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AC vs DC Keenovo heated Beds

Posted by Forum-Layman 
AC vs DC Keenovo heated Beds
June 06, 2019 04:08PM
I have been going back and forth on a new printer build I'm working on about the bed heater and whether I should go for a 200W 24V heater pad from keenovo or a 240V 400/500W heater pad from keenovo. So I've come here to seek some wisdom! As I see it the AC pad obviously heats faster but is also much higher risk and trickier to install. Ive read about the UMMD bed wiring so would probably emulate that with fuse, SSR, TCO and ground wire. Im using the Duet wifi as my controller.

AC:
Need to add a TCO to the bed (arguably important for both) but would require careful insulating on the mains system to prevent live wires being accessible / bringing the bed live.
Need to add an SSR.
Mains wiring to the bed will require conduit for protection.
More mains wiring inside the printer (all most be within conduit for protection)
Ground wire must be installed
High risk of shock

DC:
Higher current in wires (may not be an issue with correctly spec'ed wires)
Higher strain on PSU (need a higher W PSU)
More current running through the controller.
No 'killler' voltage on the surfaces you touch so much less risk.

What do you guys think? Im keen to hear the benefits of mains or DC and why one is a better option. As far as I can see currently the AC is a lot more faff to make safe and has much higher consequence of misuse in exchange for reduced wire current (which could be mitigated using thick wires)
VDX
Re: AC vs DC Keenovo heated Beds
June 06, 2019 04:36PM
... for AC -- instead of wiring to mains, you can use tansformers with "harmless" voltage levels - used to heat my fist bed with a "12V-transformer" at roughly 16 Volts AC winking smiley


Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org]
Call for the project "garbage-free seas" - [reprap.org]
Re: AC vs DC Keenovo heated Beds
June 06, 2019 04:39PM
Quote
VDX
... for AC -- instead of wiring to mains, you can use tansformers with "harmless" voltage levels - used to heat my fist bed with a "12V-transformer" at roughly 16 Volts AC winking smiley

I think that probably adds a little more complication!
Re: AC vs DC Keenovo heated Beds
June 06, 2019 05:00PM
Why does AC wiring need to be in a conduit? Billions of AC appliances don't use conduit.

Keenova sells genuine Fotek SSRs. They also provide wiring schematic.

I made 3d printed covers for all of the AC wiring inside my printer to make it impossible to get shocked by touching the contacts.
Re: AC vs DC Keenovo heated Beds
June 06, 2019 05:25PM
You can buy AC heaters from Keenovo with the TCO already built in.

Have you read [duet3d.dozuki.com] ?

Edited 1 time(s). Last edit at 06/06/2019 05:27PM by dc42.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
VDX
Re: AC vs DC Keenovo heated Beds
June 06, 2019 06:06PM
Quote
Forum-Layman
Quote
VDX
... for AC -- instead of wiring to mains, you can use tansformers with "harmless" voltage levels - used to heat my fist bed with a "12V-transformer" at roughly 16 Volts AC winking smiley

I think that probably adds a little more complication!

... why?

It's as simple as mains to transformer, then the transformer output across an AC-SSR to the heated bed ... AC-SSR's can be switched with 3.3V without further parts (=optocoupler input) ...


Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org]
Call for the project "garbage-free seas" - [reprap.org]
Re: AC vs DC Keenovo heated Beds
June 07, 2019 04:59AM
Have you seen these, [www.ebay.co.uk] Wonder how safe these are? 200w version even cheaper. Though not answering the question it's tempting me to dip my toe into 220v ideally for me a little controller box to set temp seperate from printer.

Edited 1 time(s). Last edit at 06/07/2019 05:09AM by MechaBits.


