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Complete heated bed kit for Ultimaker

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Posted · Complete heated bed kit for Ultimaker

hi

I have the 2mm JasonHK heated bed up and running, so far no problems. Before I used a heated bed I often let the printer print large model (10h or more printing time) unattended, I wasn't even at home. Haven't done it yet with the heated bed, I don't like the idea of having a 250W heater that depends on a relay and the readout of a single thermistor and let it unattended.

So I would like to add like a thermal fuse or thermal switch. Anybody else done that? Other ideas? Or am I too paranoid (got an aeronautical background, used to have redundancy ;)).

 

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Posted · Complete heated bed kit for Ultimaker

@polar, exactly my situation.

What I did to counter that:

- Installed a "main" switch that cuts the wall power and has also a fuse. I only power it if I require it.

- as you could read in my experiance, my relay got messed up. I replaced it with a MosFET. To be honest, I have no clue what happens if a MosFet blows (open or closed) but I guess it can be both.

Thermal switch / fuse is a good idea. I will also look into that.

 

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Posted · Complete heated bed kit for Ultimaker

MosFETs can generally become unpredictable if they fail.

Generally, there shouldn't be an electrical problem here, because even if the MosFET fails and is 100% on, you still have a fixed resistance (the heater) that will prevent short-circuits.

Plus, pretty much all switching power supplies have over-current protection.

But there can be a thermal problem if your heater is left 100% on all the time. The ideal thing would be something that does not rely on a mictrocontroller or computer to work, for example a bi-metal switch operating a thyristor (mains switch). But I don't know if there are bi-metal switches for these temperatures that you can mount to the heated bed (and that are affordable).

Another thing I would put a certain trust into would be an OP-Amp switch reading a thermistor. Purely analog circuit that will, if done correctly, switch off the mains in every case of failure.

It takes some time to design something like that, but I may have a look at it (I'm designing an UM1 replacement mainboard anyways...).

/edit:

TI has an interesting device: LM27

Guess I'll have to look into that one. It's basically a thermal switch that will activate an output when it measures a temperature above 140°C (there are mulltiple temperature options).

 

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Posted · Complete heated bed kit for Ultimaker

a main switch for the psu I already have, but this is of course only useful when the printer is not printing :)

the LM27 looks interesting, but as I am not an electronics guy I would rather go for a pure "mechanical" soultion like a bimetall-switch that switches directly the 24V.

I found switches in an acceptable temperature range e.g. from ESKA, also rated at at large enough currents, but this rating is only specified for 120/240 VAC. For DC it would probably not work, the switch would not "separate" but rather "stick" ?

 

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Posted · Complete heated bed kit for Ultimaker

probably burnt it's contacts together (how do you say that in proper english?)

 

Welded.

Welded the contacts together.

 

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Posted · Complete heated bed kit for Ultimaker

a main switch for the psu I already have, but this is of course only useful when the printer is not printing :)

the LM27 looks interesting, but as I am not an electronics guy I would rather go for a pure "mechanical" soultion like a bimetall-switch that switches directly the 24V.

I found switches in an acceptable temperature range e.g. from ESKA, also rated at at large enough currents, but this rating is only specified for 120/240 VAC. For DC it would probably not work, the switch would not "separate" but rather "stick" ?

 

What would such a solution cost?

The beauty about the LM27 is, it costs pretty much nothing :)

Sure, you need a PCB and some small components, so in the end you're somewhere near 20 Euros. It depends on how many PCBs you order...

 

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Posted · Complete heated bed kit for Ultimaker

maybe a stupid question, but why did we all go for DC to heat the bed? ok, DC PSUs are easier to find but using AC the relay would not "weld" so easily and tons of thermal fuses would be available. Only negative effect I can think of is EMI, but again I am not an electrics/electronics guy, maybe there is a very obvious reason to go for DC :)

 

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Posted · Complete heated bed kit for Ultimaker

I'd rather just throw all the relays away and simply use MosFETs :)

As long as you're not talking 110-240VAC, then there shouldn't be any problem with using AC. Just please don't use 240V heaters, this is dangerous...

