Advice On Powering Large (450 x 450mm) Heated Bed.

Hi everyone,

I have built a 3D Printer with a build volume of approx 460 x 460 x 260mm. Currently i am using a makeshift heated bed setup, as i havn't been able to find a suitable heated bed that fits. Currently i am using 2 240 x 200mm 12V Silicone heaters to heat an area of my print bed. These are powered in parallel using an external 12V PSU and one of these i have made: [www.youmagine.com]

Its working ok, and can get upto 110+, the MOSFET relay is obviously not designed for powering this size of heater setup, and as a result gets very hot when trying to print ABS. Previously i had used a DC-DC SSR from amazon that melted itself very quickly.

Now, i have now bought a much larger heated bed ( [www.aliexpress.com] ), which is 450 x 450mm - 240V - 1500 Watt, obviously power will now be taken from the mains. I am still very new to electrics so would like some advice on what type of relay i can now use. I have enough parts to make another MOSFET relay as before, but in a proper enclosure with cooling, however im worried the MOSFET isn't fit for purpose now: [uk.mouser.com]

Am i right in assuming the MOSFET is only rated for switching upto 30V?

Am i now able to use a DC - AC SSR such as this: [www.amazon.co.uk]

By my calcs the bed will only draw 6.25A so this SSR should be more than up to the job?

Any help is appreciated.

Thanks

Alex

Edited 1 time(s). Last edit at 09/09/2015 07:56AM by xelad.

The problem with heaters is that the resistance is lower when cold so the initial power draw could be higher. Maybe adding a fan will help to keep it cool and use thermal paste on the cooler for better heat transfer.

On the other hand, A good mosfet of 400V 50A with a low rds on can handle it, though you may have to drive it with a higher voltage than a micro controller can supply, like 12V. You'll also need an opto coupler to separate the dc from ac. A ssr is a good solution if you can find one that can handle the load, often the rating is higher than what it can actually handle. Use the rating as an absolute 30 sec max, after whi h it should be allowed to cool down to ambient temperature.

If the gate source voltage is too low the mosfet isn't driven open fully, which gives it a higher Rdson, which will consume more power, which is in turn converted into heat and must be dissipated using a proper cooler.

Switching itself also consumes more power, depending how it's driven. If you intend to use PID you'll need to make sure it's driven most efficiently. If it's driven with a impedance that's too high the ssr will switch slowly during which the resistance is high. You could use a small signal transistor or another mosfet to convert the level from 5v to 12v, or 24 depending on what you have available and the ssr can handle.

A zero-crossing DC-AC SSR is the correct solution. I use one in my large delta, see the link in my signature. That SSR-40DA will be more than up to the job, in fact a SSR-25DA with just a small heatsink would do. The voltage drop will be between 1V and 2V, so the power dissipation in the relay will be 12W or less.

---

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].

Hi, and thanks for the replies. So it is advised that I do not use the MOSFET relay? I dont mind the SSR-40DA, it's very cheap and I would rather it be more than man enough for the job.

A simple mosfet is not suitable for switching AC mains, for two reasons: it does not provide isolation between the input and output, and it can't switch AC because in the reverse direction it conducts regardless of the gate voltage. Use a DC/AC SSR.

Be sure to take appropriate safety precautions when using AC mains. In particular:

- Any metal parts in the vicinity of mains wiring must be connected to mains ground. This includes the bed and the metal frame of your printer, assuming it has one.

- If the bed will be moving, use appropriately-rated (voltage and current) highly flexible cable to connect the bed heater to the non-moving part of the printer, and provide strain relief at both ends of the cable run. The type of cable used for multimeter tests leads should be suitable.

- Use an RCD-protected plug or socket to connect the printer to the mains.

---

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].

Sure you know this part, but make sure to adjust the firmware settings for the bed heater; like in Marlin Bang bang seems to work the best unless you spend the bucks to get a super fast switching SSR.

Quote
dc42
A simple mosfet is not suitable for switching AC mains, for two reasons: it does not provide isolation between the input and output, and it can't switch AC because in the reverse direction it conducts regardless of the gate voltage. Use a DC/AC SSR.

Be sure to take appropriate safety precautions when using AC mains. In particular:

- Any metal parts in the vicinity of mains wiring must be connected to mains ground. This includes the bed and the metal frame of your printer, assuming it has one.

- If the bed will be moving, use appropriately-rated (voltage and current) highly flexible cable to connect the bed heater to the non-moving part of the printer, and provide strain relief at both ends of the cable run. The type of cable used for multimeter tests leads should be suitable.

- Use an RCD-protected plug or socket to connect the printer to the mains.

Ok i understand now thanks for explaining that, yeah i need to ground the printer frame, its 40 x 40 T-Slot Extrusion, this can be grounded using the ground in the Mains Supply for the Bed? The bed comes with quite a long stretch of cable i believe, failing that i have a roll of Thick Immersion Heater Wire, its like Hot End Heater Cartridge Cable but much thicker.

Quote
AquaticsLive
Sure you know this part, but make sure to adjust the firmware settings for the bed heater; like in Marlin Bang bang seems to work the best unless you spend the bucks to get a super fast switching SSR.

No that was a great mention thanks, im sure this is the reason my last SSR melted, i thought any SSR could run PID but most cheap ones struggle i think, the MOSFET uses PID fine, but i will switch back to Bang Bang when using the SSR.

Zero-crossing DC/AC SSRs can handle slow PWM (e.g. 1Hz frequency) just fine. Fast PWM won't work properly, but won't damage them either. But bang-bang control is normally good enough for a heated bed.

