Upgrading heat bed

I've been thinking about upgrading my heatbed,initially I was looking at going to 24VDC, but am reconsidering going to 240VAC with a SSR driven from the Duet heater output

anyone got any reccomendations, Ive found this one [www.ebay.com.au] with this SSR [www.ebay.com.au]

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The listed heater is 320mm, so I assume it is for a bed 350mm or 400mm. If this is the case, it is too powerful, which can be quite dangerous. For a bed 400mm diam, 500W is sufficient.

I am using a 280W bed heater (280mm) for a bed of 300mm diameter, which is 0.4W/cm2 , and with a good bottom insulation (ceramic fiber), this is sufficient to go higher than 150°C (measured) and cook your effector (I did that...).

Note that an overheated silicone pad will inflate with significant pressure (I also did that testing chamber heater...) and your design shall take that into account with sufficient strength in bed support. You don't want to eject your bed from supports in case of incidents when you are not present. Use fireproof insulation (NOT cardboard!). I used chimney hood insulation panel.

For safety reasons and more accurate measurement, it is recommended to not use the pad thermistor, as you cannot control the electrical insulation between the pad and thermistor and it gives a difference of 30°C between the thermistor and real surface temperature. I set the thermistor on the aluminium spreader, and as the spreader is grounded, a fault in electrical insulation between thermistor and heater may not save the control board, but it can save me !

This thermistor position will reduce the temperature difference between thermistor and bed to 10~15°C

Protect physically you main cable and SSR connections, ground the aluminium spreader and SSR baseplate, use stranded wires with crimped terminals, check your home earthing. In most installations, contact between neutral wire and ground shall trigger the 4-20mA breaker (impedant neutral).
[edit] Connect your mains ground and your DC '-' .

A 25A SSR is more than sufficient if supplied in 230V, and it shall be less costly than the listed one. Prefer one with plastic shield or print a shield. my SSR is not in contact with its support and some air is blowed on its metallic pad.

Wiring on Duet is described here :
Duet_Wiring#BED_and_chamber_heater_SSR_wiring [reprap.org]









Edited 4 time(s). Last edit at 05/24/2016 08:40AM by PRZ.

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Pierre

- Safety [reprap.org]
- Embedded help system for Duet and RepRap Firmware [forums.reprap.org]
- Enclosed delta printers Lily [rouzeau.net] and Lily Big [rouzeau.net]
- OpenScad delta printer simulator [github.com]
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This pad from jsr. industrial (diam 400mm, 500W) looks a bit more reasonable in power and is less costly.

[www.ebay.com]

My bed pad cames from this supplier. I did not notice much visual difference with another pad from Keenovo (the one which inflated while overheating). From what I can see on ebay most pad from Keenovo have larger power/surface ratio, which I feel unsafe (from experience...).

Cables are good quality with fiber sheath for both pads, but were 800mm long on jsr.industrial and 1000mm on Keenovo.

[edit] Both suppliers have a lot more choice than what is shown on ebay, so you may ask them for a given size.

Edited 1 time(s). Last edit at 05/24/2016 09:09AM by PRZ.

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Pierre

- Safety [reprap.org]
- Embedded help system for Duet and RepRap Firmware [forums.reprap.org]
- Enclosed delta printers Lily [rouzeau.net] and Lily Big [rouzeau.net]
- OpenScad delta printer simulator [github.com]
- 3D printing on my site [www.rouzeau.net]

Highly recommend these guys [www.aliexpress.com]
If they don't have what you want they will make it

I installed a 1200W 500mm one under the 600mm x 8mm ATP5 cast plate
[www.aliexpress.com]

Lesson from that...... make the pad nearly the same size as the bed to remove cool edges, I'm considering ordering a 600mm round one now

Oh I have no real issue with the thermistor in mine..... I consistently see a simple 5 degree lower thermistor reading than the measured surface temperature

Second PRZ on using the Ceramic fibre paper, i have 2 x3mm thick under mine.

