Aluminium Heat Bed

I have just changed my old reprap "printed circuit" type heat bed for an aluminium reprap type. It seems well made and a lot more rigid. Nice and flat. However it takes an age to heat up, like 30 mins+ to reach 110 deg C.
Is this normal? I am using 12v on pin 1 and pins 2 and 3 are earthed together. I was thinking about recessing the hole for the thermistor so it fits in a bit better. The spring mounts for the bed are also running hot. I was thinking of fitting fibre washers to insulate against heat losses. Any thoughts?

What power supply are you using? If it is a LED-type power supply, you can turn up the voltage to 13 or 14V to get more heating power. Be careful of doing this if you are using Arduino/RAMPS electronics and have an LCD screen attached, because the voltage regulator on the Arduino will already be running hot, and increasing the supply voltage will make it run hotter.

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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].

Increasing the voltage should get it to heat up faster, but be careful if the power supply is near its current limit. If it's good for 20A at 12V it will only be good for 17A (or less) at 14V. If you exceed the designed power limit it will die quickly. In my experience, if the power supply's cooling fan turns on, you had better turn the voltage back down because the supply won't last long if it runs hot enough to activate the fan.

Before tweaking the supply voltage try running the PID autotune on the new bed. It may be slow to heat up because it is using the old bed's PID constants.

Edited 1 time(s). Last edit at 03/01/2015 10:17PM by the_digital_dentist.

Dave3D, can you tell us which aluminium bed you bought ?
If it's got a higher than expected heater resistance it may be useful to know before buying one if somebody is using 12V primary power (or 13.8V in my case).

If your power supply can handle it and your board is well ventilated, check the power setting in your firmware and raised up to full 255.

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ggherbaz
If your power supply can handle it and your board is well ventilated, check the power setting in your firmware and raised up to full 255.

Most 3D printer firmware drives the heated bed in bang-bang mode, so the 255 max PWM setting isn't relevant.

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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].

If the controller runs on Marlin or Smoothieware (and probably others), PID control is an option that should be turned on. Bang-bang control allows relatively large swings in the bed temperature that will cause artifacts in the Z axis of the print.

Would it be possible to put a DC 12v to 24v stepup in line to the heat bed? One like this? ebay One working: Youtube

Ehhh....

- Are there any directed air currents hitting the aluminum bed while it's heating up?

Aluminium is a great thermal conductor, but if there's fan circulating air onto the plate, the air is going to absorb some of that heat. This actually can be a significant loss of heat, as evidence by the aluminum radiator in a car. Make sure fans are pointed away or turned off while things are warming up. Even if you have a huge powersupply, keeping fans turned while you're waiting for things to heat up is self defeating.

- Where is the printer at?

Related to the above point, if you're in a room that is cold, like a basement or garage, the air hitting the plate is going to have a lower temperature and have a greater impact on rate of heating. If this is the case, you might try covering the aluminum build plate with an insulating covering while it's heating up. An old glass build plate, or an oven mit (provided both are clean), would allow the build plate to heat up more quickly and can be easily removed before printing starts.

Have you measured the resistance of the bed? Most heaters made for 12V are 1ohms or less. If it is anything more than that, it would take longer to heat up. You could try putting fiberglass cloth underneath it to prevent heat from escaping.

Thanks for all your replies. I will try and answer all the points raised. I have also done some further electrical tests.

The power supply is a standard Reprap 12v 30A switching power supply that came with the printer kit, an OrdBot. To my knowledge the voltage is not adjustable, at least not from the outside. It maybe if I take it apart. I measured the no load voltage at the bed and it is 12.5v. I do not wish to increase it. I have a history of damaging Arduino boards!

My new heat bed is a Reprap Mk3

[www.ebay.co.uk]

I am controlling the bed via a solid state 40A relay. It is just on/off control. I am not using PID. I have used it for a long time satisfactorily with my other heat bed. The only thing that has changed is the aluminium bed. I have my printer in my living room, typically 20 deg C. There are no strong draughts. I have a small fan on the extruder but it was turned off for these tests. I have fitted the heat bed upside down with thermal insulation underneath. The printer bed mounting plate underneath that does not get hot. However as mentioned the levelling springs get too hot to touch. They are in contact with the aluminium bed. I need to try fitting small fibre washers both sides.