[mechabits.co.uk]
Re: AC vs DC Keenovo heated Beds
June 07, 2019 07:48AM
Before you decide AC vs DC, you should get some idea of how long it will take to heat the bed up based on the power input and mass/material of the bed. This site has a pretty accurate calculator.
You'll have to decide what is an acceptable heat-up time. If you don't mind waiting, you can get away with lower power. If you don't want to wait long at all, you'll need more power. Insulating the underside of the bed may speed heat up time a little, but will also increase cool-down time.

For heat-up time in the 5-10 minute range, expect 0.5W per cm^2. For a 300 mm x 300 mm bed that's 400-500W. As a point of reference, UMMD has an 8mm thick aluminum bed and 750W heater and gets to 100C in about 4.5 minutes. SoM has a 1/4" thick bed, the same heater, and gets to 100C in about 2.5 minutes (about as fast as the hot-end gets up to temperature for printing ABS).

Trying to use DC for more than about 200W is a little troublesome. You need a beefy power supply, heavy yet flexible cables, high current connectors, fuse, TCO, and maybe DC-DC SSR with a heatsink depending on the controller board. For line power, you won't need the big power supply, heavy gauge wire, or high current connectors, and the SSR usually doesn't need a heatsink. You're going to want to be careful with the wiring in either case- wires carrying high current DC can cause a fire as easily as wires connected to the line.

Don't under-spec the power supply. Operating electronics at the edge of specs, like anything else, is a bad idea if you want reliability. If you need a total of 300W to operate the printer, get a 500W power supply. When it comes to power supplies, don't waste money on the junk "LED power supply" crap listed on ebay. You can buy a high quality MeanWell supply for about $5 more for the same voltage/power rating, and know that it has over current, short circuit, over voltage protection.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: AC vs DC Keenovo heated Beds
June 07, 2019 07:52AM
Quote
etfrench
Why does AC wiring need to be in a conduit? Billions of AC appliances don't use conduit.

In the UK at least all cabling must be double insulated (that's why the individual strands are encased within a black (usually) plastic insulation on kettle leads) so I would be surprised if you can find something with just single insulated and has exposed mains wires (essentially you should never be able to see blue / brown wires). The stuff that comes with the keenovo is 2 wires with a single layer of insulation on each so you would need to put them some kind of insulation to protect them and plastic conduit is just easiest and nicest looking IMO. It only a requirement because if a cable breaks there is a chance it could flail and bring the frame or bed live. In the conduit it is still not ideal but it is at least contained and not making contact with other parts.

Im not sure what youre getting at with the genuine Fotek SSRs - that is probably the SSR I would buy but its more the faff of having to have all these extra parts and having mains running around the machine that concerns me!

--------------------------------------

Thanks for the link DC - I actually started my research there and its a good set of guidelines.

I cant seem to find any keenovo beds with a TCO built in.

--------------------------------------

I have heard horror stories of the off brand mains beds and im not sure I would trust it - a genuine keenovo is not much more and would likely be far more reliable!
Re: AC vs DC Keenovo heated Beds
June 07, 2019 08:22AM
Just send email to sales@keenovo.com and ask about TCOs, adhesive, etc.

I am replacing the heater in UMMD with one that I ordered without adhesive (no charge and no delay for the custom order). I'll be using high temperature silicone to mount it. I have found that the 468MP lets go of the bed plate after about 2 years of daily temperature cycling. This is the result that demonstrates why the TCO should be mounted on the heater and not the bed plate:



Here's a link to the heater mounting instructions: [keenovo.store]

And if you ever need to remove the heater: [keenovo.store] I found that the wire/fishing line technique didn't work for my large bed. I heated the bed to 110C, disconnected the power, and attacked it with a scraper while it was hot. It came off without too much of a struggle.

Edited 1 time(s). Last edit at 06/07/2019 08:22AM by the_digital_dentist.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: AC vs DC Keenovo heated Beds
June 07, 2019 11:49AM
For me the the advantage to use a smaller PSU for the electronics so that you can use a smaller fanless PSU, and the quick heat up times with a 300x300x6mm tooling plate were the biggest factors. It's probably more energy efficient since there's no PSU between the heater and mains. I've mounted a couple of fans in the enclosure at the top that blows the air around. Just before the print I can heat up the enclosure that way.


