have a DC-DC Relais but I’m not sure how to connect it (as a replacement of the PE).
Can anyone here try to explain to me how to connect it if using this relais? Look at this picture so you exactly know what I have.

I found this connection scheme in this post but I’m not sure if its correct for my ssr? It looks weird that the input (3 & 4) goes to the mainboard and not the psu because that’s where the power comes from? Or I’m thinking wrong for what the SSR is needed and what it does. If anyone could confirm this connection scheme is 100% right, I’m certain I can make it work and will probably finish the upgrade tonight :slight_smile:

Thanks in advance!


#279

Hi Tom,
Sorry I didn’t have the time to reply to your email.
Your drawing is almost correct except for the connection to the PSU.
Connect the -V of the PSU to pin 1 of the SSR and Pin 2 of the SSR to 2-3 of the bed. Pin 1 of the bed goes to V+ of the PSU.
Like in this drawing :


#280

Thanks for clearing that one out :slight_smile:
I made a new drawing of it, I would say. Check it once more but I’m pretty sure I got it now. The only thing I’m not certain about is, because you told if using 2,5mm speaker wires that you should double them from psu to SSR and SSR to heatbed.
But could I solder one cable on pad 2 and one on pad 3 (of the heatbed) and make them together connect on the 2 on the SSR, or should I solder both cables on 2 and 3 of the heatbed?

Besides that I hope this scheme helps everyone out that wants to use a SSR as well in stead of a PE. Because for me it was not that clear how to connect it at the beginning.