I ordered a 75A SSR on heatsink from [www.ebay.com.au] have bought quite a bit from that store over the last few years.
Really the rating is no doubt inflated but it's not much more than the cheaper 25A units.

Aussiephil, the reason I have different temperature between pad thermistor or aluminium spreader thermistor is that my bed is in glass (borosilicate), so with much less heat conductivity than your full aluminium bed.
An aggravating factor is that now I added a Printbite surface (fiberglass + resin) which create an insulation on top of the bed and have created troubles in temperature control (now solved with PID control instead of bang-bang). It also increase the radiative coefficient of the bed, hence tend to heat more anything in view of the bed surface (this is the combination of temperature instability, poor adhesion of ABS so increased temperature and higher radiation that leaded to the effector destruction).

[edit] Surprisingly, I have lower temperature (by 5°C) at the center of the bed than in the periphery.

Safety wise, I will be a bit concerned having an aluminium bed without insulating surface in direct contact with a heat pad, as there is no double electrical insulation and you may touch the bed.
However, on my own setup the aluminium bed retainers are easily accessed by my hands and are electrically connected to the aluminium spreader, so that is not much better. I may add wood shields or make retainers in wood (no plastic will handle 150°C).

Edited 3 time(s). Last edit at 05/24/2016 10:15AM by PRZ.

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Pierre

- Safety [reprap.org]
- Embedded help system for Duet and RepRap Firmware [forums.reprap.org]
- Enclosed delta printers Lily [rouzeau.net] and Lily Big [rouzeau.net]
- OpenScad delta printer simulator [github.com]
- 3D printing on my site [www.rouzeau.net]

Quote
aussiephil
Highly recommend these guys [www.aliexpress.com]
If they don't have what you want they will make it

I installed a 1200W 500mm one under the 600mm x 8mm ATP5 cast plate
[www.aliexpress.com]

Lesson from that...... make the pad nearly the same size as the bed to remove cool edges, I'm considering ordering a 600mm round one now

Oh I have no real issue with the thermistor in mine..... I consistently see a simple 5 degree lower thermistor reading than the measured surface temperature

Second PRZ on using the Ceramic fibre paper, i have 2 x3mm thick under mine.

I ordered a 75A SSR on heatsink from [www.ebay.com.au] have bought quite a bit from that store over the last few years.
Really the rating is no doubt inflated but it's not much more than the cheaper 25A units.

That supplier will also do one with a small hole of say 10-15 mm in the centre of the pad to allow a thermister to be mounted in the center of the bed and this is What I will be asking for when I order my 600mm diam one?

Doug

Quote
aussiephil
Highly recommend these guys [www.aliexpress.com]
If they don't have what you want they will make it

I recommend them too. They made me a 230V 350W 300mm diameter one for my delta, which I drive from the Duet using a 10A SSR (SSR-10DA). See the link to my blog for more details. As noted by others, if you get one with built-in thermistor, it will over-read because it isn't in contact with the bed plate. I have learned to correct for this, but one with a small hole in the middle for your own thermistor as Doug suggests would be better.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
PRZ
Safety wise, I will be a bit concerned having an aluminium bed without insulating surface in direct contact with a heat pad, as there is no double electrical insulation and you may touch the bed.
However, on my own setup the aluminium bed retainers are easily accessed by my hands and are electrically connected to the aluminium spreader, so that is not much better. I may add wood shields or make retainers in wood (no plastic will handle 150°C).

Electrical safety is quite easy to handle as long as you are plugged into a mains circuit protected by Earth Leakage Breakers....... (I still need to do this myself) Tap a M3 hole into the edge of the plate and connect an Earth wire. this should result in any shorting of the active wires in the heat pad to the plate near instantly tripping the earth leakage breaker.
Can I suggest this is added to the Wiki page on heated beds?