I measured the resistance of the bed (cold) at 1.3 ohms. I fitted an ammeter in series with the bed and got the following results


Time Amps Temp deg C
start 8.37A 20C
1min 7.75A -
2min 7.5A -
3min 7.25A 65C
4min 7.13A 72C
5min 7.08A 75C
6min 6.94A 81C
7min 6.87A 84C
8min 6.82A 88C
10min 6.73A 93C
18min 6.62A 100C

sorry can't do tables

Because it was taking so long to heat up, the heater cut out at just over 100 deg C with the error message "HEATER DECOUPLED". This is a safety feature in the Repetier firmware. I could have reset it and carried on. The above results were without a glass plate or any cover over the aluminium heat bed. As can be seen, the current drops off as the bed heats up, presumably as the resistance alters with temp. I checked the voltage under load when it was at 70 deg C and got 11.31v.

I also checked the bed temp with an IR gun, which gave a reading 5 to 6 deg C lower, I think because of the reflectivity of the shiny aluminium surface.

I was thinking of getting a 24v power supply unit to power just the bed. Not sure what size but twice the voltage would be twice the amps I think. I will also try insulating the springs, there is heat loss there and I will re check my wiring and solder connections.

Quote
dave3d
Thanks for all your replies. I will try and answer all the points raised. I have also done some further electrical tests.

The power supply is a standard Reprap 12v 30A switching power supply that came with the printer kit, an OrdBot. To my knowledge the voltage is not adjustable, at least not from the outside. It maybe if I take it apart. I measured the no load voltage at the bed and it is 12.5v. I do not wish to increase it. I have a history of damaging Arduino boards!

My new heat bed is a Reprap Mk3

[www.ebay.co.uk]

I am controlling the bed via a solid state 40A relay. It is just on/off control. I am not using PID. I have used it for a long time satisfactorily with my other heat bed. The only thing that has changed is the aluminium bed. I have my printer in my living room, typically 20 deg C. There are no strong draughts. I have a small fan on the extruder but it was turned off for these tests. I have fitted the heat bed upside down with thermal insulation underneath. The printer bed mounting plate underneath that does not get hot. However as mentioned the levelling springs get too hot to touch. They are in contact with the aluminium bed. I need to try fitting small fibre washers both sides.

I measured the resistance of the bed (cold) at 1.3 ohms. I fitted an ammeter in series with the bed and got the following results


Time Amps Temp deg C
start 8.37A 20C
1min 7.75A -
2min 7.5A -
3min 7.25A 65C
4min 7.13A 72C
5min 7.08A 75C
6min 6.94A 81C
7min 6.87A 84C
8min 6.82A 88C
10min 6.73A 93C
18min 6.62A 100C

sorry can't do tables

Because it was taking so long to heat up, the heater cut out at just over 100 deg C with the error message "HEATER DECOUPLED". This is a safety feature in the Repetier firmware. I could have reset it and carried on. The above results were without a glass plate or any cover over the aluminium heat bed. As can be seen, the current drops off as the bed heats up, presumably as the resistance alters with temp. I checked the voltage under load when it was at 70 deg C and got 11.31v.

I also checked the bed temp with an IR gun, which gave a reading 5 to 6 deg C lower, I think because of the reflectivity of the shiny aluminium surface.

I was thinking of getting a 24v power supply unit to power just the bed. Not sure what size but twice the voltage would be twice the amps I think. I will also try insulating the springs, there is heat loss there and I will re check my wiring and solder connections.

DC solid state relays typically have a 1-1.6 volt drop on them this could be the problem measure the on-load voltage at the heatbed connections and see what you are losing in the SSR.

dougal1957: under load from cold I I got 11.19v at the bed. At the SSR I measured 11.93v on supply side output terminal and 11.49v on the load (bed) side. Maybe I need to beef up the bed wiring.

Quote
dave3d
dougal1957: under load from cold I I got 11.19v at the bed. At the SSR I measured 11.93v on supply side output terminal and 11.49v on the load (bed) side. Maybe I need to beef up the bed wiring.

Possibly What are you using.