 

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Posted · Complete heated bed kit for Ultimaker

nono, not 240VAC, I don't like that near a metalplate I have to touch sometimes :)... would be much lower like 20VAC.

throwing the relays away and use mosfets is a good idea, however this will only solve the "stuck relay" problem, but not the "failed thermistor" or a potential the "software" problem :)

 

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Posted · Complete heated bed kit for Ultimaker

@tsp42 hmmmm...could be a good solution. Very easy and cost effective. Could be mounted to the underside of the alloy bed? How? Also I guess the R is very low to 0 / so no waste of energy. Could be just in series to the load. Don't see a problem.

I don't like that it's single use (but I guess not really a problem as it would only trigger when something goes really wrong).

 

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Posted · Complete heated bed kit for Ultimaker

A thermostat temperature switch would be a permanent safety measure. I think I'd rather have it be one time thing though, wouldn't want the bed turning back on after an issue without figuring out said issue first.

You could even use one of http://www.ebay.com/itm/NC-Thermostat-Temperature-Switch-Bimetal-Disc-150-x10-/130579778170?pt=LH_DefaultDomain_0&hash=item1e67294a7a as a (terrible) set temperature controller directly.

 

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Posted · Complete heated bed kit for Ultimaker

@martin bienz: Kapton tape might do the trick for mounting as long as the glue fails after the fuse. Heating of the fuse would go to the heated bead so it is not wasted but it may lower the current going though the heated bed but I don't expect the resistance to be very high. The tricky part is to figure out what the cutoff temperature should be.

Another question. My cheap Chinese 24V PSU only goes down to 22V with no load. Do you think that the voltage is lower when the load increases or should I use a step-down voltage converter for powering the ultimaker to be on the safe side.

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Posted · Complete heated bed kit for Ultimaker

@tsp42: this is a AC fuse, no guaranteed to work on DC, as I wrote a few posts earlier. AC fuses are easy to find, DC I am still looking for a good one.

http://il.mouser.com/ProductDetail/TE-Connectivity-Raychem/RTP200R060SA-2/?qs=n8MrIBhnc4RacQBDqxfQ7Q==

This would be ok as far as DC, amps are concerned, but it goes at at too high temperature... still looking :)

 

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Posted · Complete heated bed kit for Ultimaker

 

@Polar: From what I can see it is just a simple fuse where the conducting wire melts at the given cutoff temperature so it should work with DC.

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Posted · Complete heated bed kit for Ultimaker

180°C or 200°C is too high. 150°C sounds like a good point for "single use" fuses. I'd even lower it to 130°C for a solution that can be used continuously.

Seems like there are multiple possibilities (no particular order):

1. Thermal "melting" fuse. Simple to use, but needs replacement when triggered (which should never occur, so no big deal).

2. Bi-metal thermostat switch. Should be simple to use, too, and doesn't get damaged when triggered. Probably the best solution..

3. Integrated thermal switch (LM27 or similar). Needs circuit board design, but should give a nice result which does not interfere with the heated bed in any way (no serial connection). As an electronics guy, I prefer this one :)

4. Did I forget something?

Would be nice to see the solutions compared against each other (price, ease of use, effectivity, reliability...).

 

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Posted · Complete heated bed kit for Ultimaker

I would also go for way below 150°. In my case I would even go to 100° that's plenty hot still. Or even lower as I am not planing to print ABS (but maybe XT @ 70-80 I think).

I will probably go for 1 as (not beeing toooo paranoid) I really don't want the bed to rheat up again after the bi-metal thermostat cools down and re-enables the current.

Yesterday I found a source but had no time to look at details yet:

http://www.setfuse.com/product/over-temperature-protection/thermal-cutofftco/radial-shape/st-series.html

Could this be the way to go ?

Also, still remains how to mount it to the underside of the bed. Any ideas? Probably hot glue is not the way to go.

I WILL GO FOR 3. IF Jonny designs the circuit! :mrgreen:

 

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Posted · Complete heated bed kit for Ultimaker

@tsp42:

DC and fuses seems quite difficult, look at how they have to design DC fuses, often springloaded and other tricks. the "simply melts" is not working as far as I was told.

@Johnny+Martin: As I wrote, the one from Raychem triggers at too high temperature. I was aiming at 110-130C for triggering (which again is very simple to find for AC but not DC). Maybe I just buy a couple of them and do some testruns.