The main problem with cheap DC-DC SSRs is that they drop around 2V, and if you are switching around 20A of current, that's 40W of heat to get rid of, which isn't easy. You can get DC-DC SSRs with much lower voltage drop, but they are expensive.

---

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].

Hi guys, Now my 450 x 450 silicone heat bed has arrived from AliExpress, it was shipped saturday and arrived today so really good going i highly recommend that supplier.

Here's my next questions, as the heat bed is 240v i was expecting it to have an earth cable, it doesn't. It only has the Pos & Neg wires, and wires for the thermistor. How does this work when wiring to mains?

I thought that the wiring would go as follows:

AC Mains Live to the SSR Output +
SSR Output - to the Bed +
Bed - to AC Mains Neutral
Bed Gnd to Mains Gnd
Printer Frame to Mains Gnd

Now i think it will have to be like this:

AC Mains Live to the SSR Output +
SSR Output - to the Bed +
Bed - to AC Mains Neutral
Printer Frame & Bed Carriage to Mains Gnd

Can anyone offer some advice on this? I obviously don't want to try without grounding properly.

Thanks

Alex

Edited 1 time(s). Last edit at 09/21/2015 06:28AM by xelad.

I assume you will stick the heater on the underside of an aluminium plate that will serve as the bed or bed support. Connect mains ground to the aluminium plate, and to any any other metal parts in the vicinity of the bed heater or the mains wiring.

You should use a cable tie on the end of the cable near where you connect it to the SSR etc. so that the cable doesn't pull on the connections. Here's a photo of how I wired mine. I use a terminal block to connect the wires to/from the mains inlet, SSR, bad heater and power supply. After taking this photo, I rerouted the thermistor wires to keep them away from the SSR and other mains wiring. In my case, the aluminium bed plate is supported by metal pillars on the printer frame, and I grounded the printer frame. So the plate is grounded via the pillars and the frame.

For additional safety, I recommend you power the printer via a RCD-protected plug or socket.

Be sure to connect the SSR input wires to the electronics bed heater output with the correct polarity, otherwise you may damage the SSR.



Edited 1 time(s). Last edit at 09/21/2015 06:32AM by dc42.

---

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].

Hi dc42 and thanks for the reply, so the wiring should be done as i thought, thanks for clearing that up. I am at a bit of standstill on my bed material at the moment, the printer i have built is based on the gMax, i have attached a picture:



The Y Axis is a moving carriage, the parts that hold the V Wheel to the carriage are printed, so i need reduce the amount of heat transferring to them (otherwise they deform and the carriage stops working properly).

So the assembly of the carriage should go (from the bottom up):

- H Frame 20 x 20 V-Slot With V-Wheels.
- Some form of Rigid Sheet that will not warp when heated to 110c+ and stop heat transferring to the H Frame Carriage. (This is the part i dont know what to do about)
- 450 x 450mm Silicone Heater
- 4mm Toughened Glass.

I have a sheet of 12mm Lexan (polycarbonate), however im not sure if this will deform when heated to 110c, i had thought about using an aluminium plate but don't want the heat transferring to the carriage.

Materials i have thought about are Acetal (Delrin), Polycarbonate, Bakelite (Maybe this is the best option?) etc.

Any help would be greatly appreciated.

Thanks

Alex

Edited 1 time(s). Last edit at 09/21/2015 10:08AM by xelad.

The heater should be attached to a thermally-conductive material, to ensure uniform heating. If you use an aluminium tooling plate a little larger than the heater, you can support it at the edges using metal standoff pillars, and these will carry only a small amount of heat to the frame. Use thermal insulation (e.g. cork) underneath the heater.

As the bed will be moving in the Y direction, you need a very flexible cable to connect the moving heater to the stationary part of the printer. I suggest you do not use the cable attached to the heater for this purpose, as it is unlikely to be designed for continuous flexing. So attach a terminal block to the Y carriage, and run 3 flexible single-core cables from there to a terminal block on the stationary part of the printer. Use strain relief at both ends of the flexible cables. A suitable cable would be the type used for multimeter test leads, which is readily available.

---

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].

Thanks for the advice! I have bought a sheet of Tufnol/Bakelite to sit between the H Frame and the silicone heater. I got the heater wired up and going last night; it heats up really really quick. It got to 110c in about 35 seconds, and considering it's size i thought that was pretty good going!

The cabling and relay stayed perfectly cool when i left it to sit at 110c for about an hour.

Heres the next question, the bed is super powerful and usually overshoots its target temp by about 15c when first heating, then by about 10c when cycling around the target temp. I'm currently using bang bang, can PID be used with an SSR & AC or is it not recommended?

Thanks

Alex

You can use PID with a low PWM frequency (about 1Hz) with a DC/AC SSR, Normal Arduino PWM at ~400Hz won't work properly.

Quote
xelad
Thanks for the advice! I have bought a sheet of Tufnol/Bakelite to sit between the H Frame and the silicone heater. I got the heater wired up and going last night; it heats up really really quick. It got to 110c in about 35 seconds, and considering it's size i thought that was pretty good going!

The cabling and relay stayed perfectly cool when i left it to sit at 110c for about an hour.

Heres the next question, the bed is super powerful and usually overshoots its target temp by about 15c when first heating, then by about 10c when cycling around the target temp. I'm currently using bang bang, can PID be used with an SSR & AC or is it not recommended?

Thanks

Alex

---

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].

OK so i'll stick Bang-Bang then, thanks for all the help mate, really appreciate it.