Must admit I have had 2 Aluminium bed's now and haven't been happy with either and have reverted to a PCB One new build is having AC fed Heaters at much more wattage (220V AC 500 watts on 300mm diam and 220V 600 watts on a 400mm x 400mm heater fed with Zero Crossing SSR's.

Doug

I have one of those beds (from a different supplier). It doesn't get hot enough because the resistance is too high. It should be 1.1 - 1.2R. Worse than that, the two halves don't heat evenly when wired in parallel. I plan to try series with a 37V supply.

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[www.hydraraptor.blogspot.com]

It seems other people have had the same problem: [forums.reprap.org]

There also appears to be different types of Mk3 heat bed. This is the one I have: [www.ebay.co.uk]

and there is a Wiki which shows a slightly different one: [reprap.org]

On mine, both earth wires are joined for 12v. The one on the Wiki has the +12v joined together. Because the circuit is bonded or printed onto a solid aluminium base the temp should be even across the bed I think. Aluminium is a very good heat conductor.

I am now thinking about switching to 24v for the bed. The Wiki says it will heat up to 100 deg C in 2 mins! which would be great. Combined with a 12v step down voltage controller for the Arduino/Ramps boards maybe. Anyone tried this?

AL is a good conductor but thermal conductivity is never in the same league as electrical. It is easy to get a difference from one edge to the other when it is only 3mm thick. I recommend checking it heats evenly when wired in parallel as mine does not.

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[www.hydraraptor.blogspot.com]

Quote
dave3d
It seems other people have had the same problem: [forums.reprap.org]

There also appears to be different types of Mk3 heat bed. This is the one I have: [www.ebay.co.uk]

and there is a Wiki which shows a slightly different one: [reprap.org]

On mine, both earth wires are joined for 12v. The one on the Wiki has the +12v joined together. Because the circuit is bonded or printed onto a solid aluminium base the temp should be even across the bed I think. Aluminium is a very good heat conductor.

I am now thinking about switching to 24v for the bed. The Wiki says it will heat up to 100 deg C in 2 mins! which would be great. Combined with a 12v step down voltage controller for the Arduino/Ramps boards maybe. Anyone tried this?

Wouldn't that be a step up? 12v to 24v. I think if it was done downline from the transformer and the Ramps board it wouldn't be any extra pull as the step up is doing all the work. The draw from the board/power supply shouldn't be any different.

I think he means to get a big 24V supply to power the bed and then a smaller 24VDC- 12VDC converter to power the controller and motors.

Quote
the_digital_dentist
I think he means to get a big 24V supply to power the bed and then a smaller 24VDC- 12VDC converter to power the controller and motors.

Oh, ok. That makes sense. Thanks

I advise against powering a 12V heat bed from 24V. You will be running it at four times its design power. I suggest 14V which will give 36% more power, or 15V which will give 56% more power.

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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].

My aluminium bed is dual voltage and is designed for operating at both 12v and 24v. Connections are different depending on voltage chosen. It seems better to me to run it properly at 24v rather than putting 15v through 12v connections.

digital dentist is right. If I got a 24v power supply, the 24v bed would take most of the current, leaving a much smaller current requirement for the Arduino/Ramps boards and hotend which could be supplied with 12v from a small step down converter. Pity they don't do a dual voltage power unit in the first place for 3d printers.

If you put 24V on the 24V connections it wont get any hotter than 12V on the 12V connections (other than reducing the connection and MOSFET losses). It might heat more evenly though. You would need 30V to get the same heat as 15V on 12V connections.

Edited 1 time(s). Last edit at 03/04/2015 09:04AM by nophead.

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[www.hydraraptor.blogspot.com]

Running a 12V heater from a 24V supply will dissipate 4X the power at 100% duty cycle (maximum PID duty cycle setting of 255). If you set the duty cycle to 25% by using a maximum setting of 63 you'll get the equivalent power dissipation of a 12V supply. Since it is taking too long to heat at 12V, you can set the maximum duty cycle to maybe 70 or 75 to increase the average power to the bed. PID control is a very good and useful thing to have and use.

Quote
the_digital_dentist
PID control is a very good and useful thing to have and use.