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Imqqmi

NFAN CoreXY printer:
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Re: AC vs DC Keenovo heated Beds
June 07, 2019 02:05PM
Thanks guys,

I probably should have mentioned up top that my printer is quite small: 200 x 200 x 6/8(still deciding, perhaps someone could weigh in - cost is almost identical) mm bed so a 200 W heater is more than enough for me which makes a DC bed way more feasible. I think AC is the only way to go if your plate is any bigger than mine but I really sit on the boundary.

Are the non genuine silicone heaters ok at the lower voltages? I wouldn't consider a knock-off mains bed but at 24V im less worried. The genuine keenovo comes in at about £30 while the aliexpress alternative is £9-15.

I guess the main reason im considering DC is im more comfortable working with DC voltages and I dont need the rapid heat times or exceptionally high max temp.

the_digital_dentist: Is the Keenovo TCO bonded to the heater?

Scary stuff - what temps were you running that at?

Edited 1 time(s). Last edit at 06/07/2019 02:07PM by Forum-Layman.
Re: AC vs DC Keenovo heated Beds
June 07, 2019 04:05PM
Quote
Forum-Layman
Quote
etfrench
Why does AC wiring need to be in a conduit? Billions of AC appliances don't use conduit.

In the UK at least all cabling must be double insulated (that's why the individual strands are encased within a black (usually) plastic insulation on kettle leads) so I would be surprised if you can find something with just single insulated and has exposed mains wires (essentially you should never be able to see blue / brown wires). The stuff that comes with the keenovo is 2 wires with a single layer of insulation on each so you would need to put them some kind of insulation to protect them and plastic conduit is just easiest and nicest looking IMO.

The AC mains silicone heater that I bought from Shenzen Ali Brother came with a double insulated cable. But check the images on their web site in case that has changed.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: AC vs DC Keenovo heated Beds
June 07, 2019 04:13PM
I wouldn't get a self resetting TCO. That will allow the hazardous condition to keep cycling power to the heater. If there's a problem, you want to shut down.

I use Cantherm TCOs that cost about $1 each via digikey. Don't cheap out and buy no name, no data, no certification parts from Chinese suppliers. The TCOs self-heat so you have to look at the data sheet and select one that will be rated for a temperature higher than the temperature at which you intend to operate the bed. Glue the one-shot TCO to the heater with the same high temp silicone you use to glue the heater to the bed plate.

I operate the bed at 100C for printing ABS most of the time. The heater burned because it separated from the bed and no longer had the aluminum conducting the heat away from the heater. Those areas that were no longer touching the bed plate got extra hot.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: AC vs DC Keenovo heated Beds
June 09, 2019 05:55AM
I use a knock-off silicone 200 watt bed heater, powered through an external mosfet board (it lowers the current the main-board carries despite being quite capable of it, Azteeg X5 GT) and powered by a 500 watt Mean Well psu that also powers the board.
Though the wires had a decent silicone jacket as well an an impregnated weave on the outside, I chose to run it through silicone tubing, to work similar to a cable chain for snag prevention. (there's your full second jacket).
Keenovo does the same on one of my mains heaters that is awaiting it's machine to be built, the wires are in a thin-wall orange silicone tubing.

While my new printers will use mains heaters, this one will remain on 24 volts. It's been my ABS/Nylon/PC workhorse for two years now, and still takes about 6.5 minutes to heat the 230x230x3mm bed to 110c.

BTW, the one with the orange tubing has a temperature controller in-line for stand alone operation, but I won't be using it that way, and wouldn't recommend it. I have a genuine Fotek SSR for it (5 actually), mainly so that the board can shut off the bed at the end of the print, like it does with the external mosfet.
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