Quote
dougal1957
That supplier will also do one with a small hole of say 10-15 mm in the centre of the pad to allow a thermister to be mounted in the center of the bed and this is What I will be asking for when I order my 600mm diam one?
Doug

Been thinking about this overnight and considering David's comments below I personally believe it is better to measure the temperature towards the edge of the bed and to measure and understand how your bed heats up and spreads the heat. So personally ordering the next one it will not be with the hole in the middle.

Quote
dc42
As noted by others, if you get one with built-in thermistor, it will over-read because it isn't in contact with the bed plate. I have learned to correct for this, but one with a small hole in the middle for your own thermistor as Doug suggests would be better.

Actually mine reads 5C Lower than the temperature on top of the build plate in the main central area (460mm odd diameter).
further thoughts.......

The thermistor is part of the silicon heat pad and should effective read an accurate temp for the heat pad, this I believe is more important so that you don't end up over heating the pad because you have poor conductive connection to your heat bed or you run glass on top of aluminium or even in Pierre's case you then run another coating on top of the glass making it worse.
Surely if your taking the time and energy to migrate to an AC powered (even high powered DC) Silicon heat pad it should be relatively easy to heat map you build plate and map the differences between your thermistor reading and the build plate readings.

A nicely secured silicon heat pad that is then covered on the non build plate side with insulation such as Ceramic Fibre paper, should provide effective thermal coupling to the build plate meaning little to no difference between heat pad and build plate temps. As a matter of fact if I note the reading about where the thermistor would be my readings are within a few degrees.
This is on the premise of "effective thermal coupling" and somewhat goes out the window if you use the Silicon heat pad directly under glass...

I'm going to borrow a thermal imaging camera for the weekend to get some whole bed readings at different settings to "test" the comments above.

Cheers

Well Ive finally got a silicone 240V bed from these guys [www.aliexpress.com] , getting it was something of a drama, but all is resolved now.

The problem I've got now is the pad has double sided contact surface on one side and on the top there is a 6mm bulge where the cables are connected, which means that if I just pup a plate on top of it then it wont sit flat.

I was wondering how others resolved this issue, I guess I could mount it upside down onto an aluminium plate and use the heat resistant padding to absorb the bulge, but then I would have to drill holes into it for the mounting screws.

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RepRapPro Mendel 3 Tricolour
RepRapPro Fisher
-Carbon Arms
-Easy adjust Carriage+effector
-axis stiffness mods
HE3D -600 delta
-Duet 0.8.5
-PanelDue
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-RobotDigg metal components
Simplyfy3D
RS Design Spark CAD

Quote
bgkdavis
Well Ive finally got a silicone 240V bed from these guys [www.aliexpress.com] , getting it was something of a drama, but all is resolved now.

The problem I've got now is the pad has double sided contact surface on one side and on the top there is a 6mm bulge where the cables are connected, which means that if I just pup a plate on top of it then it wont sit flat.

I was wondering how others resolved this issue, I guess I could mount it upside down onto an aluminium plate and use the heat resistant padding to absorb the bulge, but then I would have to drill holes into it for the mounting screws.

I use an aluminium bed plate that is larger than the printable area and the silicone heater by 15mm all round. That extra 15mm gives me space to attach mounting pillars, and space to clip a glass printing plate on top of the aluminium without the clips getting in the way of the print. The flat side of the silicone heater with the adhesive sheet is stuck to the underside of the aluminium plate, and the cork sheet underneath the heater absorbs the bulge caused by the cable. See [miscsolutions.wordpress.com] for more details.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

yeah upside down I guess it is then

Fortunately I can see where the heated tracks are through the silicon, so whilst its really not to be recommended I reckon I can put some holes through at the very edge

---

RepRapPro Mendel 3 Tricolour
RepRapPro Fisher
-Carbon Arms
-Easy adjust Carriage+effector
-axis stiffness mods
HE3D -600 delta
-Duet 0.8.5
-PanelDue
-DC42 Height probe
-RobotDigg metal components
Simplyfy3D
RS Design Spark CAD