I don't entirely agree with you in respect of the heated bed. There are a few issues with using PID control of the heated bed:

1. The power dissipation in the controlling mosfet will be greater, because of finite on/off switching times and/or avalanching. How much greater will depend on the PWM frequency and the mosfet driver circuit. For some controller boards, this may cause excessinve heating of the mosfet.

2. Using PID will result in increased EMI generation from the heated bed and associated wiring.

3. No heated bed driver circuit that I have seen a schematic for includes a flyback diode. But heated beds and the associated wiring may be significanty inductive. As a result, the driver mosfet is likely to avalanche on every PWM cycle, as is evident here (measured on an Ormerod with a 200x210mm 12V 1.2 ohm heated bed):



Note the jagged top of the 50V approx. peak voltage, indicating that avalnaching is taking place, and the ringing that occurs after avalanching due to the inductance and capacitance of the circuit.

Power mosfets generally have a repetitive avalanche rating (as well as a higher non-repetitive avalanche rating), but is the avalanche energy within that rating? Speaking as professonal electronics engineer, I can't recommend using PWM to control the bed unless the amount of avalanching is measured and found to be well within the limits of the mosfet, or precautions are taken to avoid it, and preferably to limit the amount of EMI generated as well.

PID control of the bed is only of benefit if the bed heater is powerful enough that significant temperature overshoot occurs. With the typical Mk2 or Mk3 heated bed used in diy 3D printers, this is not the case unless they are operated with a higher than normal voltage. But some other heated beds do have significant overshoot, and benefit from PID.

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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].

Marlin software PID is only a few Hz, so not much different in terms of EMI, etc as bang-bang. It does stop the PCB flexing as it can do with bang-bang.

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[www.hydraraptor.blogspot.com]

Quote
Nophead wrote:
If you put 24V on the 24V connections it wont get any hotter than 12V on the 12V connections (other than reducing the connection and MOSFET losses). It might heat more evenly though. You would need 30V to get the same heat as 15V on 12V connections.

My aluminium bed has connections for a 12v supply and connections for a 24v supply. One would assume it is constructed with two parallel heating circuits etched into the board. When on 12v only one circuit is used with the other redundant. On 24v both are used in parallel so twice the current and twice the heat generation rate.

Quote
the digital dentist wrote:
Running a 12V heater from a 24V supply will dissipate 4X the power at 100% duty cycle

The heat generated is directly proportional to the current. Twice the voltage equals twice the current equals twice the heat generated. Twice the current through a circuit designed for 12v maybe overloading it.

Quote
dc42 wrote:
PID control of the bed is only of benefit if the bed heater is powerful enough that significant temperature overshoot occurs. With the typical Mk2 or Mk3 heated bed used in diy 3D printers, this is not the case unless they are operated with a higher than normal voltage. But some other heated beds do have significant overshoot, and benefit from PID.

Simple on/off control worked well on my old heat bed. There was a bit of initial overshoot but it settled down to very close control around the set point. There wasn't much cycling. This was no doubt due to the thermal inertia of the system - heating the bed and 6mm glass. I did not see any need for PID control and I subsequently modified it by adding a SSR. It worked very well.

However if the heat output of the new aluminium bed is significantly increased and I did away with the glass plate, then I admit temperature cycling could be a problem.

Quote

The heat generated is directly proportional to the current. Twice the voltage equals twice the current equals twice the heat generated.

No the heat generated is directly proportional to power, which is equal to current * voltage and current ^ 2 * resistance or voltage ^ 2 / resistance.

Edited 1 time(s). Last edit at 03/04/2015 05:45PM by nophead.

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[www.hydraraptor.blogspot.com]

Your scope photo is impressive, but the avalanche is over in under 250ns and the ringing voltage has dropped below 20Vpk in under 1 us. The FETs in the RAMPS board are rated for 50A continuous and the SOA curve shows that they can handle >100A for 100 us (that's 400x the duration of the avalanche in your scope image). I don't think avalanche is going to be an issue. Marlin PID operates at a maximum of about 7Hz, plenty of time for the die to cool between those brief, in-spec avalanche events. SmoothieWare PID operates at a much higher frequency by default, so it is slightly more possible for it to cause a problem for the FET, but you can easily turn down the frequency with a config.txt variable if you're worried about it or if your FET heats up and you don't want to put a heat sink on it. I'd be more concerned about burning up the PCB traces than the MOSFET.