 

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Posted · Complete heated bed kit for Ultimaker

@Polar: the AC fuses may fail earlier due to the alloy inside the fuse when used with DC(1) but it is better than they don't triggers when reaching the cutoff temperature. Some of the fuses are rated for both AC and DC and it looks like the max. amps increases(2) when going from 250VAC to 50VDC.

1) http://www.google.com/patents/EP1583125A1?cl=en

2) http://www.cci-tco.com/elcut-22-series-rohs-axial

http://www.setfuse.com/product/over-temperature-protection/thermal-cutofftco/axial-shape/r-series.html

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Posted · Complete heated bed kit for Ultimaker

@tsp42

I was told that under DC there is the possibility that you get like an electrical "ark" that is not estinguished.

for the elcut the amps increase, for the setfuse they dont :). But be that as it may, your setfuse r series are rated for 50VDC, so this is ok for me.

Btw, the elcut 22 I also found but could not find a store to purchase them. Now it seems to be the same which the setfuse or do you have a store?

 

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Posted · Complete heated bed kit for Ultimaker

Arcing happens when you put two physical electrical contacts together (for example when you switch a mechanical relay on).

With AC, the arc will be extinguished when the AC sine wave passes zero.

DC is an "always on" current that doesn't periodically change it's direction, so it never passes zero. This means that an Arc can remain much longer without being extinguished.

Personally, I wouldn't run a DC through any physical switching element. Any solution which works around that would be preferable imho ;)

 

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Posted · Complete heated bed kit for Ultimaker

 

@polar: I couldn't found a distributor for setfuse neither so I ordered a SEFUSE SF139E instead. It is a 250 VAC/15A fuse with a 142C threshold. I intent to test if it work with DC current and hope that 24V is low enough voltage to prevent arcing.

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Posted · Complete heated bed kit for Ultimaker

@tsp42: When I google SEFUSE SF139E it shows up with 10A?

But never mind, I will try THERMODISC 16A 250V, 121C. The manufacturer writes the following in the datasheet:

Direct Current (DC) Applications

MICROTEMP® thermal cutoffs do not have published electrical ratings for direct current (DC)

applications. Current interruption capacity in DC circuits is highly application sensitive.

Therm-O-Disc recommends thorough testing of DC electrical applications using the testing

guidelines in Therm-O-Disc’s MICROTEMP® thermal cutoff technical information section.

So I will do some testing.

 

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Posted · Complete heated bed kit for Ultimaker

@polar: I couldn't found a distributor for setfuse neither so I ordered a SEFUSE SF139E instead. It is a 250 VAC/15A fuse with a 142C threshold. I intent to test if it work with DC current and hope that 24V is low enough voltage to prevent arcing.

 

It depends on the physical distance that is made with an "open" switch. 24V actually arcs pretty awfully - I've arc-welded some test probes together before :p (24V, 10mA max.)

 

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Posted · Complete heated bed kit for Ultimaker

I did my tests with the thermodisc 16A 104C. As they are cylindircal I machined a small aluminium part to have a good heat conductivity. To fix it to the heated bed I used on of the 3mm holes at the corners. Between the bed and the alu part I put some thermal paste. Around the fuse a little Kapton tape as the fuse-casing is electrically conductive and "connected" to the power (dont know why, found this rather strange). Between alu and Kapton again some thermal paste, cant hurt ;)

I blew several fuses on purpose. I monitored the temperature given by the thermistor and also measured the temp as close to the fuse as possible with a thermocouple.

Cutoff themp was always between 104 and 106C at the fuse, which corresponded to a bed temp of 130C, starting form a 70C bed temp (which was is like 65-68 at the fuse). My bed heats up quite quickly due to the 320W PSU. This explains the large difference between the center temperature and the temperature at the fuse, it takes some time for the heat to heat up the aluminium part and "flow" to the fuse :)

All in all it worked perfectly and I consider it to be a good safety system. (I am not happy with the cable routing, but I will change that soon. Also another connector, 90degree or something totally different) :)

2014-07-2617.57.57.jpg

2014-07-2717.43.54.jpg

 

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