I have never heard of anyone complaining about EMI from a 3D printer. I'm not saying it doesn't exist, I just haven't heard of anyone complaining about it. If I were trying to make a product to sell to consumers and had to pass UL, CSA, FCC, and/or TUV certification I might be concerned. If the EMI interfered with my wife's ability to watch Gilligan's Island reruns because it interferes with the TV or wifi signal I would be concerned about it. If EMI affected print quality I would definitely be concerned about it. But I'm not making a commercial product and it doesn't do either of those other things, so I am not concerned about it. I doubt anyone else is either.

If you want optimum print quality you have to eliminate (ideal) or reduce (real life) any uncontrolled motion in the extruder carriage relative to the print bed. Likewise, wide temperature swings in the bed and hot-end will affect print quality, so it is best to eliminate wide temperature swings. PID control is the way you prevent temperature swings that occur with bang-bang control which degrade print quality. Regulated bed temperature prevents z artifacts that occur when bang-bang is used. My print bed temperature varies by about +/- 1C with PID control and swings about +/-10C with bang-bang as reported by both Marlin and SmoothieWare which are certainly not laboratory grade instrument readings, but they're close enough to tell me what I need to know.

AFAIK, PID control is built into every 3D printer controller board and firmware, and it seems silly not to use it when it's there waiting to be used. Of course you need to pay attention to the current drain and supply voltage, more for the sake of the circuit board than the MOSFET. You can always use an SSR or external MOSFET if you're trying to switch higher currents than your circuit board can handle. I'd suggest people check the recommendations of the manufacturer of their circuit board if they're unable to understand the relevant specs.

Quote
the_digital_dentist
Your scope photo is impressive, but the avalanche is over in under 250ns and the ringing voltage has dropped below 20Vpk in under 1 us. The FETs in the RAMPS board are rated for 50A continuous and the SOA curve shows that they can handle >100A for 100 us (that's 400x the duration of the avalanche in your scope image).

Are you sure that SOA curve is 100A avalanche current for 100us? Please link to the datasheet you found it on.

Quote
the_digital_dentist
Marlin PID operates at a maximum of about 7Hz, plenty of time for the die to cool between those brief, in-spec avalanche events. SmoothieWare PID operates at a much higher frequency by default, so it is slightly more possible for it to cause a problem for the FET, but you can easily turn down the frequency with a config.txt variable if you're worried about it or if your FET heats up and you don't want to put a heat sink on it.

I agree that low frequency PID is a better option than fast PID for the heated bed. It's also compatible with mains-powered heated beds switched by zero-crossing SSRs.

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the_digital_dentist
I have never heard of anyone complaining about EMI from a 3D printer. I'm not saying it doesn't exist, I just haven't heard of anyone complaining about it. If I were trying to make a product to sell to consumers and had to pass UL, CSA, FCC, and/or TUV certification I might be concerned. If the EMI interfered with my wife's ability to watch Gilligan's Island reruns because it interferes with the TV or wifi signal I would be concerned about it. If EMI affected print quality I would definitely be concerned about it. But I'm not making a commercial product and it doesn't do either of those other things, so I am not concerned about it. I doubt anyone else is either.

RepRapPro had to go to a lot of trouble to get their Ormerod 2 kits meeting CE industrial standards for EMI when correctly assembled. My understanding is that it doesn't yet meet consumer CE standards, but with the next iteration of the electronics it may.

Quote
the_digital_dentist
My print bed temperature varies by about +/- 1C with PID control and swings about +/-10C with bang-bang as reported by both Marlin and SmoothieWare which are certainly not laboratory grade instrument readings, but they're close enough to tell me what I need to know.

Now I understand why you are so strongly in favour of PID control for the heated bed! If you get 10C swings with bang-bang control, I think there is something seriously wrong with your heated bed construction. Both my printers use bang-bang. One has temperature swings of +0.7 -0.3C, the other has +0.3 -0.1C. I suspect that your thermistor is not in good thermal contact with the bed, and that you have no heat spreader.

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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].